BPFTRACE(8)

NAME
bpftrace - a high-level tracing language

SYNOPSIS

bpftrace [OPTIONS] FILENAME
bpftrace [OPTIONS] -e 'program code'

When FILENAME is "-", bpftrace will read program code from stdin.

DESCRIPTION

bpftrace is a high-level tracing language and runtime for Linux based on eBPF. It supports static and dynamic tracing for both the kernel and user-space.

EXAMPLES

Trace processes calling sleep

    # bpftrace -e 'kprobe:do_nanosleep { printf("%d sleeping\n", pid); }'

Trace processes calling sleep while spawning sleep 5 as a child process

    # bpftrace -e 'kprobe:do_nanosleep { printf("%d sleeping\n", pid); }' -c 'sleep 5'

List all probes with "sleep" in their name

    # bpftrace -l '*sleep*'

List all the probes attached in the program

    # bpftrace -l -e 'kprobe:do_nanosleep { printf("%d sleeping\n", pid); }'

SUPPORTED ARCHITECTURES

x86_64, arm64, s390x, arm32, loongarch64, mips64, ppc64, riscv64

OPTIONS

  -B MODE

Set the buffer mode for stdout.

Valid values are
    none No buffering. Each I/O is written as soon as possible
    line Data is written on the first newline or when the buffer is full. This is the default mode.
    full Data is written once the buffer is full.

  -c COMMAND

Run COMMAND as a child process. When the child terminates bpftrace will also terminate, as if 'exit()' had been called. If bpftrace terminates before the child process does the child process will be terminated with a SIGTERM. If used, 'USDT' probes will only be attached to the child process. To avoid a race condition when using 'USDTs', the child is stopped after 'execve' using 'ptrace(2)' and continued when all 'USDT' probes are attached. The child PID is available to programs as the 'cpid' builtin. The child process runs with the same privileges as bpftrace itself (usually root).

  -d

Enable debug mode. For more details see the Debug Output section.

  -dd

Enable verbose debug mode. For more details see the Debug Output section.

  -e PROGRAM

Execute PROGRAM instead of reading the program from a file or stdin.

  -f FORMAT

Set the output format.

Valid values are
    json
    text

Note: the json output is ndjson, meaning each line of the streamed output is a single blob of valid json.

  -h, --help

Print the help summary.

  -I DIR

Add the directory DIR to the search path for C headers. This option can be used multiple times. For more details see the Preprocessor Options section.

  --include FILENAME

Add FILENAME as an include for the pre-processor. This is equal to adding '#include FILENAME' at the top of the program. This option can be used multiple times. For more details see the Preprocessor Options section.

  --info

Print detailed information about features supported by the kernel and the bpftrace build.

  -k

Errors from bpf-helpers(7) are silently ignored by default which can lead to strange results.

This flag enables the detection of errors (except for errors from 'probe_read_*' BPF helpers). When errors occur bpftrace will log an error containing the source location and the error code:

    stdin:48-57: WARNING: Failed to probe_read_user_str: Bad address (-14)
    u:lib.so:"fn(char const*)" { printf("arg0:%s\n", str(arg0));}
                                                     ~~~~~~~~~

  -kk

Same as '-k' but also includes the errors from 'probe_read_*' BPF helpers.

  -l [SEARCH|FILENAME]

List all probes that match the SEARCH pattern. If the pattern is omitted all probes will be listed. This pattern supports wildcards in the same way that probes do. E.g. '-l kprobe:*file*' to list all 'kprobes' with 'file' in the name. This can be used with a program, which will list all probes in that program. For more details see the Listing Probes section.

  --no-feature feature,feature,...

Disable detected features, valid values are
    uprobe_multi to disable uprobe_multi link
    kprobe_multi to disable kprobe_multi link

  --no-warnings

Suppress all warning messages created by bpftrace.

  -o FILENAME

Write bpftrace tracing output to FILENAME instead of stdout. This doesn’t include child process (-c option) output. Errors are still written to stderr.

  -p PID

Attach to the process with PID. If the process terminates, bpftrace will also terminate. When using USDT probes, uprobes, and uretprobes they will be attached to only this process. For listing uprobes/uretprobes set the target to '*' and the process’s address space will be searched for the symbols.

  -q

Keep messages quiet.

  --unsafe

Some calls, like 'system', are marked as unsafe as they can have dangerous side effects ('system("rm -rf")') and are disabled by default. This flag allows their use.

  --usdt-file-activation

Activate usdt semaphores based on file path.

  -V, --version

Print bpftrace version information.

  -v

Enable verbose messages. For more details see the Verbose Output section.

TERMINOLOGY
BPF | Berkeley Packet Filter: a kernel technology originally developed for optimizing the processing of packet filters (eg, tcpdump expressions).
BPF map | A BPF memory object, which is used by bpftrace to create many higher-level objects.
BTF | BPF Type Format: the metadata format which encodes the debug info related to BPF program/map.
dynamic tracing | Also known as dynamic instrumentation, this is a technology that can instrument any software event, such as function calls and returns, by live modification of instruction text. Target software usually does not need special capabilities to support dynamic tracing, other than a symbol table that bpftrace can read. Since this instruments all software text, it is not considered a stable API, and the target functions may not be documented outside of their source code.
eBPF | Enhanced BPF: a kernel technology that extends BPF so that it can execute more generic programs on any events, such as the bpftrace programs listed below. It makes use of the BPF sandboxed virtual machine environment. Also note that eBPF is often just referred to as BPF.
kprobes | A Linux kernel technology for providing dynamic tracing of kernel functions.
probe | An instrumentation point in software or hardware, that generates events that can execute bpftrace programs.
static tracing | Hard-coded instrumentation points in code. Since these are fixed, they may be provided as part of a stable API, and documented.
tracepoints | A Linux kernel technology for providing static tracing.
uprobes | A Linux kernel technology for providing dynamic tracing of user-level functions.
USDT | User Statically-Defined Tracing: static tracing points for user-level software. Some applications support USDT.

PROGRAM FILES

Programs saved as files are often called scripts and can be executed by specifying their file name. We use a .bt file extension, short for bpftrace, but the extension is not required.

For example, listing the sleepers.bt file using cat -n:

    # cat -n sleepers.bt
    1 tracepoint:syscalls:sys_enter_nanosleep
    2 {
    3   printf("%s is sleeping.\n", comm);
    4 }

And then calling it:

    # bpftrace sleepers.bt
    Attaching 1 probe...
    iscsid is sleeping.
    iscsid is sleeping.

It can also be made executable to run stand-alone. Start by adding an interpreter line at the top (#!) with either the path to your installed bpftrace (/usr/local/bin is the default) or the path to env (usually just /usr/bin/env) followed by bpftrace (so it will find bpftrace in your $PATH):

    # cat -n sleepers.bt
    1 #!/usr/local/bin/bpftrace
    2
    3 tracepoint:syscalls:sys_enter_nanosleep
    4 {
    5   printf("%s is sleeping.\n", comm);
    6 }

Then make it executable:

    # chmod 755 sleepers.bt
    # ./sleepers.bt
    Attaching 1 probe...
    iscsid is sleeping.
    iscsid is sleeping.

BPFTRACE LANGUAGE

The bpftrace (bt) language is inspired by the D language used by dtrace and uses the same program structure. Each script consists of a preamble and one or more action blocks.

    preamble
    actionblock1
    actionblock2

Preprocessor and type definitions take place in the preamble:

    #include <linux/socket.h>
    #define RED "\033[31m"
    struct S {
      int x;
    }

Each action block consists of three parts:

    probe[,probe]
    /predicate/ {
      action
    }

Probes
    A probe specifies the event and event type to attach too. Probes list.

Predicate
    The predicate is an optional condition that must be met for the action to be executed.

Action
      Actions are the programs that run when an event fires (and the predicate is met). An action is a semicolon (;) separated list of statements and always enclosed by brackets {}.

A program will continue running until Ctrl-C is hit, or an exit function is called. When a program exits, all populated maps are printed (this behavior and maps are explained in later sections).

A basic script that traces the open(2) and openat(2) system calls can be written as follows:

    BEGIN {
            printf("Tracing open syscalls... Hit Ctrl-C to end.\n");
    }
    tracepoint:syscalls:sys_enter_open,
    tracepoint:syscalls:sys_enter_openat {
            printf("%-6d %-16s %s\n", pid, comm, str(args.filename));
    }

The above script has two action blocks and a total of 3 probes.

The first action block uses the special BEGIN probe, which fires once during bpftrace startup. This probe is used to print a header, indicating that the tracing has started.

The second action block uses two probes, one for open and one for openat, and defines an action that prints the file being open ed as well as the pid and comm of the process that execute the syscall. See the PROBES section for details on the available probe types.

  Arrays

bpftrace supports accessing one-dimensional arrays like those found in C.

Constructing arrays from scratch, like int a[] = {1,2,3} in C, is not supported. They can only be read into a variable from a pointer.

The [] operator is used to access elements.

    struct MyStruct {
      int y[4];
    }
    kprobe:dummy {
      $s = (struct MyStruct *) arg0;
      print($s->y[0]);
    }

  Comments

Both single line and multi line comments are supported.

    // A single line comment
    i:s:1 { // can also be used to comment inline
    /*
     a multi line comment
    */
      print(/* inline comment block */ 1);
    }

  Conditionals

Conditional expressions are supported in the form of if/else statements and the ternary operator.

The ternary operator consists of three operands: a condition followed by a ?, the expression to execute when the condition is true followed by a : and the expression to execute if the condition is false.

    condition ? ifTrue : ifFalse

Both the ifTrue and ifFalse expressions must be of the same type, mixing types is not allowed.

The ternary operator can be used as part of an assignment.

    $a == 1 ? print("true") : print("false");
    $b = $a > 0 ? $a : -1;

If/else statements, like the one in C, are supported.

    if (condition) {
      ifblock
    } else if (condition) {
      if2block
    } else {
      elseblock
    }

  Config Block

To improve script portability, you can set bpftrace CONFIG VARIABLES via the config block, which can only be placed at the top of the script before any probes (even BEGIN).

    config = {
        stack_mode=perf;
        max_map_keys=2
    }
    BEGIN { ... }
    uprobe:./testprogs/uprobe_test:uprobeFunction1 { ... }

The names of the config variables can be in the format of environment variables or their lowercase equivalent without the BPFTRACE_ prefix. For example, BPFTRACE_STACK_MODE, STACK_MODE, and stack_mode are equivalent.

Note: Environment variables for the same config take precedence over those set inside a script config block.

List of All Config Variables

  Data Types

The following fundamental integer types are provided by the language. Integers are internally represented as 64 bit signed. If you need another representation, you may cast to the following built in types:
Type | Description
uint8 | Unsigned 8 bit integer
int8 | Signed 8 bit integer
uint16 | Unsigned 16 bit integer
int16 | Signed 16 bit integer
uint32 | Unsigned 32 bit integer
int32 | Signed 32 bit integer
uint64 | Unsigned 64 bit integer
int64 | Signed 64 bit integer

    BEGIN { $x = 1<<16; printf("%d %d\n", (uint16)$x, $x); }
    /*
     * Output:
     * 0 65536
     */

  Filtering

Filters (also known as predicates) can be added after probe names. The probe still fires, but it will skip the action unless the filter is true.

    kprobe:vfs_read /arg2 < 16/ {
      printf("small read: %d byte buffer\n", arg2);
    }
    kprobe:vfs_read /comm == "bash"/ {
      printf("read by %s\n", comm);
    }

  Floating-point

Floating-point numbers are not supported by BPF and therefore not by bpftrace.

  Identifiers

Identifiers must match the following regular expression: [_a-zA-Z][_a-zA-Z0-9]*

  Literals

Integer, char, and string literals are supported.

Integer literals can be defined in the following formats:

    • decimal (base 10)

    • octal (base 8)

    • hexadecimal (base 16)

    • scientific (base 10)

Octal literals have to be prefixed with a 0 e.g. 0123. Hexadecimal literals start with either 0x or 0X e.g. 0x10. Scientific literals are written in the <m>e<n> format which is a shorthand for m*10^n e.g. $i = 2e3;. Note that scientific literals are integer only due to the lack of floating point support e.g. 1e-3 is not valid.

To improve the readability of big literals an underscore _ can be used as field separator e.g. 1_000_123_000.

Integer suffixes as found in the C language are parsed by bpftrace to ensure compatibility with C headers/definitions but they’re not used as size specifiers. 123UL, 123U and 123LL all result in the same integer type with a value of 123.

Character literals can be defined by enclosing the character in single quotes e.g. $c = 'c';.

String literals can be defined by enclosing the character string in double quotes e.g. $str = "Hello world";.

Characters and strings support the following escape sequences:
\n | Newline
\t | Tab
\0nn | Octal value nn
\xnn | Hexadecimal value nn

  Loops

  For

With Linux 5.13 and later, for loops can be used to iterate over elements in a map.

    for ($kv : @map) {
      block;
    }

The variable declared in the for loop will be initialised on each iteration with a tuple containing a key and a value from the map, i.e. $kv = (key, value).

    @map[10] = 20;
    for ($kv : @map) {
      print($kv.0); // key
      print($kv.1); // value
    }

When a map has multiple keys, the loop variable will be initialised with nested tuple of the form: ((key1, key2, ...), value)

    @map[10,11] = 20;
    for ($kv : @map) {
      print($kv.0.0); // key 1
      print($kv.0.1); // key 2
      print($kv.1);   // value
    }

  While

Since kernel 5.3 BPF supports loops as long as the verifier can prove they’re bounded and fit within the instruction limit.

In bpftrace, loops are available through the while statement.

    while (condition) {
      block;
    }

Within a while-loop the following control flow statements can be used:
continue | skip processing of the rest of the block and jump back to the evaluation of the conditional
break | Terminate the loop

    i:s:1 {
      $i = 0;
      while ($i <= 100) {
        printf("%d ", $i);
        if ($i > 5) {
          break;
        }
        $i++
      }
      printf("\n");
    }

  Unroll

Loop unrolling is also supported with the unroll statement.

    unroll(n) {
      block;
    }

The compiler will evaluate the block n times and generate the BPF code for the block n times. As this happens at compile time n must be a constant greater than 0 (n > 0).

The following two probes compile into the same code:

    i:s:1 {
      unroll(3) {
        print("Unrolled")
      }
    }
    i:s:1 {
      print("Unrolled")
      print("Unrolled")
      print("Unrolled")
    }

  Operators and Expressions

  Arithmetic Operators

The following operators are available for integer arithmetic:
+ | integer addition
- | integer subtraction
* | integer multiplication
/ | integer division
% | integer modulo

  Logical Operators
&& | Logical AND
|| | Logical OR
! | Logical NOT

  Bitwise Operators
& | AND
| | OR
^ | XOR
<< | Left shift the left-hand operand by the number of bits specified by the right-hand expression value
>> | Right shift the left-hand operand by the number of bits specified by the right-hand expression value

  Relational Operators

The following relational operators are defined for integers and pointers.
< | left-hand expression is less than right-hand
<= | left-hand expression is less than or equal to right-hand
> | left-hand expression is bigger than right-hand
>= | left-hand expression is bigger or equal to than right-hand
== | left-hand expression equal to right-hand
!= | left-hand expression not equal to right-hand

The following relation operators are available for comparing strings and integer arrays.
== | left-hand string equal to right-hand
!= | left-hand string not equal to right-hand

  Assignment Operators

The following assignment operators can be used on both map and scratch variables:
 | Assignment, assign the right-hand expression to the left-hand variable
<<= | Update the variable with its value left shifted by the number of bits specified by the right-hand expression value
>>= | Update the variable with its value right shifted by the number of bits specified by the right-hand expression value
+= | Increment the variable by the right-hand expression value
-= | Decrement the variable by the right-hand expression value
*= | Multiple the variable by the right-hand expression value
/= | Divide the variable by the right-hand expression value
%= | Modulo the variable by the right-hand expression value
&= | Bitwise AND the variable by the right-hand expression value
|= | Bitwise OR the variable by the right-hand expression value
^= | Bitwise XOR the variable by the right-hand expression value

All these operators are syntactic sugar for combining assignment with the specified operator. @ -= 5 is equal to @ = @ - 5.

  Increment and Decrement Operators

The increment (+`) and decrement (`--`) operators can be used on integer and pointer variables to increment their value by one. They can only be used on variables and can either be applied as prefix or suffix. The difference is that the expression `x+ returns the original value of x, before it got incremented while ++x returns the value of x post increment.

    $x = 10;
    $y = $x--; // y = 10; x = 9
    $a = 10;
    $b = --$a; // a = 9; b = 9

Note that maps will be implicitly declared and initialized to 0 if not already declared or defined. Scratch variables must be initialized before using these operators.

Note ++/-- on a shared global variable can lose updates. See count() for more details.

  Pointers

Pointers in bpftrace are similar to those found in C.

  Structs

C like structs are supported by bpftrace. Fields are accessed with the . operator. Fields of a pointer to a struct can be accessed with the -> operator.

Custom structs can be defined in the preamble.

Constructing structs from scratch, like struct X var = {.f1 = 1} in C, is not supported. They can only be read into a variable from a pointer.

    struct MyStruct {
      int a;
    }
    kprobe:dummy {
      $ptr = (struct MyStruct *) arg0;
      $st = *$ptr;
      print($st.a);
      print($ptr->a);
    }

  Tuples

bpftrace has support for immutable N-tuples (n > 1). A tuple is a sequence type (like an array) where, unlike an array, every element can have a different type.

Tuples are a comma separated list of expressions, enclosed in brackets, (1,2) Individual fields can be accessed with the . operator. Tuples are zero indexed like arrays are.

    i:s:1 {
      $a = (1,2);
      $b = (3,4, $a);
      print($a);
      print($b);
      print($b.0);
    }
    /*
     * Sample output:
     * (1, 2)
     * (3, 4, (1, 2))
     * 3
     */

  Type conversion

Integer and pointer types can be converted using explicit type conversion with an expression like:

    $y = (uint32) $z;
    $py = (int16 *) $pz;

Integer casts to a higher rank are sign extended. Conversion to a lower rank is done by zeroing leading bits.

It is also possible to cast between integers and integer arrays using the same syntax:

    $a = (uint8[8]) 12345;
    $x = (uint64) $a;

Both the cast and the destination type must have the same size. When casting to an array, it is possible to omit the size which will be determined automatically from the size of the cast value.

Integers are internally represented as 64 bit signed. If you need another representation, you may cast to the supported Data Types.

  Array casts

It is possible to cast between integer arrays and integers. Both the source and the destination type must have the same size. The main purpose of this is to allow casts from/to byte arrays.

    BEGIN {
      $a = (int8[8])12345;
      printf("%x %x\n", $a[0], $a[1]);
      printf("%d\n", (uint64)$a);
    }
    /*
     * Output:
     * 39 30
     * 12345
     */

When casting to an array, it is possible to omit the size which will be determined automatically from the size of the cast value.

This feature is especially useful when working with IP addresses since various libraries, builtins, and parts of the kernel use different approaches to represent addresses (usually byte arrays vs. integers). Array casting allows seamless comparison of such representations:

    kfunc:tcp_connect {
        if (args->sk->__sk_common.skc_daddr == (uint32)pton("127.0.0.1"))
            ...
    }

  Variables and Maps

bpftrace knows two types of variables, 'scratch' and 'map'.

'scratch' variables are kept on the BPF stack and only exists during the execution of the action block and cannot be accessed outside of the program. Scratch variable names always start with a $, e.g. $myvar.

'map' variables use BPF 'maps'. These exist for the lifetime of bpftrace itself and can be accessed from all action blocks and user-space. Map names always start with a @, e.g. @mymap.

All valid identifiers can be used as name.

The data type of a variable is automatically determined during first assignment and cannot be changed afterwards.

  Associative Arrays

Associative arrays are a collection of elements indexed by a key, similar to the hash tables found in languages like C++ (std::map) and Python (dict). They’re a variant of 'map' variables.

    @name[key] = expression
    @name[key1,key2] = expression

Just like with any variable the type is determined on first use and cannot be modified afterwards. This applies to both the key(s) and the value type.

The following snippet creates a map with key signature [int64, string[16]] and a value type of int64:

    @[pid, comm]++

  Per-Thread Variables

These can be implemented as an associative array keyed on the thread ID. For example, @start[tid]:

    kprobe:do_nanosleep {
      @start[tid] = nsecs;
    }
    kretprobe:do_nanosleep /@start[tid] != 0/ {
      printf("slept for %d ms\n", (nsecs - @start[tid]) / 1000000);
      delete(@start[tid]);
    }
    /*
     * Sample output:
     * slept for 1000 ms
     * slept for 1009 ms
     * slept for 2002 ms
     * ...
     */

BUILTINS

Builtins are special variables built into the language. Unlike scratch and map variables they don’t need a $ or @ as prefix (except for the positional parameters). The 'Kernel' column indicates the minimum kernel version required and the 'BPF Helper' column indicates the raw BPF helper function used for this builtin.
Variable | Type | Kernel | BPF Helper | Description
$1, $2, ...$n | int64 | n/a | n/a | The nth positional parameter passed to the bpftrace program. If less than n parameters are passed this evaluates to 0. For string arguments use the str() call to retrieve the value.
$# | int64 | n/a | n/a | Total amount of positional parameters passed.
arg0, arg1, ...argn | int64 | n/a | n/a | nth argument passed to the function being traced. These are extracted from the CPU registers. The amount of args passed in registers depends on the CPU architecture. (kprobes, uprobes, usdt).
args | struct args | n/a | n/a | The struct of all arguments of the traced function. Available in tracepoint, kfunc, and uprobe (with DWARF) probes. Use args.x to access argument x or args to get a record with all arguments.
cgroup | uint64 | 4.18 | get_current_cgroup_id | ID of the cgroup the current process belongs to. Only works with cgroupv2.
comm | string[16] | 4.2 | get_current_comm | Name of the current thread
cpid | uint32 | n/a | n/a | Child process ID, if bpftrace is invoked with -c
cpu | uint32 | 4.1 | raw_smp_processor_id | ID of the processor executing the BPF program
curtask | uint64 | 4.8 | get_current_task | Pointer to struct task_struct of the current task
elapsed | uint64 | (see nsec) | ktime_get_ns / ktime_get_boot_ns | Nanoseconds elapsed since bpftrace initialization, based on nsecs
func | string | n/a | n/a | Name of the current function being traced (kprobes,uprobes)
gid | uint64 | 4.2 | get_current_uid_gid | Group ID of the current thread, as seen from the init namespace
jiffies | uint64 | 5.9 | get_jiffies_64 | Jiffies of the kernel. In 32-bit system, using this builtin might be slower.
numaid | uint32 | 5.8 | numa_node_id | ID of the NUMA node executing the BPF program
pid | uint64 | 4.2 | get_current_pid_tgid | Process ID of the current thread (aka thread group ID), as seen from the init namespace
probe | string | n/na | n/a | Name of the current probe
rand | uint32 | 4.1 | get_prandom_u32 | Random number
return | n/a | n/a | n/a | The return keyword is used to exit the current probe. This differs from exit() in that it doesn’t exit bpftrace.
retval | int64 | n/a | n/a | Value returned by the function being traced (kretprobe, uretprobe, kretfunc)
tid | uint64 | 4.2 | get_current_pid_tgid | Thread ID of the current thread, as seen from the init namespace
uid | uint64 | 4.2 | get_current_uid_gid | User ID of the current thread, as seen from the init namespace

  Positional Parameters

variants

    • $1, $2, ..., $N, $#

These are the positional parameters to the bpftrace program, also referred to as command line arguments. If the parameter is numeric (entirely digits), it can be used as a number. If it is non-numeric, it must be used as a string in the str() call. If a parameter is used that was not provided, it will default to zero for numeric context, and "" for string context. Positional parameters may also be used in probe argument and will be treated as a string parameter.

If a positional parameter is used in str(), it is interpreted as a pointer to the actual given string literal, which allows to do pointer arithmetic on it. Only addition of a single constant, less or equal to the length of the supplied string, is allowed.

$# returns the number of positional arguments supplied.

This allows scripts to be written that use basic arguments to change their behavior. If you develop a script that requires more complex argument processing, it may be better suited for bcc instead, which supports Python’s argparse and completely custom argument processing.

    # bpftrace -e 'BEGIN { printf("I got %d, %s (%d args)\n", $1, str($2), $#); }' 42 "hello"
    I got 42, hello (2 args)
    # bpftrace -e 'BEGIN { printf("%s\n", str($1 + 1)) }' "hello"
    ello

Script example, bsize.d:

    #!/usr/local/bin/bpftrace
    BEGIN
    {
            printf("Tracing block I/O sizes > %d bytes\n", $1);
    }
    tracepoint:block:block_rq_issue
    /args.bytes > $1/
    {
            @ = hist(args.bytes);
    }

When run with a 65536 argument:

    # ./bsize.bt 65536
    Tracing block I/O sizes > 65536 bytes
    ^C
    @:
    [512K, 1M)             1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

It has passed the argument in as $1 and used it as a filter.

With no arguments, $1 defaults to zero:

    # ./bsize.bt
    Attaching 2 probes...
    Tracing block I/O sizes > 0 bytes
    ^C
    @:
    [4K, 8K)             115 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
    [8K, 16K)             35 |@@@@@@@@@@@@@@@                                     |
    [16K, 32K)             5 |@@                                                  |
    [32K, 64K)             3 |@                                                   |
    [64K, 128K)            1 |                                                    |
    [128K, 256K)           0 |                                                    |
    [256K, 512K)           0 |                                                    |
    [512K, 1M)             1 |                                                    |

FUNCTIONS
Function Name | Description | Sync/Async/Compile Time
bswap(uint[8|\16|32|64] n) | Reverse byte order | Sync
buf(void *d [, int length]) | Returns a hex-formatted string of the data pointed to by d | Sync
cat(char *filename) | Print file content | Async
cgroupid(char *path) | Resolve cgroup ID | Compile Time
cgroup_path(int cgroupid, string filter) | Convert cgroup id to cgroup path | Sync
exit() | Quit bpftrace | Async
join(char *arr[] [, char *delim]) | Print the array | Async
kaddr(char *name) | Resolve kernel symbol name | Compile Time
kptr(void *p) | Annotate as kernelspace pointer | Sync
kstack([StackMode mode, ][int level]) | Kernel stack trace | Sync
ksym(void *p) | Resolve kernel address | Async
macaddr(char[6] addr) | Convert MAC address data | Sync
nsecs([TimestampMode mode]) | Timestamps and Time Deltas | Sync
ntop([int af, ]int|char[4|16] addr) | Convert IP address data to text | Sync
offsetof(struct, element) | Offset of element in structure | Compile Time
override(u64 rc) | Override return value | Sync
path(struct path *path) | Return full path | Sync
print(...) | Print a non-map value with default formatting | Async
printf(char *fmt, ...) | Print formatted | Async
pton(const string *addr) | Convert text IP address to byte array | Compile Time
reg(char *name) | Returns the value stored in the named register | Sync
signal(char[] signal | u32 signal) | Send a signal to the current process | Sync
sizeof(...) | Return size of a type or expression | Sync
skboutput(const string p, struct sk_buff *s, ...) | Write skb 's data section into a PCAP file | Async
str(char *s [, int length]) | Returns the string pointed to by s | Sync
strcontains(const char *haystack, const char *needle) | Compares whether the string haystack contains the string needle. | Sync
strerror(uint64 error) | Get error message for errno code | Sync
strftime(char *format, int nsecs) | Return a formatted timestamp | Async
strncmp(char *s1, char *s2, int length) | Compare first n characters of two strings | Sync
system(char *fmt) | Execute shell command | Async
time(char *fmt) | Print formatted time | Async
uaddr(char *name) | Resolve user-level symbol name | Compile Time
uptr(void *p) | Annotate as userspace pointer | Sync
ustack([StackMode mode, ][int level]) | User stack trace | Sync
usym(void *p) | Resolve user space address | Async

Functions that are marked async are asynchronous which can lead to unexpected behaviour, see the Invocation Mode section for more information.

compile time functions are evaluated at compile time, a static value will be compiled into the program.

unsafe functions can have dangerous side effects and should be used with care, the --unsafe flag is required for use.

  bswap

variants

    • uint8 bswap(uint8 n)

    • uint16 bswap(uint16 n)

    • uint32 bswap(uint32 n)

    • uint64 bswap(uint64 n)

bswap reverses the order of the bytes in integer n. In case of 8 bit integers, n is returned without being modified. The return type is an unsigned integer of the same width as n.

  buf

variants

    • buf_t buf(void * data, [int64 length])

buf reads length amount of bytes from address data. The maximum value of length is limited to the BPFTRACE_MAX_STRLEN variable. For arrays the length is optional, it is automatically inferred from the signature.

buf is address space aware and will call the correct helper based on the address space associated with data.

The buf_t object returned by buf can safely be printed as a hex encoded string with the %r format specifier.

Bytes with values >=32 and <=126 are printed using their ASCII character, other bytes are printed in hex form (e.g. \x00). The %rx format specifier can be used to print everything in hex form, including ASCII characters. The similar %rh format specifier prints everything in hex form without \x and with spaces between bytes (e.g. 0a fe).

    i:s:1 {
      printf("%r\n", buf(kaddr("avenrun"), 8));
    }

    \x00\x03\x00\x00\x00\x00\x00\x00
    \xc2\x02\x00\x00\x00\x00\x00\x00

  cat

variants

    • void cat(string namefmt, [...args])

async

Dump the contents of the named file to stdout. cat supports the same format string and arguments that printf does. If the file cannot be opened or read an error is printed to stderr.

    t:syscalls:sys_enter_execve {
      cat("/proc/%d/maps", pid);
    }

    55f683ebd000-55f683ec1000 r--p 00000000 08:01 1843399                    /usr/bin/ls
    55f683ec1000-55f683ed6000 r-xp 00004000 08:01 1843399                    /usr/bin/ls
    55f683ed6000-55f683edf000 r--p 00019000 08:01 1843399                    /usr/bin/ls
    55f683edf000-55f683ee2000 rw-p 00021000 08:01 1843399                    /usr/bin/ls
    55f683ee2000-55f683ee3000 rw-p 00000000 00:00 0

  cgroupid

variants

    • uint64 cgroupid(const string path)

compile time

cgroupid retrieves the cgroupv2 ID of the cgroup available at path.

    BEGIN {
      print(cgroupid("/sys/fs/cgroup/system.slice"));
    }

  cgroup_path

variants

    • cgroup_path cgroup_path(int cgroupid, string filter)

Convert cgroup id to cgroup path. This is done asynchronously in userspace when the cgroup_path value is printed, therefore it can resolve to a different value if the cgroup id gets reassigned. This also means that the returned value can only be used for printing.

A string literal may be passed as an optional second argument to filter cgroup hierarchies in which the cgroup id is looked up by a wildcard expression (cgroup2 is always represented by "unified", regardless of where it is mounted).

The currently mounted hierarchy at /sys/fs/cgroup is used to do the lookup. If the cgroup with the given id isn’t present here (e.g. when running in a Docker container), the cgroup path won’t be found (unlike when looking up the cgroup path of a process via /proc/.../cgroup).

    BEGIN {
      $cgroup_path = cgroup_path(3436);
      print($cgroup_path);
      print($cgroup_path); /* This may print a different path */
      printf("%s %s", $cgroup_path, $cgroup_path); /* This may print two different paths */
    }

  exit

variants

    • void exit()

async

Terminate bpftrace, as if a SIGTERM was received. The END probe will still trigger (if specified) and maps will be printed.

  join

variants

    • void join(char *arr[], [char * sep = ' '])

async

join joins all the string array arr with sep as separator into one string. This string will be printed to stdout directly, it cannot be used as string value.

The concatenation of the array members is done in BPF and the printing happens in userspace.

    tracepoint:syscalls:sys_enter_execve {
      join(args.argv);
    }

  kaddr

variants

    • uint64 kaddr(const string name)

compile time

Get the address of the kernel symbol name.

The following script:

  kptr

variants

    • T * kptr(T * ptr)

Marks ptr as a kernel address space pointer. See the address-spaces section for more information on address-spaces. The pointer type is left unchanged.

  kstack

variants

    • kstack([StackMode mode, ][int limit])

These are implemented using BPF stack maps.

    kprobe:ip_output { @[kstack()] = count(); }
    /*
     * Sample output:
     * @[
     *  ip_output+1
     *  tcp_transmit_skb+1308
     *  tcp_write_xmit+482
     *  tcp_release_cb+225
     *  release_sock+64
     *  tcp_sendmsg+49
     *  sock_sendmsg+48
     *  sock_write_iter+135
     *   __vfs_write+247
     *  vfs_write+179
     *  sys_write+82
     *   entry_SYSCALL_64_fastpath+30
     * ]: 1708
     */

Sampling only three frames from the stack (limit = 3):

    kprobe:ip_output { @[kstack(3)] = count(); }
    /*
     * Sample output:
     * @[
     *  ip_output+1
     *  tcp_transmit_skb+1308
     *  tcp_write_xmit+482
     * ]: 1708
     */

You can also choose a different output format. Available formats are bpftrace, perf, and raw (no symbolication):

    kprobe:ip_output { @[kstack(perf, 3)] = count(); }
    /*
     * Sample output:
     * @[
     *  ffffffffb4019501 do_mmap+1
     *  ffffffffb401700a sys_mmap_pgoff+266
     *  ffffffffb3e334eb sys_mmap+27
     * ]: 1708
     */

  ksym

variants

    • ksym_t ksym(uint64 addr)

async

Retrieve the name of the function that contains address addr. The address to name mapping happens in user-space.

The ksym_t type can be printed with the %s format specifier.

    kprobe:do_nanosleep
    {
      printf("%s\n", ksym(reg("ip")));
    }
    /*
     * Sample output:
     * do_nanosleep
     */

  macaddr

variants

    • macaddr_t macaddr(char [6] mac)

Create a buffer that holds a macaddress as read from mac This buffer can be printed in the canonical string format using the %s format specifier.

    kprobe:arp_create {
      $stack_arg0 = *(uint8*)(reg("sp") + 8);
      $stack_arg1 = *(uint8*)(reg("sp") + 16);
      printf("SRC %s, DST %s\n", macaddr($stack_arg0), macaddr($stack_arg1));
    }
    /*
     * Sample output:
     * SRC 18:C0:4D:08:2E:BB, DST 74:83:C2:7F:8C:FF
     */

  nsecs

variants

    • nsecs([TimestampMode mode])

Returns a timestamp in nanoseconds, as given by the requested kernel clock. Defaults to boot if no clock is explicitly requested.

    • nsecs(monotonic) - nanosecond timestamp since boot, exclusive of time the system spent suspended (CLOCK_MONOTONIC)

    • nsecs(boot) - nanoseconds since boot, inclusive of time the system spent suspended (CLOCK_BOOTTIME)

    • nsecs(tai) - TAI timestamp in nanoseconds (CLOCK_TAI)

    • nsecs(sw_tai) - approximation of TAI timestamp in nanoseconds, is obtained through the "triple vdso sandwich" method. For older kernels without direct TAI timestamp access in BPF.

    i:s:1 {
      $sw_tai1 = nsecs(sw_tai);
      $tai = nsecs(tai);
      $sw_tai2 = nsecs(sw_tai);
      printf("sw_tai precision: %lldns\n", ($sw_tai1 + $sw_tai2)/2 - $tai);
    }
    /*
     * Sample output:
     * sw_tai precision: -98ns
     * sw_tai precision: -99ns
     * ...
     */

  ntop

variants

    • inet_t ntop([int64 af, ] int addr)

    • inet_t ntop([int64 af, ] char addr[4])

    • inet_t ntop([int64 af, ] char addr[16])

ntop returns the string representation of an IPv4 or IPv6 address. ntop will infer the address type (IPv4 or IPv6) based on the addr type and size. If an integer or char[4] is given, ntop assumes IPv4, if a char[16] is given, ntop assumes IPv6. You can also pass the address type (e.g. AF_INET) explicitly as the first parameter.

  offsetof

variants

    • offsetof(STRUCT, FIELD)

    • offsetof(EXPRESSION, FIELD)

compile time

Returns offset of the field offset bytes in struct. Similar to kernel offsetof operator. Note that subfields are not yet supported.

  override

variants

    • override(uint64 rc)

unsafe

Kernel 4.16

Helper bpf_override

Supported probes

    • kprobe

When using override the probed function will not be executed and instead rc will be returned.

    k:__x64_sys_getuid
    /comm == "id"/ {
      override(2<<21);
    }

    uid=4194304 gid=0(root) euid=0(root) groups=0(root)

This feature only works on kernels compiled with CONFIG_BPF_KPROBE_OVERRIDE and only works on functions tagged ALLOW_ERROR_INJECTION.

bpftrace does not test whether error injection is allowed for the probed function, instead if will fail to load the program into the kernel:

    ioctl(PERF_EVENT_IOC_SET_BPF): Invalid argument
    Error attaching probe: 'kprobe:vfs_read'

  path

variants

    • char * path(struct path * path)

Kernel 5.10

Helper bpf_d_path

Return full path referenced by struct path pointer in argument.

This function can only be used by functions that are allowed to, these functions are contained in the btf_allowlist_d_path set in the kernel.

  print

variants

    • void print(T val)

async

variants

    • void print(T val)

    • void print(@map)

    • void print(@map, uint64 top)

    • void print(@map, uint64 top, uint64 div)

print prints a the value, which can be a map or a scalar value, with the default formatting for the type.

    i:s:1 {
      print(123);
      print("abc");
      exit();
    }
    /*
     * Sample output:
     * 123
     * abc
     */

    i:ms:10 { @=hist(rand); }
    i:s:1 {
      print(@);
      exit();
    }

Prints:

    @:
    [16M, 32M)             3 |@@@                                                 |
    [32M, 64M)             2 |@@                                                  |
    [64M, 128M)            1 |@                                                   |
    [128M, 256M)           4 |@@@@                                                |
    [256M, 512M)           3 |@@@                                                 |
    [512M, 1G)            14 |@@@@@@@@@@@@@@                                      |
    [1G, 2G)              22 |@@@@@@@@@@@@@@@@@@@@@@                              |
    [2G, 4G)              51 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

Declared maps and histograms are automatically printed out on program termination.

Note that maps are printed by reference while scalar values are copied. This means that updating and printing maps in a fast loop will likely result in bogus map values as the map will be updated before userspace gets the time to dump and print it.

The printing of maps supports the optional top and div arguments. top limits the printing to the top N entries with the highest integer values

    BEGIN {
      $i = 11;
      while($i) {
        @[$i] = --$i;
      }
      print(@, 2);
      clear(@);
      exit()
    }
    /*
     * Sample output:
     * @[9]: 9
     * @[10]: 10
     */

The div argument scales the values prior to printing them. Scaling values before storing them can result in rounding errors. Consider the following program:

    k:f {
      @[func] += arg0/10;
    }

With the following sequence as numbers for arg0: 134, 377, 111, 99. The total is 721 which rounds to 72 when scaled by 10 but the program would print 70 due to the rounding of individual values.

Changing the print call to print(@, 5, 2) will take the top 5 values and scale them by 2:

    @[6]: 3
    @[7]: 3
    @[8]: 4
    @[9]: 4
    @[10]: 5

  printf

variants

    • void printf(const string fmt, args...)

async

printf() formats and prints data. It behaves similar to printf() found in C and many other languages.

The format string has to be a constant, it cannot be modified at runtime. The formatting of the string happens in user space. Values are copied and passed by value.

bpftrace supports all the typical format specifiers like %llx and %hhu. The non-standard ones can be found in the table below:
Specifier | Type | Description
r | buffer | Hex-formatted string to print arbitrary binary content returned by the buf function.
rh | buffer | Prints in hex-formatted string without \x and with spaces between bytes (e.g. 0a fe)

Supported escape sequences

Colors are supported too, using standard terminal escape sequences:

    print("\033[31mRed\t\033[33mYellow\033[0m\n")

  pton

variants

    • char addr[4] pton(const string *addr_v4)

    • char addr[16] pton(const string *addr_v6)

compile time

pton converts a text representation of an IPv4 or IPv6 address to byte array. pton infers the address family based on . or : in the given argument. pton comes in handy when we need to select packets with certain IP addresses.

  reg

variants

    • reg(const string name)

Supported probes

    • kprobe

    • uprobe

Get the contents of the register identified by name. Valid names depend on the CPU architecture.

  signal

variants

    • signal(const string sig)

    • signal(uint32 signum)

unsafe

Kernel 5.3

Helper bpf_send_signal

Probe types: k(ret)probe, u(ret)probe, USDT, profile

Send a signal to the process being traced. The signal can either be identified by name, e.g. SIGSTOP or by ID, e.g. 19 as found in kill -l.

    kprobe:__x64_sys_execve
    /comm == "bash"/ {
      signal(5);
    }

    $ ls
    Trace/breakpoint trap (core dumped)

  sizeof

variants

    • sizeof(TYPE)

    • sizeof(EXPRESSION)

compile time

Returns size of the argument in bytes. Similar to C/C++ sizeof operator. Note that the expression does not get evaluated.

  skboutput

variants

    • uint32 skboutput(const string path, struct sk_buff *skb, uint64 length, const uint64 offset)

Kernel 5.5

Helper bpf_skb_output

Write sk_buff skb 's data section to a PCAP file in the path, starting from offset to offset + length.

The PCAP file is encapsulated in RAW IP, so no ethernet header is included. The data section in the struct skb may contain ethernet header in some kernel contexts, you may set offset to 14 bytes to exclude ethernet header.

Each packet’s timestamp is determined by adding nsecs and boot time, the accuracy varies on different kernels, see nsecs.

This function returns 0 on success, or a negative error in case of failure.

Environment variable BPFTRACE_PERF_RB_PAGES should be increased in order to capture large packets, or else these packets will be dropped.

Usage

    # cat dump.bt
    kfunc:napi_gro_receive {
      $ret = skboutput("receive.pcap", args.skb, args.skb->len, 0);
    }
    kfunc:dev_queue_xmit {
      // setting offset to 14, to exclude ethernet header
      $ret = skboutput("output.pcap", args.skb, args.skb->len, 14);
      printf("skboutput returns %d\n", $ret);
    }
    # export BPFTRACE_PERF_RB_PAGES=1024
    # bpftrace dump.bt
    ...
    # tcpdump -n -r ./receive.pcap  | head -3
    reading from file ./receive.pcap, link-type RAW (Raw IP)
    dropped privs to tcpdump
    10:23:44.674087 IP 22.128.74.231.63175 > 192.168.0.23.22: Flags [.], ack 3513221061, win 14009, options [nop,nop,TS val 721277750 ecr 3115333619], length 0
    10:23:45.823194 IP 100.101.2.146.53 > 192.168.0.23.46619: 17273 0/1/0 (130)
    10:23:45.823229 IP 100.101.2.146.53 > 192.168.0.23.46158: 45799 1/0/0 A 100.100.45.106 (60)

  str

variants

    • str(char * data [, uint32 length)

Helper probe_read_str, probe_read_{kernel,user}_str

str reads a NULL terminated (\0) string from data. The maximum string length is limited by the BPFTRACE_MAX_STRLEN env variable, unless length is specified and shorter than the maximum. In case the string is longer than the specified length only length - 1 bytes are copied and a NULL byte is appended at the end.

When available (starting from kernel 5.5, see the --info flag) bpftrace will automatically use the kernel or user variant of probe_read_{kernel,user}_str based on the address space of data, see [Address-spaces] for more information.

  strcontains

variants

    • int64 strcontains(const char *haystack, const char *needle)

strcontains compares whether the string haystack contains the string needle. If needle is contained 1 is returned, else zero is returned.

bpftrace doesn’t read past the length of the shortest string.

  strerror

variants

    • strerror strerror(int error)

Convert errno code to string. This is done asynchronously in userspace when the strerror value is printed, hence the returned value can only be used for printing.

    #include <errno.h>
    BEGIN {
      print(strerror(EPERM));
    }

  strftime

variants

    • strtime_t strftime(const string fmt, int64 timestamp_ns)

async

Format the nanoseconds since boot timestamp timestamp_ns according to the format specified by fmt. The time conversion and formatting happens in user space, therefore the timestr_t value returned can only be used for printing using the %s format specifier.

bpftrace uses the strftime(3) function for formatting time and supports the same format specifiers.

    i:s:1 {
      printf("%s\n", strftime("%H:%M:%S", nsecs));
    }

bpftrace also supports the following format string extensions:
Specifier | Description
%f | Microsecond as a decimal number, zero-padded on the left

  strncmp

variants

    • int64 strncmp(char * s1, char * s2, int64 n)

strncmp compares up to n characters string s1 and string s2. If they’re equal 0 is returned, else a non-zero value is returned.

bpftrace doesn’t read past the length of the shortest string.

The use of the == and != operators is recommended over calling strncmp directly.

  system

variants

    • void system(string namefmt [, ...args])

unsafe async

system lets bpftrace run the specified command (fork and exec) until it completes and print its stdout. The command is run with the same privileges as bpftrace and it blocks execution of the processing threads which can lead to missed events and delays processing of async events.

    i:s:1 {
      time("%H:%M:%S: ");
      printf("%d\n", @++);
    }
    i:s:10 {
      system("/bin/sleep 10");
    }
    i:s:30 {
      exit();
    }

Note how the async time and printf first print every second until the i:s:10 probe hits, then they print every 10 seconds due to bpftrace blocking on sleep.

    Attaching 3 probes...
    08:50:37: 0
    08:50:38: 1
    08:50:39: 2
    08:50:40: 3
    08:50:41: 4
    08:50:42: 5
    08:50:43: 6
    08:50:44: 7
    08:50:45: 8
    08:50:46: 9
    08:50:56: 10
    08:50:56: 11
    08:50:56: 12
    08:50:56: 13
    08:50:56: 14
    08:50:56: 15
    08:50:56: 16
    08:50:56: 17
    08:50:56: 18
    08:50:56: 19

system supports the same format string and arguments that printf does.

    t:syscalls:sys_enter_execve {
      system("/bin/grep %s /proc/%d/status", "vmswap", pid);
    }

  time

variants

    • void time(const string fmt)

async

Format the current wall time according to the format specifier fmt and print it to stdout. Unlike strftime() time() doesn’t send a timestamp from the probe, instead it is the time at which user-space processes the event.

bpftrace uses the strftime(3) function for formatting time and supports the same format specifiers.

  uaddr

variants

    • T * uaddr(const string sym)

Supported probes

    • uprobes

    • uretprobes

    • USDT

Does not work with ASLR, see issue

The uaddr function returns the address of the specified symbol. This lookup happens during program compilation and cannot be used dynamically.

The default return type is uint64*. If the ELF object size matches a known integer size (1, 2, 4 or 8 bytes) the return type is modified to match the width (uint8*, uint16*, uint32* or uint64* resp.). As ELF does not contain type info the type is always assumed to be unsigned.

    uprobe:/bin/bash:readline {
      printf("PS1: %s\n", str(*uaddr("ps1_prompt")));
    }

  uptr

variants

    • T * uptr(T * ptr)

Marks ptr as a user address space pointer. See the address-spaces section for more information on address-spaces. The pointer type is left unchanged.

  ustack

variants

    • ustack([StackMode mode, ][int limit])

These are implemented using BPF stack maps.

    kprobe:do_sys_open /comm == "bash"/ { @[ustack()] = count(); }
    /*
     * Sample output:
     * @[
     *  __open_nocancel+65
     *  command_word_completion_function+3604
     *  rl_completion_matches+370
     *  bash_default_completion+540
     *  attempt_shell_completion+2092
     *  gen_completion_matches+82
     *  rl_complete_internal+288
     *  rl_complete+145
     *  _rl_dispatch_subseq+647
     *  _rl_dispatch+44
     *  readline_internal_char+479
     *  readline_internal_charloop+22
     *  readline_internal+23
     *  readline+91
     *  yy_readline_get+152
     *  yy_readline_get+429
     *  yy_getc+13
     *  shell_getc+469
     *  read_token+251
     *  yylex+192
     *  yyparse+777
     *  parse_command+126
     *  read_command+207
     *  reader_loop+391
     *  main+2409
     *  __libc_start_main+231
     *  0x61ce258d4c544155
     * ]: 9
     */

Sampling only three frames from the stack (limit = 3):

    kprobe:ip_output { @[ustack(3)] = count(); }
    /*
     * Sample output:
     * @[
     *  __open_nocancel+65
     *  command_word_completion_function+3604
     *  rl_completion_matches+370
     * ]: 20
     */

You can also choose a different output format. Available formats are bpftrace, perf, and raw (no symbolication):

    kprobe:ip_output { @[ustack(perf, 3)] = count(); }
    /*
     * Sample output:
     * @[
     *  5649feec4090 readline+0 (/home/mmarchini/bash/bash/bash)
     *  5649fee2bfa6 yy_readline_get+451 (/home/mmarchini/bash/bash/bash)
     *  5649fee2bdc6 yy_getc+13 (/home/mmarchini/bash/bash/bash)
     * ]: 20
     */

Note that for these examples to work, bash had to be recompiled with frame pointers.

  usym

variants

    • usym_t usym(uint64 * addr)

async

Supported probes

    • uprobes

    • uretprobes

Equal to ksym but resolves user space symbols.

If ASLR is enabled, user space symbolication only works when the process is running at either the time of the symbol resolution or the time of the probe attachment. The latter requires BPFTRACE_CACHE_USER_SYMBOLS to be set to PER_PID, and might not work with older versions of BCC. A similar limitation also applies to dynamically loaded symbols.

    uprobe:/bin/bash:readline
    {
      printf("%s\n", usym(reg("ip")));
    }
    /*
     * Sample output:
     * readline
     */

  unwatch

variants

    • void unwatch(void * addr)

async

Removes a watchpoint

MAP FUNCTIONS

Map functions are built-in functions who’s return value can only be assigned to maps. The data type associated with these functions are only for internal use and are not compatible with the (integer) operators.

Functions that are marked async are asynchronous which can lead to unexpected behavior, see the Invocation Mode section for more information.
Function Name | Description | Sync/async
avg(int64 n) | Calculate the running average of n between consecutive calls. | Sync
clear(map m) | Clear all keys/values from a map. | Async
count() | Count how often this function is called. | Sync
delete(mapkey k, ...) | Delete a single key from a map. For a single value map this deletes the only element. For an associative-array the key to delete has to be specified. Multiple arguments can be passed to delete many keys at once. | Sync
hist(int64 n[, int k]) | Create a log2 histogram of n using buckets per power of 2, 0 ⇐ k ⇐ 5, defaults to 0. | Sync
len(map m) | Return the number of elements in a map. | Sync
lhist(int64 n, int64 min, int64 max, int64 step) | Create a linear histogram of n. lhist creates M ((max - min) / step) buckets in the range [min,max) where each bucket is step in size. | Sync
max(int64 n) | Update the map with n if n is bigger than the current value held. | Sync
min(int64 n) | Update the map with n if n is smaller than the current value held. | Sync
stats(int64 n) | Combines the count, avg and sum calls into one. | Sync
sum(int64 n) | Calculate the sum of all n passed. | Sync
zero(map m) | Set all values for all keys to zero. | Async

  avg

variants

    • avg(int64 n)

Calculate the running average of n between consecutive calls.

    i:s:1 {
      @x++;
      @y = avg(@x);
      print(@x);
      print(@y);
    }

Internally this keeps two values in the map: value count and running total. The average is computed in user-space when printing by dividing the total by the count.

  clear

variants

    • clear(map m)

async

Clear all keys/values from map m.

    i:ms:100 {
      @[rand % 10] = count();
    }
    i:s:10 {
      print(@);
      clear(@);
    }

  count

variants

    • count()

Count how often this function is called.

Using @=count() is conceptually similar to @`. The difference is that the `count()` function uses a map type optimized for writing (PER_CPU), increasing performance and correctness. However, sync reads can be expensive as bpftrace needs to iterate over all the cpus to collect and sum these values. Note: In contrast to hash maps (e.g. `@), multiple writers to a shared global var might lose counts as bpftrace doesn’t update them atomically.

    i:ms:100 {
      @ = count();
    }
    i:s:10 {
      // async read
      print(@);
      // sync read
      if (@ > 10) {
        print(("hello"));
      }
      clear(@);
    }

  delete

variants

    • delete(mapkey k, ...)

Delete a single key from a map. For a single value map this deletes the only element. For an associative-array the key to delete has to be specified. Multiple arguments can be passed to delete many keys at once.

    k:dummy {
      @scalar = 1;
      @associative[1,2] = 1;
      delete(@scalar);
      delete(@associative[1,2]);
      // alternatively, you can delete both at once
      delete(@scalar, @associative[1,2]);
      delete(@associative); // error
    }

  hist

variants

    • hist(int64 n[, int k])

Create a log2 histogram of n using $2^k$ buckets per power of 2, 0 ⇐ k ⇐ 5, defaults to 0.

    kretprobe:vfs_read {
      @bytes = hist(retval);
    }

Prints:

    @:
    [1M, 2M)               3 |                                                    |
    [2M, 4M)               2 |                                                    |
    [4M, 8M)               2 |                                                    |
    [8M, 16M)              6 |                                                    |
    [16M, 32M)            16 |                                                    |
    [32M, 64M)            27 |                                                    |
    [64M, 128M)           48 |@                                                   |
    [128M, 256M)          98 |@@@                                                 |
    [256M, 512M)         191 |@@@@@@                                              |
    [512M, 1G)           394 |@@@@@@@@@@@@@                                       |
    [1G, 2G)             820 |@@@@@@@@@@@@@@@@@@@@@@@@@@@                         |

  len

variants

    • len(map m)

Return the number of elements in the map.

  lhist

variants

    • lhist(int64 n, int64 min, int64 max, int64 step)

Create a linear histogram of n. lhist creates M ((max - min) / step) buckets in the range [min,max) where each bucket is step in size. Values in the range (-inf, min) and (max, inf) get their get their own bucket too, bringing the total amount of buckets created to M+2.

    i:ms:1 {
      @ = lhist(rand %10, 0, 10, 1);
    }
    i:s:5 {
      exit();
    }

Prints:

    @:
    [0, 1)               306 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@         |
    [1, 2)               284 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@            |
    [2, 3)               294 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@          |
    [3, 4)               318 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@       |
    [4, 5)               311 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@        |
    [5, 6)               362 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
    [6, 7)               336 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@    |
    [7, 8)               326 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@      |
    [8, 9)               328 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@     |
    [9, 10)              318 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@       |

  max

variants

    • max(int64 n)

Update the map with n if n is bigger than the current value held. Similar to count this uses a PER_CPU map (fast writes, slow reads).

  min

variants

    • min(int64 n)

Update the map with n if n is smaller than the current value held. Similar to count this uses a PER_CPU map (fast writes, slow reads).

  stats

variants

    • stats(int64 n)

stats combines the count, avg and sum calls into one.

    kprobe:vfs_read {
      @bytes[comm] = stats(arg2);
    }

    @bytes[bash]: count 7, average 1, total 7
    @bytes[sleep]: count 5, average 832, total 4160
    @bytes[ls]: count 7, average 886, total 6208
    @

  sum

variants

    • sum(int64 n)

Calculate the sum of all n passed.

Using @=sum(5) is conceptually similar to @=5`. The difference is that the `sum()` function uses a map type optimized for writing (PER_CPU), increasing performance and correctness. However, sync reads can be expensive as bpftrace needs to iterate over all the cpus to collect and sum these values. Note: In contrast to hash maps (e.g. `@=5), multiple writers to a shared global var might lose updates as bpftrace doesn’t update them atomically.

    i:ms:100 {
      @ = sum(5);
    }
    i:s:10 {
      // async read
      print(@);
      // sync read
      if (@ > 10) {
        print(("hello"));
      }
      clear(@);
    }

  zero

variants

    • zero(map m)

async

Set all values for all keys to zero.

PROBES

bpftrace supports various probe types which allow the user to attach BPF programs to different types of events. Each probe starts with a provider (e.g. kprobe) followed by a colon (:) separated list of options. The amount of options and their meaning depend on the provider and are detailed below. The valid values for options can depend on the system or binary being traced, e.g. for uprobes it depends on the binary. Also see Listing Probes.

It is possible to associate multiple probes with a single action as long as the action is valid for all specified probes. Multiple probes can be specified as a comma (,) separated list:

    kprobe:tcp_reset,kprobe:tcp_v4_rcv {
      printf("Entered: %s\n", probe);
    }

Wildcards are supported too:

    kprobe:tcp_* {
      printf("Entered: %s\n", probe);
    }

Both can be combined:

    kprobe:tcp_reset,kprobe:*socket* {
      printf("Entered: %s\n", probe);
    }

Most providers also support a short name which can be used instead of the full name, e.g. kprobe:f and k:f are identical.
Probe Name | Description | Kernel/User Level
BEGIN/END | Built-in events | Kernel/User
hardware | Processor-level events | Kernel
interval | Timed output | Kernel/User
iter | Iterators tracing | Kernel
kfunc/kretfunc | Kernel functions tracing with BTF support | Kernel
kprobe/kretprobe | Kernel function start/return | Kernel
profile | Timed sampling | Kernel/User
rawtracepoint | Kernel static tracepoints with raw arguments | Kernel
software | Kernel software events | Kernel
tracepoint | Kernel static tracepoints | Kernel
uprobe/uretprobe | User-level function start/return | User
usdt | User-level static tracepoints | User
watchpoint/asyncwatchpoint | Memory watchpoints | Kernel

  BEGIN/END

These are special built-in events provided by the bpftrace runtime. BEGIN is triggered before all other probes are attached. END is triggered after all other probes are detached.

Note that specifying an END probe doesn’t override the printing of 'non-empty' maps at exit. To prevent printing all used maps need be cleared in the END probe:

    END {
        clear(@map1);
        clear(@map2);
    }

  hardware

variants

    • hardware:event_name:

    • hardware:event_name:count

short name

    • h

These are the pre-defined hardware events provided by the Linux kernel, as commonly traced by the perf utility. They are implemented using performance monitoring counters (PMCs): hardware resources on the processor. There are about ten of these, and they are documented in the perf_event_open(2) man page. The event names are:

    • cpu-cycles or cycles

    • instructions

    • cache-references

    • cache-misses

    • branch-instructions or branches

    • branch-misses

    • bus-cycles

    • frontend-stalls

    • backend-stalls

    • ref-cycles

The count option specifies how many events must happen before the probe fires (sampling interval). If count is left unspecified a default value is used.

This will fire once for every 1,000,000 cache misses.

    hardware:cache-misses:1e6 { @[pid] = count(); }

  interval

variants

    • interval:us:count

    • interval:ms:count

    • interval:s:count

    • interval:hz:rate

short name

    • i

The interval probe fires at a fixed interval as specified by its time spec. Interval fires on one CPU at a time, unlike profile probes.

This prints the rate of syscalls per second.

    tracepoint:raw_syscalls:sys_enter { @syscalls = count(); }
    interval:s:1 { print(@syscalls); clear(@syscalls); }

  iterator

variants

    • iter:task

    • iter:task:pin

    • iter:task_file

    • iter:task_file:pin

    • iter:task_vma

    • iter:task_vma:pin

short name

    • it

Warning this feature is experimental and may be subject to interface changes.

These are eBPF iterator probes that allow iteration over kernel objects. Iterator probe can’t be mixed with any other probe, not even another iterator. Each iterator probe provides a set of fields that could be accessed with the ctx pointer. Users can display the set of available fields for each iterator via -lv options as described below.

    iter:task { printf("%s:%d\n", ctx->task->comm, ctx->task->pid); }
    /*
     * Sample output:
     * systemd:1
     * kthreadd:2
     * rcu_gp:3
     * rcu_par_gp:4
     * kworker/0:0H:6
     * mm_percpu_wq:8
     */

    iter:task_file {
      printf("%s:%d %d:%s\n", ctx->task->comm, ctx->task->pid, ctx->fd, path(ctx->file->f_path));
    }
    /*
     * Sample output:
     * systemd:1 1:/dev/null
     * systemd:1 3:/dev/kmsg
     * ...
     * su:1622 2:/dev/pts/1
     * ...
     * bpftrace:1892 2:/dev/pts/1
     * bpftrace:1892 6:anon_inode:bpf-prog
     */

    iter:task_vma {
      printf("%s %d %lx-%lx\n", comm, pid, ctx->vma->vm_start, ctx->vma->vm_end);
    }
    /*
     * Sample output:
     * bpftrace 119480 55b92c380000-55b92c386000
     * ...
     * bpftrace 119480 7ffd55dde000-7ffd55de2000
     */

It’s possible to pin an iterator by specifying the optional probe ':pin' part, that defines the pin file. It can be specified as an absolute or relative path to /sys/fs/bpf.

relative pin

    iter:task:list { printf("%s:%d\n", ctx->task->comm, ctx->task->pid); }
    /*
     * Sample output:
     * Program pinned to /sys/fs/bpf/list
     */

absolute pin

    iter:task_file:/sys/fs/bpf/files {
      printf("%s:%d %s\n", ctx->task->comm, ctx->task->pid, path(ctx->file->f_path));
    }
    /*
     * Sample output:
     * Program pinned to /sys/fs/bpf/files
     */

  kfunc and kretfunc

variants

    • kfunc[:module]:fn

    • fentry[:module]:fn

    • kretfunc[:module]:fn

    • fexit[:module]:fn

short names

    • f (kfunc or fentry)

    • fr (kretfunc or fexit)

requires (--info)

    • Kernel features:BTF

    • Probe types:kfunc

kfuncs attach to kernel functions similar to kprobe and kretprobe. They make use of eBPF trampolines which allow kernel code to call into BPF programs with near zero overhead. kfunc and kretfunc are aliased as fentry and fexit to match how these are referenced in the kernel.

kfuncs make use of BTF type information to derive the type of function arguments at compile time. This removes the need for manual type casting and makes the code more resilient against small signature changes in the kernel. The function arguments are available in the args struct which can be inspected by doing verbose listing (see Listing Probes). These arguments are also available in the return probe (kretfunc), unlike kretprobe.

    # bpftrace -lv 'kfunc:tcp_reset'
    kfunc:tcp_reset
        struct sock * sk
        struct sk_buff * skb

    kfunc:x86_pmu_stop {
      printf("pmu %s stop\n", str(args.event->pmu->name));
    }

The fget function takes one argument as file descriptor and you can access it via args.fd and the return value is accessible via retval:

    kretfunc:fget {
      printf("fd %d name %s\n", args.fd, str(retval->f_path.dentry->d_name.name));
    }
    /*
     * Sample output:
     * fd 3 name ld.so.cache
     * fd 3 name libselinux.so.1
     */

  kprobe and kretprobe

variants

    • kprobe[:module]:fn

    • kprobe[:module]:fn+offset

    • kretprobe[:module]:fn

short names

    • k

    • kr

kprobes allow for dynamic instrumentation of kernel functions. Each time the specified kernel function is executed the attached BPF programs are ran.

    kprobe:tcp_reset {
      @tcp_resets = count()
    }

Function arguments are available through the argN for register args. Arguments passed on stack are available using the stack pointer, e.g. $stack_arg0 = (int64)reg("sp") + 16. Whether arguments passed on stack or in a register depends on the architecture and the number or arguments used, e.g. on x86_64 the first 6 non-floating point arguments are passed in registers and all following arguments are passed on the stack. Note that floating point arguments are typically passed in special registers which don’t count as argN arguments which can cause confusion. Consider a function with the following signature:

    void func(int a, double d, int x)

Due to d being a floating point, x is accessed through arg1 where one might expect arg2.

bpftrace does not detect the function signature so it is not aware of the argument count or their type. It is up to the user to perform Type conversion when needed, e.g.

    #include <linux/path.h>
    #include <linux/dcache.h>
    kprobe:vfs_open
    {
            printf("open path: %s\n", str(((struct path *)arg0)->dentry->d_name.name));
    }

Here arg0 was cast as a (struct path *), since that is the first argument to vfs_open. The struct support is the same as bcc and based on available kernel headers. This means that many, but not all, structs will be available, and you may need to manually define structs.

If the kernel has BTF (BPF Type Format) data, all kernel structs are always available without defining them. For example:

    kprobe:vfs_open {
      printf("open path: %s\n", str(((struct path *)arg0)->dentry->d_name.name));
    }

You can optionally specify a kernel module, either to include BTF data from that module, or to specify that the traced function should come from that module.

    kprobe:kvm:x86_emulate_insn
    {
      $ctxt = (struct x86_emulate_ctxt *) arg0;
      printf("eip = 0x%lx\n", $ctxt->eip);
    }

See BTF Support for more details.

kprobe s are not limited to function entry, they can be attached to any instruction in a function by specifying an offset from the start of the function.

kretprobe s trigger on the return from a kernel function. Return probes do not have access to the function (input) arguments, only to the return value (through retval). A common pattern to work around this is by storing the arguments in a map on function entry and retrieving in the return probe:

    kprobe:d_lookup
    {
            $name = (struct qstr *)arg1;
            @fname[tid] = $name->name;
    }
    kretprobe:d_lookup
    /@fname[tid]/
    {
            printf("%-8d %-6d %-16s M %s\n", elapsed / 1e6, pid, comm,
                str(@fname[tid]));
    }

  profile

variants

    • profile:us:count

    • profile:ms:count

    • profile:s:count

    • profile:hz:rate

short name

    • p

Profile probes fire on each CPU on the specified interval. These operate using perf_events (a Linux kernel facility, which is also used by the perf command).

    profile:hz:99 { @[tid] = count(); }

  rawtracepoint

variants

    • rawtracepoint:event

short name

    • rt

The hook point triggered by tracepoint and rawtracepoint is the same. tracepoint and rawtracepoint are nearly identical in terms of functionality. The only difference is in the program context. rawtracepoint offers raw arguments to the tracepoint while tracepoint applies further processing to the raw arguments. The additional processing is defined inside the kernel.

    rawtracepoint:block_rq_insert {
      printf("%llx %llx\n", arg0, arg1);
    }

Tracepoint arguments are available via the argN builtins. Each arg is a 64-bit integer. The available arguments can be found in the relative path of the kernel source code include/trace/events/. For example:

    include/trace/events/block.h
    DEFINE_EVENT(block_rq, block_rq_insert,
            TP_PROTO(struct request_queue *q, struct request *rq),
            TP_ARGS(q, rq)
    );

  software

variants

    • software:event:

    • software:event:count

short name

    • s

These are the pre-defined software events provided by the Linux kernel, as commonly traced via the perf utility. They are similar to tracepoints, but there is only about a dozen of these, and they are documented in the perf_event_open(2) man page. If the count is not provided, a default is used.

The event names are:

    • cpu-clock or cpu

    • task-clock

    • page-faults or faults

    • context-switches or cs

    • cpu-migrations

    • minor-faults

    • major-faults

    • alignment-faults

    • emulation-faults

    • dummy

    • bpf-output

    software:faults:100 { @[comm] = count(); }

This roughly counts who is causing page faults, by sampling the process name for every one in one hundred faults.

  tracepoint

variants

    • tracepoint:subsys:event

short name

    • t

Tracepoints are hooks into events in the kernel. Tracepoints are defined in the kernel source and compiled into the kernel binary which makes them a form of static tracing. Unlike kprobe s, new tracepoints cannot be added without modifying the kernel.

The advantage of tracepoints is that they generally provide a more stable interface than kprobe s do, they do not depend on the existence of a kernel function.

    tracepoint:syscalls:sys_enter_openat {
      printf("%s %s\n", comm, str(args.filename));
    }

Tracepoint arguments are available in the args struct which can be inspected with verbose listing, see the Listing Probes section for more details.

    # bpftrace -lv "tracepoint:*"
    tracepoint:xhci-hcd:xhci_setup_device_slot
      u32 info
      u32 info2
      u32 tt_info
      u32 state
    ...

Alternatively members for each tracepoint can be listed from their /format file in /sys.

Apart from the filename member, we can also print flags, mode, and more. After the "common" members listed first, the members are specific to the tracepoint.

Additional information

    •

  uprobe, uretprobe

variants

    • uprobe:binary:func

    • uprobe:binary:func+offset

    • uprobe:binary:offset

    • uretprobe:binary:func

short names

    • u

    • ur

uprobe s or user-space probes are the user-space equivalent of kprobe s. The same limitations that apply kprobe and kretprobe also apply to uprobe s and uretprobe s, namely: arguments are available via the argN and sargN builtins and can only be accessed with a uprobe (sargN is more common for older versions of golang). retval is the return value for the instrumented function and can only be accessed with a uretprobe.

    uprobe:/bin/bash:readline { printf("arg0: %d\n", arg0); }

What does arg0 of readline() in /bin/bash contain? I don’t know, so I’ll need to look at the bash source code to find out what its arguments are.

When tracing libraries, it is sufficient to specify the library name instead of a full path. The path will be then automatically resolved using /etc/ld.so.cache:

    uprobe:libc:malloc { printf("Allocated %d bytes\n", arg0); }

If the traced binary has DWARF included, function arguments are available in the args struct which can be inspected with verbose listing, see the Listing Probes section for more details.

    # bpftrace -lv 'uprobe:/bin/bash:rl_set_prompt'
    uprobe:/bin/bash:rl_set_prompt
        const char* prompt

When tracing C++ programs, it’s possible to turn on automatic symbol demangling by using the :cpp prefix:

    # bpftrace:cpp:"bpftrace::BPFtrace::add_probe" { ... }

It is important to note that for uretprobe s to work the kernel runs a special helper on user-space function entry which overrides the return address on the stack. This can cause issues with languages that have their own runtime like Golang:

example.go

    func myprint(s string) {
      fmt.Printf("Input: %s\n", s)
    }
    func main() {
      ss := []string{"a", "b", "c"}
      for _, s := range ss {
        go myprint(s)
      }
      time.Sleep(1*time.Second)
    }

bpftrace

    # bpftrace -e 'uretprobe:./test:main.myprint { @=count(); }' -c ./test
    runtime: unexpected return pc for main.myprint called from 0x7fffffffe000
    stack: frame={sp:0xc00008cf60, fp:0xc00008cfd0} stack=[0xc00008c000,0xc00008d000)
    fatal error: unknown caller pc

  usdt

variants

    • usdt:binary_path:probe_name

    • usdt:binary_path:[probe_namespace]:probe_name

    • usdt:library_path:probe_name

    • usdt:library_path:[probe_namespace]:probe_name

short name

    • U

Where probe_namespace is optional if probe_name is unique within the binary.

You can target the entire host (or an entire process’s address space by using the -p arg) by using a single wildcard in place of the binary_path/library_path:

    usdt:*:loop { printf("hi\n"); }

Please note that if you use wildcards for the probe_name or probe_namespace and end up targeting multiple USDTs for the same probe you might get errors if you also utilize the USDT argument builtin (e.g. arg0) as they could be of different types.

Arguments are available via the argN builtins:

    usdt:/root/tick:loop { printf("%s: %d\n", str(arg0), arg1); }

bpftrace also supports USDT semaphores. If both your environment and bpftrace support uprobe refcounts, then USDT semaphores are automatically activated for all processes upon probe attachment (and --usdt-file-activation becomes a noop). You can check if your system supports uprobe refcounts by running:

    # bpftrace --info 2>&1 | grep "uprobe refcount"
    bcc bpf_attach_uprobe refcount: yes
      uprobe refcount (depends on Build:bcc bpf_attach_uprobe refcount): yes

If your system does not support uprobe refcounts, you may activate semaphores by passing in -p $PID or --usdt-file-activation. --usdt-file-activation looks through /proc to find processes that have your probe’s binary mapped with executable permissions into their address space and then tries to attach your probe. Note that file activation occurs only once (during attach time). In other words, if later during your tracing session a new process with your executable is spawned, your current tracing session will not activate the new process. Also note that --usdt-file-activation matches based on file path. This means that if bpftrace runs from the root host, things may not work as expected if there are processes execved from private mount namespaces or bind mounted directories. One workaround is to run bpftrace inside the appropriate namespaces (i.e. the container).

  watchpoint and asyncwatchpoint

variants

    • watchpoint:absolute_address:length:mode

    • watchpoint:function+argN:length:mode

short names

    • w

    • aw

This feature is experimental and may be subject to interface changes. Memory watchpoints are also architecture dependent.

These are memory watchpoints provided by the kernel. Whenever a memory address is written to (w), read from (r), or executed (x), the kernel can generate an event.

In the first form, an absolute address is monitored. If a pid (-p) or a command (-c) is provided, bpftrace takes the address as a userspace address and monitors the appropriate process. If not, bpftrace takes the address as a kernel space address.

In the second form, the address present in argN when function is entered is monitored. A pid or command must be provided for this form. If synchronous (watchpoint), a SIGSTOP is sent to the tracee upon function entry. The tracee will be SIGCONTed after the watchpoint is attached. This is to ensure events are not missed. If you want to avoid the SIGSTOP + SIGCONT use asyncwatchpoint.

Note that on most architectures you may not monitor for execution while monitoring read or write.

    # bpftrace -e 'watchpoint:0x10000000:8:rw { printf("hit!\n"); }' -c ./testprogs/watchpoint

Print the call stack every time the jiffies variable is updated:

    watchpoint:0x$(awk '$3 == "jiffies" {print $1}' /proc/kallsyms):8:w {
      @[kstack] = count();
    }

"hit" and exit when the memory pointed to by arg1 of increment is written to:

    # cat wpfunc.c
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    __attribute__((noinline))
    void increment(__attribute__((unused)) int _, int *i)
    {
      (*i)++;
    }
    int main()
    {
      int *i = malloc(sizeof(int));
      while (1)
      {
        increment(0, i);
        (*i)++;
        usleep(1000);
      }
    }

    # bpftrace -e 'watchpoint:increment+arg1:4:w { printf("hit!\n"); exit() }' -c ./wpfunc

CONFIG VARIABLES

Some behavior can only be controlled through config variables, which are listed here. These can be set via the Config Block directly in a script (before any probes) or via their environment variable equivalent, which is upper case and includes the BPFTRACE_ prefix e.g. stack_mode's environment variable would be BPFTRACE_STACK_MODE.

  cache_user_symbols

Default: PER_PROGRAM if ASLR disabled or -c option given, PER_PID otherwise.

    • PER_PROGRAM - each program has its own cache. If there are more processes with enabled ASLR for a single program, this might produce incorrect results.

    • PER_PID - each process has its own cache. This is accurate for processes with ASLR enabled, and enables bpftrace to preload caches for processes running at probe attachment ti me. If there are many processes running, it will consume a lot of a memory.

    • NONE - caching disabled. This saves the most memory, but at the cost of speed.

  cpp_demangle

Default: 1

C++ symbol demangling in userspace stack traces is enabled by default.

This feature can be turned off by setting the value of this environment variable to 0.

  lazy_symbolication

Default: 0

For user space symbols, symbolicate lazily/on-demand (1) or symbolicate everything ahead of time (0).

  log_size

Default: 1000000

Log size in bytes.

  max_bpf_progs

Default: 512

This is the maximum number of BPF programs (functions) that bpftrace can generate. The main purpose of this limit is to prevent bpftrace from hanging since generating a lot of probes takes a lot of resources (and it should not happen often).

  max_cat_bytes

Default: 10000

Maximum bytes read by cat builtin.

  max_map_keys

Default: 4096

This is the maximum number of keys that can be stored in a map. Increasing the value will consume more memory and increase startup times. There are some cases where you will want to, for example: sampling stack traces, recording timestamps for each page, etc.

  max_probes

Default: 512

This is the maximum number of probes that bpftrace can attach to. Increasing the value will consume more memory, increase startup times, and can incur high performance overhead or even freeze/crash the system.

  max_strlen

Default: 64

Number of bytes allocated on the BPF stack for the string returned by str(). Make this larger if you wish to read bigger strings with str(). Beware that the BPF stack is small (512 bytes), and that you pay the toll again inside printf() (whilst it composes a perf event output buffer). So in practice you can only grow this to about 200 bytes.

Support for even larger strings is being discussed:

  max_type_res_iterations

Default: 0

Maximum number of levels of nested field accesses for tracepoint args. 0 is unlimited.

  missing_probes

Default: warn

Controls handling of probes with multiple kprobe or uprobe attach points which cannot be attached to some functions because they do not exist in the kernel or in the traced binary.

The possible options are: - error - always fail on missing probes - warn - print a warning but continue execution - ignore - silently ignore missing probes

  perf_rb_pages

Default: 64

Number of pages to allocate per CPU perf ring buffer. The value must be a power of 2. If you’re getting a lot of dropped events bpftrace may not be processing events in the ring buffer fast enough. It may be useful to bump the value higher so more events can be queued up. The tradeoff is that bpftrace will use more memory.

  stack_mode

Default: bpftrace

Output format for ustack and kstack builtins. Available modes/formats:

    • bpftrace

    • perf

    • raw: no symbolication

This can be overwritten at the call site.

  str_trunc_trailer

Default: ..

Trailer to add to strings that were truncated. Set to empty string to disable truncation trailers.

ENVIRONMENT VARIABLES

These are not available as part of the standard set of CONFIG VARIABLES and can only be set as environment variables.

  BPFTRACE_BTF

Default: None

The path to a BTF file. By default, bpftrace searches several locations to find a BTF file. See src/btf.cpp for the details.

  BPFTRACE_DEBUG_OUTPUT

Default: 0

Outputs bpftrace’s runtime debug messages to the trace_pipe. This feature can be turned on by setting the value of this environment variable to 1.

  BPFTRACE_KERNEL_BUILD

Default: /lib/modules/$(uname -r)

Only used with BPFTRACE_KERNEL_SOURCE if it is out-of-tree Linux kernel build.

  BPFTRACE_KERNEL_SOURCE

Default: /lib/modules/$(uname -r)

bpftrace requires kernel headers for certain features, which are searched for in this directory.

  BPFTRACE_VMLINUX

Default: None

This specifies the vmlinux path used for kernel symbol resolution when attaching kprobe to offset. If this value is not given, bpftrace searches vmlinux from pre defined locations. See src/attached_probe.cpp:find_vmlinux() for details.

OPTIONS EXPANDED

  Debug Output

The -d option produces debug output and does not run the program. This is mostly useful for debugging issues with bpftrace itself. You can also use -dd to produce a more verbose debug output, which will also print unoptimized IR.

Note: This is primarily used for bpftrace developers.

The output begins with Program and then an abstract syntax tree (AST) representation of the program.

    # bpftrace -d -e 'tracepoint:syscalls:sys_enter_nanosleep { printf("%s is sleeping.\n", comm); }'
    Program
     tracepoint:syscalls:sys_enter_nanosleep
      call: printf
       string: %s is sleeping.\n
       builtin: comm
    [...]

Continued:

    [...]
    %printf_t = type { i64, [16 x i8] }
    [...]
    define i64 @"tracepoint:syscalls:sys_enter_nanosleep"(i8*) local_unnamed_addr section "s_tracepoint:syscalls:sys_enter_nanosleep" {
    entry:
      %comm = alloca [16 x i8], align 1
      %printf_args = alloca %printf_t, align 8
      %1 = bitcast %printf_t* %printf_args to i8*
      call void @llvm.lifetime.start.p0i8(i64 -1, i8* nonnull %1)
      %2 = getelementptr inbounds [16 x i8], [16 x i8]* %comm, i64 0, i64 0
      %3 = bitcast %printf_t* %printf_args to i8*
      call void @llvm.memset.p0i8.i64(i8* nonnull %3, i8 0, i64 24, i32 8, i1 false)
      call void @llvm.lifetime.start.p0i8(i64 -1, i8* nonnull %2)
      call void @llvm.memset.p0i8.i64(i8* nonnull %2, i8 0, i64 16, i32 1, i1 false)
      %get_comm = call i64 inttoptr (i64 16 to i64 (i8*, i64)*)([16 x i8]* nonnull %comm, i64 16)
      %4 = getelementptr inbounds %printf_t, %printf_t* %printf_args, i64 0, i32 1, i64 0
      call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull %4, i8* nonnull %2, i64 16, i32 1, i1 false)
      %pseudo = call i64 @llvm.bpf.pseudo(i64 1, i64 1)
      %get_cpu_id = call i64 inttoptr (i64 8 to i64 ()*)()
      %perf_event_output = call i64 inttoptr (i64 25 to i64 (i8*, i8*, i64, i8*, i64)*)(i8* %0, i64 %pseudo, i64 %get_cpu_id, %printf_t* nonnull %printf_args, i64 24)
      call void @llvm.lifetime.end.p0i8(i64 -1, i8* nonnull %1)
      ret i64 0
    [...]

This section shows the llvm intermediate representation (IR) assembly, which is then compiled into BPF.

  Listing Probes

Probe listing is the method to discover which probes are supported by the current system. Listing supports the same syntax as normal attachment does and alternatively can be combined with -e or filename args to see all the probes that a program would attach to.

    # bpftrace -l 'kprobe:*'
    # bpftrace -l 't:syscalls:*openat*
    # bpftrace -l 'kprobe:tcp*,trace
    # bpftrace -l 'k:*socket*,tracepoint:syscalls:*tcp*'
    # bpftrace -l -e 'tracepoint:xdp:mem_* { exit(); }'
    # bpftrace -l my_script.bt
    # bpftrace -lv 'enum cpu_usage_stat'

The verbose flag (-v) can be specified to inspect arguments (args) for providers that support it:

    # bpftrace -l 'fr:tcp_reset,t:syscalls:sys_enter_openat' -v
    kretfunc:tcp_reset
        struct sock * sk
        struct sk_buff * skb
    tracepoint:syscalls:sys_enter_openat
        int __syscall_nr
        int dfd
        const char * filename
        int flags
        umode_t mode
    # bpftrace -l 'uprobe:/bin/bash:rl_set_prompt' -v    # works only if /bin/bash has DWARF
    uprobe:/bin/bash:rl_set_prompt
        const char *prompt
    # bpftrace -lv 'struct css_task_iter'
    struct css_task_iter {
            struct cgroup_subsys *ss;
            unsigned int flags;
            struct list_head *cset_pos;
            struct list_head *cset_head;
            struct list_head *tcset_pos;
            struct list_head *tcset_head;
            struct list_head *task_pos;
            struct list_head *cur_tasks_head;
            struct css_set *cur_cset;
            struct css_set *cur_dcset;
            struct task_struct *cur_task;
            struct list_head iters_node;
    };

  Preprocessor Options

The -I option can be used to add directories to the list of directories that bpftrace uses to look for headers. Can be defined multiple times.

    # cat program.bt
    #include <foo.h>
    BEGIN { @ = FOO }
    # bpftrace program.bt
    definitions.h:1:10: fatal error: 'foo.h' file not found
    # /tmp/include
    foo.h
    # bpftrace -I /tmp/include program.bt
    Attaching 1 probe...

The --include option can be used to include headers by default. Can be defined multiple times. Headers are included in the order they are defined, and they are included before any other include in the program being executed.

    # bpftrace --include linux/path.h --include linux/dcache.h \
        -e 'kprobe:vfs_open { printf("open path: %s\n", str(((struct path *)arg0)->dentry->d_name.name)); }'
    Attaching 1 probe...
    open path: .com.google.Chrome.ASsbu2
    open path: .com.google.Chrome.gimc10
    open path: .com.google.Chrome.R1234s

  Verbose Output

The -v option prints more information about the program as it is run:

    # bpftrace -v -e 'tracepoint:syscalls:sys_enter_nanosleep { printf("%s is sleeping.\n", comm); }'
    Attaching 1 probe...
    The verifier log:
    0: (bf) r6 = r1
    1: (b7) r1 = 0
    2: (7b) *(u64 *)(r10 -24) = r1
    3: (7b) *(u64 *)(r10 -32) = r1
    4: (7b) *(u64 *)(r10 -40) = r1
    5: (7b) *(u64 *)(r10 -8) = r1
    6: (7b) *(u64 *)(r10 -16) = r1
    7: (bf) r1 = r10
    8: (07) r1 += -16
    9: (b7) r2 = 16
    10: (85) call bpf_get_current_comm#16
    11: (79) r1 = *(u64 *)(r10 -16)
    12: (7b) *(u64 *)(r10 -32) = r1
    13: (79) r1 = *(u64 *)(r10 -8)
    14: (7b) *(u64 *)(r10 -24) = r1
    15: (18) r7 = 0xffff9044e65f1000
    17: (85) call bpf_get_smp_processor_id#8
    18: (bf) r4 = r10
    19: (07) r4 += -40
    20: (bf) r1 = r6
    21: (bf) r2 = r7
    22: (bf) r3 = r0
    23: (b7) r5 = 24
    24: (85) call bpf_perf_event_output#25
    25: (b7) r0 = 0
    26: (95) exit
    processed 26 insns (limit 131072), stack depth 40
    Attaching tracepoint:syscalls:sys_enter_nanosleep
    iscsid is sleeping.
    iscsid is sleeping.
    [...]

This includes The verifier log: and then the log message from the in-kernel verifier.

ADVANCED TOPICS

  Address Spaces

Kernel and user pointers live in different address spaces which, depending on the CPU architecture, might overlap. Trying to read a pointer that is in the wrong address space results in a runtime error. This error is hidden by default but can be enabled with the -kk flag:

    stdin:1:9-12: WARNING: Failed to probe_read_user: Bad address (-14)
    BEGIN { @=*uptr(kaddr("do_poweroff")) }
            ~~~

bpftrace tries to automatically set the correct address space for a pointer based on the probe type, but might fail in cases where it is unclear. The address space can be changed with the kptrs and uptr functions.

  BTF Support

If the kernel version has BTF support, kernel types are automatically available and there is no need to include additional headers to use them. It is not recommended to mix definitions from multiple sources (ie. BTF and header files). If your program mixes definitions, bpftrace will do its best but can easily get confused due to redefinition conflicts. Prefer to exclusively use BTF as it can never get out of sync on a running system. BTF is also less susceptible to parsing failures (C is constantly evolving). Almost all current linux deployments will support BTF.

To allow users to detect this situation in scripts, the preprocessor macro BPFTRACE_HAVE_BTF is defined if BTF is detected. See tools/ for examples of its usage.

Requirements for using BTF for vmlinux:

    • Linux 4.18+ with CONFIG_DEBUG_INFO_BTF=y

        • Building requires dwarves with pahole v1.13+

    • bpftrace v0.9.3+ with BTF support (built with libbpf v0.0.4+)

Additional requirements for using BTF for kernel modules:

    • Linux 5.11+ with CONFIG_DEBUG_INFO_BTF_MODULES=y

        • Building requires dwarves with pahole v1.19+

See kernel documentation for more information on BTF.

  Clang Environment Variables

bpftrace parses header files using libclang, the C interface to Clang. Thus environment variables affecting the clang toolchain can be used. For example, if header files are included from a non-default directory, the CPATH or C_INCLUDE_PATH environment variables can be set to allow clang to locate the files. See clang documentation for more information on these environment variables and their usage.

  Complex Tools

bpftrace can be used to create some powerful one-liners and some simple tools. For complex tools, which may involve command line options, positional parameters, argument processing, and customized output, consider switching to bcc. bcc provides Python (and other) front-ends, enabling usage of all the other Python libraries (including argparse), as well as a direct control of the kernel BPF program. The down side is that bcc is much more verbose and laborious to program. Together, bpftrace and bcc are complimentary.

An expected development path would be exploration with bpftrace one-liners, then and ad hoc scripting with bpftrace, then finally, when needed, advanced tooling with bcc.

As an example of bpftrace vs bcc differences, the bpftrace xfsdist.bt tool also exists in bcc as xfsdist.py. Both measure the same functions and produce the same summary of information. However, the bcc version supports various arguments:

    # ./xfsdist.py -h
    usage: xfsdist.py [-h] [-T] [-m] [-p PID] [interval] [count]
    Summarize XFS operation latency
    positional arguments:
      interval            output interval, in seconds
      count               number of outputs
    optional arguments:
      -h, --help          show this help message and exit
      -T, --notimestamp   don't include timestamp on interval output
      -m, --milliseconds  output in milliseconds
      -p PID, --pid PID   trace this PID only
    examples:
        ./xfsdist            # show operation latency as a histogram
        ./xfsdist -p 181     # trace PID 181 only
        ./xfsdist 1 10       # print 1 second summaries, 10 times
        ./xfsdist -m 5       # 5s summaries, milliseconds

The bcc version is 131 lines of code. The bpftrace version is 22.

  Errors

    1. Looks like the BPF stack limit of 512 bytes is exceeded BPF programs that operate on many data items may hit this limit. There are a number of things you can try to stay within the limit:

        1. Find ways to reduce the size of the data used in the program. Eg, avoid strings if they are unnecessary: use pid instead of comm. Use fewer map keys.

        2. Split your program over multiple probes.

        3. Check the status of the BPF stack limit in Linux (it may be increased in the future, maybe as a tuneable).

        4. (advanced): Run -d and examine the LLVM IR, and look for ways to optimize src/ast/codegen_llvm.cpp.

    2. Kernel headers not found bpftrace requires kernel headers for certain features, which are searched for by default in: /lib/modules/$(uname -r). The default search directory can be overridden using the environment variable BPFTRACE_KERNEL_SOURCE and also BPFTRACE_KERNEL_BUILD if it is out-of-tree Linux kernel build.

  Invocation Mode

There are three invocation modes for bpftrace built-in functions.
Mode | Description | Example functions
Synchronous | The value/effect of the built-in function is determined/handled right away by the bpf program in the kernel space. | reg(), str(), ntop()
Asynchronous | The value/effect of the built-in function is determined/handled later by the bpftrace process in the user space. | printf(), clear(), exit()
Compile-time | The value of the built-in function is determined before bpf programs are running. | kaddr(), cgroupid(), offsetof()

While BPF in the kernel can do a lot there are still things that can only be done from user space, like the outputting (printing) of data. The way bpftrace handles this is by sending events from the BPF program which user-space will pick up some time in the future (usually in milliseconds). Operations that happen in the kernel are 'synchronous' ('sync') and those that are handled in user space are 'asynchronous' ('async')

The asynchronous behaviour can lead to some unexpected behavior as updates can happen before user space had time to process the event. The following situations may occur:

    • event loss: when using printf(), the amount of data printed may be less than the actual number of events generated by the kernel during BPF program’s execution.

    • delayed exit: when using the exit() to terminate the program, bpftrace needs to handle the exit signal asynchronously causing the BPF program may continue to run for some additional time.

One example is updating a map value in a tight loop:

    BEGIN {
        @=0;
        unroll(10) {
          print(@);
          @++;
        }
        exit()
    }

Maps are printed by reference not by value and as the value gets updated right after the print user-space will likely only see the final value once it processes the event:

    @: 10
    @: 10
    @: 10
    @: 10
    @: 10
    @: 10
    @: 10
    @: 10
    @: 10
    @: 10

Therefore, when you need precise event statistics, it is recommended to use synchronous functions (e.g. count() and hist()) to ensure more reliable and accurate results.

  Systemd support

To run bpftrace in the background using systemd

    # systemd-run --unit=bpftrace --service-type=notify bpftrace -e 'kprobe:do_nanosleep { printf("%d sleeping\n", pid); }'

In the above example, systemd-run will not finish until bpftrace has attached its probes, so you can be sure that all following commands will be traced. To stop tracing, run systemctl stop bpftrace.

To debug early boot issues, bpftrace can be invoked via a systemd service ordered before the service that needs to be traced. A basic unit file to run bpftrace before another service looks as follows::

    [Unit]
    Before=service-i-want-to-trace.service
    [Service]
    Type=notify
    ExecStart=bpftrace -e 'kprobe:do_nanosleep { printf("%d sleeping\n", pid); }'

Similarly to the systemd-run example, the service to be traced will not start until bpftrace started by the systemd unit has attached its probes.