NAME¶

trace - Trace a function and print its arguments or return value, optionally evaluating a filter. Uses Linux eBPF/bcc.

SYNOPSIS¶

trace [-h] [-b BUFFER_PAGES] [-p PID] [-L TID] [--uid UID] [-v] [-Z STRING_SIZE] [-S] [-s SYM_FILE_LIST]
[-M MAX_EVENTS] [-t] [-u] [-T] [-C] [-K] [-U] [-a] [-I header] [-A]
probe [probe ...]

DESCRIPTION¶

trace probes functions you specify and displays trace messages if a particular condition is met. You can control the message format to display function arguments and return values.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS¶

CONFIG_BPF and bcc.

OPTIONS¶

-h

Print usage message.

-p PID

Trace only functions in the process PID.

-L TID

Trace only functions in the thread TID.

--uid UID

Trace only functions from user UID.

-v

Display the generated BPF program, for debugging purposes.

-z STRING_SIZE

When collecting string arguments (of type char*), collect up to STRING_SIZE characters. Longer strings will be truncated.

-s SYM_FILE_LIST

When collecting stack trace in build id format, use the comma separated list for symbol resolution.

-S

If set, trace messages from trace's own process. By default, this is off to avoid tracing storms -- for example, if you trace the write system call, and consider that trace is writing to the standard output.

-M MAX_EVENTS

Print up to MAX_EVENTS trace messages and then exit.

-t

Print times relative to the beginning of the trace (offsets), in seconds.

-u

Print UNIX timestamps instead of offsets from trace beginning, requires -t.

-T

Print the time column.

-C

Print CPU id.

-c CGROUP_PATH

Trace only functions in processes under CGROUP_PATH hierarchy.

-n NAME

Only print process names containing this name.

-f MSG_FILTER

Only print message of event containing this string.

-B

Treat argument of STRCMP helper as a binary value

-K

Print the kernel stack for each event.

-U

Print the user stack for each event.

-a

Print virtual address in kernel and user stacks.

-I header

Additional header files to include in the BPF program. This is needed if your filter or print expressions use types or data structures that are not available in the standard headers. For example: 'linux/mm.h'

-A

Print aggregated amount of each trace. This should be used with -M/--max-events together.

probe [probe ...]

One or more probes that attach to functions, filter conditions, and print information. See PROBE SYNTAX below.

PROBE SYNTAX¶

The general probe syntax is as follows:

[{p,r}]:[library]:function[+offset][(signature)] [(predicate)] ["format string"[, arguments]]

{t:category:event,u:library:probe} [(predicate)] ["format string"[, arguments]]

{[{p,r}],t,u}

Probe type - "p" for function entry, "r" for function return, "t" for kernel tracepoint, "u" for USDT probe. The default probe type is "p".

[library]

Library containing the probe. Specify the full path to the .so or executable file where the function to probe resides. Alternatively, you can specify just the lib name: for example, "c" refers to libc. If no library name is specified, the kernel is assumed. Also, you can specify an executable name (without a full path) if it is in the PATH. For example, "bash".

category

The tracepoint category. For example, "sched" or "irq".

function

The function to probe. offset The offset after the address of the function where the probe should injected. For example "kfree_skb+56" in decimal or hexadecimal "kfree_skb+0x38" format. Only works with kprobes and uprobes. Zero if omitted.

signature

The optional signature of the function to probe. This can make it easier to access the function's arguments, instead of using the "arg1", "arg2" etc. argument specifiers. For example, "(struct timespec *ts)" in the signature position lets you use "ts" in the filter or print expressions.

event

The tracepoint event. For example, "block_rq_complete".

probe

The USDT probe name. For example, "pthread_create".

[(predicate)]

The filter applied to the captured data. Only if the filter evaluates as true, the trace message will be printed. The filter can use any valid C expression that refers to the argument values: arg1, arg2, etc., or to the return value retval in a return probe. If necessary, use C cast operators to coerce the arguments to the desired type. For example, if arg1 is of type int, use the expression ((int)arg1 < 0) to trace only invocations where arg1 is negative. Note that only arg1-arg6 are supported, and only if the function is using the standard x86_64 convention where the first six arguments are in the RDI, RSI, RDX, RCX, R8, R9 registers. If no predicate is specified, all function invocations are traced.

The predicate expression may also use the STRCMP pseudo-function to compare a predefined string to a string argument. For example: STRCMP("test", arg1). The order of arguments is important: the first argument MUST be a quoted literal string, and the second argument can be a runtime string, most typically an argument.

["format string"[, arguments]]

A printf-style format string that will be used for the trace message. You can use the following format specifiers: %s, %d, %u, %lld, %llu, %hd, %hu, %c, %x, %llx -- with the same semantics as printf's. Make sure to pass the exact number of arguments as there are placeholders in the format string. The format specifier replacements may be any C expressions, and may refer to the same special keywords as in the predicate (arg1, arg2, etc.).

In addition to the above format specifiers, you can also use %K and %U when the expression is an address that potentially points to executable code (i.e., a symbol). trace will resolve %K specifiers to a kernel symbol, such as vfs__read, and will resolve %U specifiers to a user-space symbol in that process, such as sprintf.

In tracepoints, both the predicate and the arguments may refer to the tracepoint format structure, which is stored in the special "args" variable. For example, the block:block_rq_complete tracepoint can print or filter by args->nr_sector. To discover the format of your tracepoint, use the tplist tool.

In USDT probes, the arg1, ..., argN variables refer to the probe's arguments. To determine which arguments your probe has, use the tplist tool.

The predicate expression and the format specifier replacements for printing may also use the following special keywords: $pid, $tgid to refer to the current process' pid and tgid; $uid, $gid to refer to the current user's uid and gid; $cpu to refer to the current processor number.

EXAMPLES¶

Trace all invocations of the open system call with the name of the file (from userspace) being opened:

# trace '::do_sys_open "%s", arg2@user'

Trace all invocations of the read system call where the number of bytes requested is greater than 20,000:

# trace '::sys_read (arg3 > 20000) "read %d bytes", arg3'

Trace all malloc calls and print the size of the requested allocation:

# trace ':c:malloc "size = %d", arg1'

Trace returns from the readline function in bash and print the return value as a string:

# trace 'r:bash:readline "%s", retval'

Trace the block:block_rq_complete tracepoint and print the number of sectors completed:

# trace 't:block:block_rq_complete "%d sectors", args->nr_sector'

Trace the pthread_create USDT probe from the pthread library and print the address of the thread's start function:

# trace 'u:pthread:pthread_create "start addr = %llx", arg3'

Trace the nanosleep system call and print the sleep duration in nanoseconds:

# trace 'p::SyS_nanosleep(struct timespec *ts) sleep for %lld ns , ts->tv_nsec'

Trace the inet_pton system call using build id mechanism and print the stack

# trace -s /lib/x86_64-linux-gnu/libc.so.6,/bin/ping 'p:c:inet_pton' -U

SOURCE¶

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS¶

Linux

AUTHOR¶

Sasha Goldshtein