tools/trace_example.txt

Demonstrations of trace.


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.

For example, suppose you want to trace all commands being exec'd across the
system:

# trace 'sys_execve "%s", arg1'
PID    COMM         FUNC             -
4402   bash         sys_execve       /usr/bin/man
4411   man          sys_execve       /usr/local/bin/less
4411   man          sys_execve       /usr/bin/less
4410   man          sys_execve       /usr/local/bin/nroff
4410   man          sys_execve       /usr/bin/nroff
4409   man          sys_execve       /usr/local/bin/tbl
4409   man          sys_execve       /usr/bin/tbl
4408   man          sys_execve       /usr/local/bin/preconv
4408   man          sys_execve       /usr/bin/preconv
4415   nroff        sys_execve       /usr/bin/locale
4416   nroff        sys_execve       /usr/bin/groff
4418   groff        sys_execve       /usr/bin/grotty
4417   groff        sys_execve       /usr/bin/troff
^C

The ::sys_execve syntax specifies that you want an entry probe (which is the
default), in a kernel function (which is the default) called sys_execve. Next,
the format string to print is simply "%s", which prints a string. Finally, the
value to print is the first argument to the sys_execve function, which happens
to be the command that is exec'd. The above trace was generated by executing
"man ls" in a separate shell. As you see, man executes a number of additional
programs to finally display the man page.

Next, suppose you are looking for large reads across the system. Let's trace
the read system call and inspect the third argument, which is the number of
bytes to be read:

# trace 'sys_read (arg3 > 20000) "read %d bytes", arg3'
PID    COMM         FUNC             -
4490   dd           sys_read         read 1048576 bytes
4490   dd           sys_read         read 1048576 bytes
4490   dd           sys_read         read 1048576 bytes
4490   dd           sys_read         read 1048576 bytes
^C

During the trace, I executed "dd if=/dev/zero of=/dev/null bs=1M count=4".
The individual reads are visible, with the custom format message printed for
each read. The parenthesized expression "(arg3 > 20000)" is a filter that is
evaluated for each invocation of the probe before printing anything.

You can also trace user functions. For example, let's simulate the bashreadline
script, which attaches to the readline function in bash and prints its return
value, effectively snooping all bash shell input across the system:

# trace 'r:bash:readline "%s", retval'
PID    COMM         FUNC             -
2740   bash         readline         echo hi!
2740   bash         readline         man ls
^C

The special retval keywords stands for the function's return value, and can
be used only in a retprobe, specified by the 'r' prefix. The next component
of the probe is the library that contains the desired function. It's OK to
specify executables too, as long as they can be found in the PATH. Or, you
can specify the full path to the executable (e.g. "/usr/bin/bash").

Multiple probes can be combined on the same command line. For example, let's
trace failed read and write calls on the libc level, and include a time column:

# trace 'r:c:read ((int)retval < 0) "read failed: %d", retval' \
        'r:c:write ((int)retval < 0) "write failed: %d", retval' -T
TIME     PID    COMM         FUNC             -
05:31:57 3388   bash         write            write failed: -1
05:32:00 3388   bash         write            write failed: -1
^C

Note that the retval variable must be cast to int before comparing to zero.
The reason is that the default type for argN and retval is an unsigned 64-bit
integer, which can never be smaller than 0.

trace has also some basic support for kernel tracepoints. For example, let's
trace the block:block_rq_complete tracepoint and print out the number of sectors
transferred:

# trace 't:block:block_rq_complete "sectors=%d", args->nr_sector' -T
TIME     PID    COMM         FUNC             -
01:23:51 0      swapper/0    block_rq_complete sectors=8
01:23:55 10017  kworker/u64: block_rq_complete sectors=1
01:23:55 0      swapper/0    block_rq_complete sectors=8
^C

To discover the tracepoint structure format (which you can refer to as the "args"
pointer variable), use the tplist tool. For example:

# tplist -v block:block_rq_complete
block:block_rq_complete
    dev_t dev;
    sector_t sector;
    unsigned int nr_sector;
    int errors;
    char rwbs[8];

This output tells you that you can use "args->dev", "args->sector", etc. in your
predicate and trace arguments.


More and more high-level libraries are instrumented with USDT probe support.
These probes can be traced by trace just like kernel tracepoints. For example,
trace new threads being created and their function name:

# trace 'u:pthread:pthread_create "%U", arg3' -T
TIME     PID    COMM         FUNC             -
02:07:29 4051   contentions  pthread_create   primes_thread+0x0
02:07:29 4051   contentions  pthread_create   primes_thread+0x0
02:07:29 4051   contentions  pthread_create   primes_thread+0x0
02:07:29 4051   contentions  pthread_create   primes_thread+0x0
^C

The "%U" format specifier tells trace to resolve arg3 as a user-space symbol,
if possible. Similarly, use "%K" for kernel symbols.

Ruby, Node, and OpenJDK are also instrumented with USDT. For example, let's
trace Ruby methods being called (this requires a version of Ruby built with 
the --enable-dtrace configure flag):

# trace 'u:ruby:method__entry "%s.%s", arg1, arg2' -p $(pidof irb) -T
TIME     PID    COMM         FUNC             -
12:08:43 18420  irb          method__entry    IRB::Context.verbose?
12:08:43 18420  irb          method__entry    RubyLex.ungetc
12:08:43 18420  irb          method__entry    RuxyLex.debug?
^C

In the previous invocation, arg1 and arg2 are the class name and method name
for the Ruby method being invoked.

You can also trace exported functions from shared libraries, or an imported
function on the actual executable:

# sudo ./trace.py 'r:/usr/lib64/libtinfo.so:curses_version "Version=%s", retval'
# tput -V

PID    TID    COMM         FUNC             -
21720  21720  tput         curses_version   Version=ncurses 6.0.20160709
^C


Occasionally, it can be useful to filter specific strings. For example, you
might be interested in open() calls that open a specific file:

# trace 'p:c:open (STRCMP("test.txt", arg1)) "opening %s", arg1' -T
TIME     PID    COMM         FUNC             -
01:43:15 10938  cat          open             opening test.txt
01:43:20 10939  cat          open             opening test.txt
^C


In the preceding example, as well as in many others, readability may be
improved by providing the function's signature, which names the arguments and
lets you access structure sub-fields, which is hard with the "arg1", "arg2"
convention. For example:

# trace 'p:c:open(char *filename) "opening %s", filename'
PID    TID    COMM         FUNC             -
17507  17507  cat          open             opening FAQ.txt
^C

# trace 'p::SyS_nanosleep(struct timespec *ts) "sleep for %lld ns", ts->tv_nsec'
PID    TID    COMM         FUNC             -
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
777    785    automount    SyS_nanosleep    sleep for 500000000 ns
^C

Remember to use the -I argument include the appropriate header file. We didn't
need to do that here because `struct timespec` is used internally by the tool,
so it always includes this header file.


As a final example, let's trace open syscalls for a specific process. By
default, tracing is system-wide, but the -p switch overrides this:

# trace -p 2740 'do_sys_open "%s", arg2' -T
TIME     PID    COMM         FUNC             -
05:36:16 15872  ls           do_sys_open      /etc/ld.so.cache
05:36:16 15872  ls           do_sys_open      /lib64/libselinux.so.1
05:36:16 15872  ls           do_sys_open      /lib64/libcap.so.2
05:36:16 15872  ls           do_sys_open      /lib64/libacl.so.1
05:36:16 15872  ls           do_sys_open      /lib64/libc.so.6
05:36:16 15872  ls           do_sys_open      /lib64/libpcre.so.1
05:36:16 15872  ls           do_sys_open      /lib64/libdl.so.2
05:36:16 15872  ls           do_sys_open      /lib64/libattr.so.1
05:36:16 15872  ls           do_sys_open      /lib64/libpthread.so.0
05:36:16 15872  ls           do_sys_open      /usr/lib/locale/locale-archive
05:36:16 15872  ls           do_sys_open      /home/vagrant
^C

In this example, we traced the "ls ~" command as it was opening its shared
libraries and then accessing the /home/vagrant directory listing.


Lastly, if a high-frequency event is traced you may overflow the perf ring
buffer. This shows as "Lost N samples":

# trace sys_open
5087   5087   pgrep        sys_open
5087   5087   pgrep        sys_open
5087   5087   pgrep        sys_open
5087   5087   pgrep        sys_open
5087   5087   pgrep        sys_open
Lost 764896 samples
Lost 764896 samples
Lost 764896 samples

The perf ring buffer size can be changed with -b. The unit is size per-CPU buffer
size and is measured in pages. The value must be a power of two and defaults to
64 pages.


USAGE message:

usage: trace [-h] [-b BUFFER_PAGES] [-p PID] [-L TID] [-v] [-Z STRING_SIZE]
             [-S] [-M MAX_EVENTS] [-t] [-T] [-K] [-U] [-I header]
             probe [probe ...]

Attach to functions and print trace messages.

positional arguments:
  probe                 probe specifier (see examples)

optional arguments:
  -h, --help            show this help message and exit
  -b BUFFER_PAGES, --buffer-pages BUFFER_PAGES
                        number of pages to use for perf_events ring buffer
                        (default: 64)
  -p PID, --pid PID     id of the process to trace (optional)
  -L TID, --tid TID     id of the thread to trace (optional)
  -v, --verbose         print resulting BPF program code before executing
  -Z STRING_SIZE, --string-size STRING_SIZE
                        maximum size to read from strings
  -S, --include-self    do not filter trace's own pid from the trace
  -M MAX_EVENTS, --max-events MAX_EVENTS
                        number of events to print before quitting
  -t, --timestamp       print timestamp column (offset from trace start)
  -T, --time            print time column
  -K, --kernel-stack    output kernel stack trace
  -U, --user-stack      output user stack trace
  -I header, --include header
                        additional header files to include in the BPF program
                        as either full path, or relative to '/usr/include'

EXAMPLES:

trace do_sys_open
        Trace the open syscall and print a default trace message when entered
trace 'do_sys_open "%s", arg2'
        Trace the open syscall and print the filename being opened
trace 'sys_read (arg3 > 20000) "read %d bytes", arg3'
        Trace the read syscall and print a message for reads >20000 bytes
trace 'r::do_sys_open "%llx", retval'
        Trace the return from the open syscall and print the return value
trace 'c:open (arg2 == 42) "%s %d", arg1, arg2'
        Trace the open() call from libc only if the flags (arg2) argument is 42
trace 'c:malloc "size = %d", arg1'
        Trace malloc calls and print the size being allocated
trace 'p:c:write (arg1 == 1) "writing %d bytes to STDOUT", arg3'
        Trace the write() call from libc to monitor writes to STDOUT
trace 'r::__kmalloc (retval == 0) "kmalloc failed!"'
        Trace returns from __kmalloc which returned a null pointer
trace 'r:c:malloc (retval) "allocated = %x", retval'
        Trace returns from malloc and print non-NULL allocated buffers
trace 't:block:block_rq_complete "sectors=%d", args->nr_sector'
        Trace the block_rq_complete kernel tracepoint and print # of tx sectors
trace 'u:pthread:pthread_create (arg4 != 0)'
        Trace the USDT probe pthread_create when its 4th argument is non-zero
trace 'p::SyS_nanosleep(struct timespec *ts) "sleep for %lld ns", ts->tv_nsec'
        Trace the nanosleep syscall and print the sleep duration in ns