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.

Event message filter is useful while you only interesting the specific event.
Like the program open thousands file and you only want to see the "temp" file
and print stack.

# trace 'do_sys_open "%s", arg2@user' -UK -f temp
PID     TID     COMM            FUNC             -
9557    9557    a.out           do_sys_open      temp.1
        do_sys_open+0x1 [kernel]
        do_syscall_64+0x5b [kernel]
        entry_SYSCALL_64_after_hwframe+0x44 [kernel]
        __open_nocancel+0x7 [libc-2.17.so]
        __libc_start_main+0xf5 [libc-2.17.so]
9558    9558    a.out           do_sys_open      temp.2
        do_sys_open+0x1 [kernel]
        do_syscall_64+0x5b [kernel]
        entry_SYSCALL_64_after_hwframe+0x44 [kernel]
        __open_nocancel+0x7 [libc-2.17.so]
        __libc_start_main+0xf5 [libc-2.17.so]

Process name filter is porting from tools/opensnoop

# trace 'do_sys_open "%s", arg2@user' -UK -n out
PID     TID     COMM            FUNC             -
9557    9557    a.out           do_sys_open      temp.1
        do_sys_open+0x1 [kernel]
        do_syscall_64+0x5b [kernel]
        entry_SYSCALL_64_after_hwframe+0x44 [kernel]
        __open_nocancel+0x7 [libc-2.17.so]
        __libc_start_main+0xf5 [libc-2.17.so]

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 keyword 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").

Sometimes it can be useful to see where in code the events happen. There are
flags to print the kernel stack (-K), the user stack (-U) and optionally
include the virtual address in the stacks as well (-a):

# trace.py -U -a 'r::sys_futex "%d", retval'
PID     TID     COMM            FUNC             -
793922  793951  poller          sys_futex        0
        7f6c72b6497a __lll_unlock_wake+0x1a [libpthread-2.23.so]
              627fef folly::FunctionScheduler::run()+0x46f [router]
        7f6c7345f171 execute_native_thread_routine+0x21 [libstdc++.so.6.0.21]
        7f6c72b5b7a9 start_thread+0xd9 [libpthread-2.23.so]
        7f6c7223fa7d clone+0x6d [libc-2.23.so]

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

Suppose that you want to trace a system-call in a short-lived process, you can use
the -s option to trace. The option is followed by list of libraries/executables to
use for symbol resolution.
# trace -s /lib/x86_64-linux-gnu/libc.so.6,/bin/ping 'p:c:inet_pton' -U
Note: Kernel bpf will report stack map with ip/build_id
PID     TID     COMM            FUNC
4175    4175    ping            inet_pton
        inet_pton+0x136340 [libc.so.6]
        getaddrinfo+0xfb510 [libc.so.6]
        _init+0x2a08 [ping]

During the trace, 'ping -c1 google.com' was executed to obtain the above results

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, include time column
and on which CPU it happened:

# trace 'u:pthread:pthread_create "%U", arg3' -T -C
TIME     CPU PID     TID     COMM            FUNC             -
13:22:01 25  2627    2629    automount       pthread_create   expire_proc_indirect+0x0 [automount]
13:22:01 5   21360   21414   osqueryd        pthread_create   [unknown] [osqueryd]
13:22:03 25  2627    2629    automount       pthread_create   expire_proc_indirect+0x0 [automount]
13:22:04 15  21360   21414   osqueryd        pthread_create   [unknown] [osqueryd]
13:22:07 25  2627    2629    automount       pthread_create   expire_proc_indirect+0x0 [automount]
13:22:07 4   21360   21414   osqueryd        pthread_create   [unknown] [osqueryd]
^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@user' -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.

# trace.py 'sys_setsockopt(int fd, int level, int optname, char* optval, int optlen)(level==0 && optname == 1 && STRCMP("{0x6C, 0x00, 0x00, 0x00}", optval))' -U -M 1 --bin_cmp
PID     TID     COMM            FUNC             -
1855611 1863183 worker          sys_setsockopt   found

In this example we are catching setsockopt syscall to change IPv4 IP_TOS
value only for the cases where new TOS value is equal to 108. we are using
STRCMP helper in binary mode (--bin_cmp flag) to compare optval array
against int value of 108 (parametr of setsockopt call) in hex representation
(little endian format)

For advanced users there is a possibility to insert the kprobes or uprobes
after a certain offset, rather than the start of the function call
This is useful for tracing register values at different places of the
execution of a function. Lets consider the following example:

int main()
{
	int val = 0xdead;
	printf("%d\n", val);
	val = 0xbeef;
	printf("%d\n", val);
}

After compiling the code with -O3 optimization the object code looks
like the following (with GCC 10 and x86_64 architecture):

objdump --disassemble=main --prefix-addresses a.out

0000000000001060 <main> endbr64
0000000000001064 <main+0x4> sub    $0x8,%rsp
0000000000001068 <main+0x8> mov    $0xdead,%edx
000000000000106d <main+0xd> mov    $0x1,%edi
0000000000001072 <main+0x12> xor    %eax,%eax
0000000000001074 <main+0x14> lea    0xf89(%rip),%rsi
000000000000107b <main+0x1b> callq  0000000000001050 <__printf_chk@plt>
0000000000001080 <main+0x20> mov    $0xbeef,%edx
0000000000001085 <main+0x25> lea    0xf78(%rip),%rsi
000000000000108c <main+0x2c> xor    %eax,%eax
000000000000108e <main+0x2e> mov    $0x1,%edi
0000000000001093 <main+0x33> callq  0000000000001050 <__printf_chk@plt>
0000000000001098 <main+0x38> xor    %eax,%eax
000000000000109a <main+0x3a> add    $0x8,%rsp
000000000000109e <main+0x3e> retq

The 0xdead and later the 0xbeef values are moved into the edx register.
As the disassembly shows the edx register contains the 0xdead value
after the 0xd offset and 0xbeef after the 0x25 offset. To verify this
with trace lets insert probes to those offsets. The following
command inserts two uprobe one after the 0xd offset and another one
after the 0x25 offset of the main function. The probe print the
value of the edx register which will show us the correct values.

trace 'p:/tmp/a.out:main+0xd "%x", ctx->dx' 'p:/tmp/a.out:main+0x25 "%x", ctx->dx'
PID     TID     COMM            FUNC             -
25754   25754   a.out           main             dead
25754   25754   a.out           main             beef


USAGE message:

usage: trace [-h] [-b BUFFER_PAGES] [-p PID] [-L TID] [-v] [-Z STRING_SIZE]
             [-S] [-M MAX_EVENTS] [-t] [-T] [-K] [-U] [-a] [-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 SYM_FILE_LIST      when collecting stack trace in build id format,
                        use the coma separated list for symbol resolution
  -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
  -n NAME, --name NAME  only print process names containing this name
  -f MSG_FILTER, --msg-filter MSG_FILTER
                        only print message of event containing this string
  -C, --print_cpu       print CPU id
  -B, --bin_cmp         allow to use STRCMP with binary values
  -K, --kernel-stack    output kernel stack trace
  -U, --user-stack      output user stack trace
  -a, --address         print virtual address in stacks
  -I header, --include header
                        additional header files to include in the BPF program
                        as either full path, or relative to current working directory,
                        or relative to default kernel header search path

EXAMPLES:

trace do_sys_open
        Trace the open syscall and print a default trace message when entered
trace kfree_skb+0x12
        Trace the kfree_skb kernel function after the instruction on the 0x12 offset
trace 'do_sys_open "%s", arg2@user'
        Trace the open syscall and print the filename being opened. @user is
        added to arg2 in kprobes to ensure that char * should be copied from
        the userspace stack to the bpf stack. If not specified, previous
        behaviour is expected.
trace 'do_sys_open "%s", arg2@user' -n main
        Trace the open syscall and only print event that process names containing "main"
trace 'do_sys_open "%s", arg2@user' -f config
        Trace the open syscall and print the filename being opened filtered by "config"
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
trace -c /sys/fs/cgroup/system.slice/workload.service '__x64_sys_nanosleep' '__x64_sys_clone'
        Trace nanosleep/clone syscall calls only under workload.service
        cgroup hierarchy.
trace -I 'linux/fs.h' \
      'p::uprobe_register(struct inode *inode) "a_ops = %llx", inode->i_mapping->a_ops'
        Trace the uprobe_register inode mapping ops, and the symbol can be found
        in /proc/kallsyms
trace -I 'kernel/sched/sched.h' \
      'p::__account_cfs_rq_runtime(struct cfs_rq *cfs_rq) "%d", cfs_rq->runtime_remaining'
        Trace the cfs scheduling runqueue remaining runtime. The struct cfs_rq is defined
        in kernel/sched/sched.h which is in kernel source tree and not in kernel-devel
        package.  So this command needs to run at the kernel source tree root directory
        so that the added header file can be found by the compiler.
trace -I 'net/sock.h' \\
      'udpv6_sendmsg(struct sock *sk) (sk->sk_dport == 13568)'
        Trace udpv6 sendmsg calls only if socket's destination port is equal
        to 53 (DNS; 13568 in big endian order)
trace -I 'linux/fs_struct.h' 'mntns_install "users = %d", $task->fs->users'
        Trace the number of users accessing the file system of the current task
trace -s /lib/x86_64-linux-gnu/libc.so.6,/bin/ping 'p:c:inet_pton' -U
        Trace inet_pton system call and use the specified libraries/executables for
        symbol resolution.
"