strace – the Linux syscall tracer

strace is a diagnostic, debugging and instructional userspace utility for Linux. It is used to monitor and tamper with interactions between processes and the Linux kernel, which include system calls, signal deliveries, and changes of process state.

System administrators, diagnosticians, and troubleshooters will find it invaluable for solving problems with programs for which the source is not readily available since they do not need to be recompiled in order to trace them.

The operation of strace is made possible by the kernel feature known as ptrace.

Some of the features

• Attach to an already running process

  $ strace -p 26380
  strace: Process 26380 attached
  ...

   

• Filter by type of syscall

  -e trace=%desc     Trace all file descriptor related system calls.
           %file     Trace all system calls which take a file name as an argument.
           %fstat    Trace fstat and fstatat syscall variants.
           %fstatfs  Trace fstatfs, fstatfs64, fstatvfs, osf_fstatfs, and osf_fstatfs64 system calls.
           %ipc      Trace all IPC related system calls.
           %lstat    Trace lstat syscall variants.
           %memory   Trace all memory mapping related system calls.
           %network  Trace all the network related system calls.
           %process  Trace all system calls which involve process management.
           %pure     Trace syscalls that always succeed and have no arguments.
           %signal   Trace all signal related system calls.
           %stat     Trace stat syscall variants.
           %statfs   Trace statfs, statfs64, statvfs, osf_statfs, and osf_statfs64 system calls.
           %%stat    Trace syscalls used for requesting file status.
           %%statfs  Trace syscalls related to file system statistics.

   

• Trace only system calls accessing given path

  $ strace -P /etc/ld.so.cache ls /var/empty
  open("/etc/ld.so.cache", O_RDONLY) = 3
  fstat(3, {st_mode=S_IFREG|0644, st_size=22446, ...}) = 0
  mmap(NULL, 22446, PROT_READ, MAP_PRIVATE, 3, 0) = 0x2b7ac2ba9000
  close(3) = 0
  +++ exited with 0 +++

   

• Perform a full hexadecimal and ASCII dump of all the data read from/written to file descriptors

  $ strace -ewrite=1 -e trace=sendmsg ./recvmsg > /dev/null
  sendmsg(1, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="012", iov_len=3},
  {iov_base="34567", iov_len=5}, {iov_base="89abcde", iov_len=7}], msg_iovlen=3,
  msg_controllen=0, msg_flags=0}, 0) = 15
   * 3 bytes in buffer 0
   | 00000  30 31 32                                          012              |
   * 5 bytes in buffer 1
   | 00000  33 34 35 36 37                                    34567            |
   * 7 bytes in buffer 2
   | 00000  38 39 61 62 63 64 65                              89abcde          |
  +++ exited with 0 +++

   

• Perform a syscall fault injection

  $ strace -e trace=open -e fault=open cat
  open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = -1 ENOSYS (Function not implemented) (INJECTED)
  open("/lib/x86_64-linux-gnu/tls/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOSYS (Function not implemented) (INJECTED)
  open("/lib/x86_64-linux-gnu/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOSYS (Function not implemented) (INJECTED)
  open("/lib/x86_64-linux-gnu/x86_64/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOSYS (Function not implemented) (INJECTED)
  open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOSYS (Function not implemented) (INJECTED)
  cat: error while loading shared libraries: libc.so.6: cannot open shared object file: Error 38
  +++ exited with 127 +++

   

• Count time, calls, and errors for each system call

  $ strace -c ls > /dev/null
  % time     seconds  usecs/call     calls    errors syscall
  ------ ----------- ----------- --------- --------- ----------------
   89.76    0.008016           4      1912           getdents
    8.71    0.000778           0     11778           lstat
    0.81    0.000072           0      8894           write
    0.60    0.000054           0       943           open
    0.11    0.000010           0       942           close
    0.00    0.000000           0         1           read
    0.00    0.000000           0       944           fstat
    0.00    0.000000           0         8           mmap
    0.00    0.000000           0         4           mprotect
    0.00    0.000000           0         1           munmap
    0.00    0.000000           0         7           brk
    0.00    0.000000           0         3         3 access
    0.00    0.000000           0         1           execve
    0.00    0.000000           0         1           sysinfo
    0.00    0.000000           0         1           arch_prctl
  ------ ----------- ----------- --------- --------- ----------------
  100.00    0.008930                 25440         3 total

   

Changelog v4.26

• Changes in behavior
  • If strace is interrupted by a signal it handles (currently
    one of HUP, INT, QUIT, PIPE, or TERM), strace terminates
    itself with this signal.
• Improvements
  • Implemented obtainment of system call information using
    PTRACE_GET_SYSCALL_INFO ptrace API.
  • Implemented decoding of PTRACE_GET_SYSCALL_INFO ptrace request.
  • Implemented decoding of BLKGETZONESZ, BLKGETNRZONES, and RND* ioctl
    commands.
  • Enhanced xlat styles support configured by -X option.
  • Enhanced internet address formatting in structureless contexts.
  • Enhanced decoding of getsockopt SO_ERROR option.
  • Enhanced error diagnostics when the first exec fails.
  • Added %net as a short form of %network in syscall specifications.
  • Updated lists of ABS_*, BPF_*, FAN_*, IFA_*, IFLA_*, KVM_CAP_*, NETLINK_*,
    NTF_*, PR_SPEC_*, REL_*, SOL_*, TCA_*, and V4L2_* constants.
  • Updated lists of ioctl commands from Linux 4.20.
  • Enhanced manual page.
• Bug fixes
  • Fixed “xlat_idx: Unexpected xlat value 0” warnings.
  • Fixed sthyi output format on s390.
  • Documented -X option in strace -h output.

Install

git clone https://github.com/strace/strace.git
cd strace
./configure && make && make install

Tutorial

Copyright (c) 1991, 1992 Paul Kranenburg <pk@cs.few.eur.nl>
Copyright (c) 1993 Branko Lankester <branko@hacktic.nl>
Copyright (c) 1993 Ulrich Pegelow <pegelow@moorea.uni-muenster.de>
Copyright (c) 1995, 1996 Michael Elizabeth Chastain <mec@duracef.shout.net>
Copyright (c) 1993, 1994, 1995, 1996 Rick Sladkey <jrs@world.std.com>
Copyright (c) 1998-2001 Wichert Akkerman <wakkerma@deephackmode.org>
Copyright (c) 2001-2018 The strace developers.
All rights reserved.

Source: https://github.com/strace/