Reverse Engineering

strace v5.0 releases: diagnostic, debugging and instructional userspace utility

do son March 20, 2019 No Comments strace

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 v5.0

  • Changes in behavior
    • -D option now implies -I4: by default strace ignores SIGHUP, SIGINT,
      SIGQUIT, SIGPIPE, SIGTERM, SIGTTIN, SIGTTOU, and SIGTSTP when -D option
      is used.
    • Diagnostic message is no longer printed if an attempt to restart a process
      has failed with ESRCH. (Addresses RHBZ bug #1662936.)
    • ASCII dump no longer tries to read the whole buffer at once. As result,
      it can fail with a diagnostic message after dumping only a part.
      Amount of preceding zeroes printed for offset is now chosen in a way
      to accommodate all the digits of all the offsets being printed.
  • Improvements
    • Changed the way tracees are handled: all pending tracees are now waited for
      before further processing, this improves fairness of tracee processing.
      (Addresses RHBZ bugs #478419, #526740, #851457, #1609318, #1610774,
      and maybe some others.)
    • Enhanced xlat styles support configured by -X option.
    • Enhanced decoding of bpf syscall.
    • Enhanced decoding of PTRACE_PEEKUSER and PTRACE_POKEUSER on hppa.
    • Enhanced pid formatting in strace-log-merge output.
    • Wired up kexec_file_load and rseq syscalls on aarch64, arc, metag, nios2,
      or1k, riscv, and tile architectures.
    • Updated lists of BPF_*, BTRFS_*, FAN_*, IFLA_*, KERN_*, KVM_CAP_*, NDA_*,
      NETNSA_*, NT_*, PR_*, REL_*, SECCOMP_*, SCTP_*, UDP_*, V4L2_*, and *_MAGIC
      constants.
    • Updated lists of ioctl commands from Linux 5.0.
  • Bug fixes
    • Fixed ordering of sockaddr_in6 fields.
    • Fixed strace-k test on alpha.
    • Fixed build on mips o32.
    • Fixed build on NOMMU architectures.
    • Fixed build in –with-libiberty=yes mode.
    • Fixed potential NULL dereference during printing of line continuation
      for syscalls that haven’t been decoded.
    • Fixed strace-log-merge error diagnostics.

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/

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