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 in 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 the 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
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