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| author | Denys Vlasenko <vda.linux@gmail.com> | 2012-08-03 06:28:46 +0200 |
|---|---|---|
| committer | Michael Kerrisk <mtk.manpages@gmail.com> | 2012-08-03 10:33:49 +0200 |
| commit | ca302d0ee368b699ffec4cd465ee02f3ed9fe1a1 (patch) | |
| tree | 7a076533058b7dfb62e1a12f5dc0eeb6d1a2456e | |
| parent | 65cee725a913678fa8cf4f3ab0d92bcc148937b2 (diff) | |
| download | man-pages-ca302d0ee368b699ffec4cd465ee02f3ed9fe1a1.tar.gz | |
ptrace.2: Explain WNOHANG behavior and EINTR bug
I didn't like ithe "SIGKILL operates similarly, with exceptions"
phrase (if it's different, then it's not "similar", right?),
and now I got around to changing it. Now it says simply:
"SIGKILL does not generate signal-delivery-stop and therefore
the tracer can't suppress it."
Replaced "why WNOHANG is not reliable" example with a more
realistic one (the one which actually inspired to add this
information to man page in the first place): we got
ESRCH - process is gone! - but waitpid(WNOHANG) can still
confusingly return 0 "no processes to wait for".
Replaced "This means that unneeded trailing arguments may
be omitted" part with a much better recommendation
to never do that and to supply zero arguments instead.
(The part about "undocumentedness" of gcc behavior was bogus,
btw - deleted).
Expanded BUGS section with the explanation and an example
of visible strace behavior on the buggy syscalls which
exit with EINTR on ptrace attach. I hope this will lead
to people submitting better bug reports to lkml about
such syscalls.
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
| -rw-r--r-- | man2/ptrace.2 | 72 |
1 files changed, 61 insertions, 11 deletions
diff --git a/man2/ptrace.2 b/man2/ptrace.2 index 5d9d4d359a..efaf59711a 100644 --- a/man2/ptrace.2 +++ b/man2/ptrace.2 @@ -46,7 +46,7 @@ .\" FIXME Linux 3.1 adds PTRACE_SEIZE, PTRACE_INTERRUPT, .\" and PTRACE_LISTEN. .\" -.TH PTRACE 2 2012-04-26 "Linux" "Linux Programmer's Manual" +.TH PTRACE 2 2012-08-03 "Linux" "Linux Programmer's Manual" .SH NAME ptrace \- process trace .SH SYNOPSIS @@ -593,10 +593,8 @@ tracees within a multithreaded process. (The term "signal-delivery-stop" is explained below.) .LP .B SIGKILL -operates similarly, with exceptions. -No signal-delivery-stop is generated for -.B SIGKILL -and therefore the tracer can't suppress it. +does not generate signal-delivery-stop and +therefore the tracer can't suppress it. .B SIGKILL kills even within system calls (syscall-exit-stop is not generated prior to death by @@ -728,8 +726,13 @@ even if the tracer knows there should be a notification. Example: .nf - kill(tracee, SIGKILL); - waitpid(tracee, &status, __WALL | WNOHANG); + errno = 0; + ptrace(PTRACE_CONT, pid, 0L, 0L); + if (errno == ESRCH) { + /* tracee is dead */ + r = waitpid(tracee, &status, __WALL | WNOHANG); + /* r can still be 0 here! */ + } .fi .\" FIXME: .\" waitid usage? WNOWAIT? @@ -1645,10 +1648,12 @@ glibc currently declares as a variadic function with only the .I request argument fixed. -This means that unneeded trailing arguments may be omitted, -though doing so makes use of undocumented -.BR gcc (1) -behavior. +It is recommended to always supply four arguments, +even if the requested operation does not use them, +setting unused/ignored arguments to +.I 0L +or +.IR "(void\ *)\ 0". .LP In Linux kernels before 2.6.26, .\" See commit 00cd5c37afd5f431ac186dd131705048c0a11fdb @@ -1743,6 +1748,51 @@ and from an .BR inotify (7) file descriptor. +The usual symptom of this bug is that when you attach to +a quiescent process with the command + + strace -p <process-ID> + +then, instead of the usual +and expected one-line output such as +.nf + + restart_syscall(<... resuming interrupted call ...>_ + +.fi +or +.nf + + select(6, [5], NULL, [5], NULL_ + +.fi +('_' denotes the cursor position), you observe more than one line. +For example: +.nf + + clock_gettime(CLOCK_MONOTONIC, {15370, 690928118}) = 0 + epoll_wait(4,_ + +.fi +What is not visible here is that the process was blocked in +.BR epoll_wait (2) +before +.BR strace (1) +has attached to it. +Attaching caused +.BR epoll_wait (2) +to return to userspace with the error +.BR EINTR . +In this particular case, the program reacted to +.B EINTR +by checking ithe current time, and then executing +.BR epoll_wait (2) +again. +(Programs which do not expect such "stray" +.BR EINTR +errors may behave in an unintended way upon an +.BR strace (1) +attach.) .SH "SEE ALSO" .BR gdb (1), .BR strace (1), |