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2.5.1: kill(pid, sig) before waitpid() returns -1 for sig != 0
- From: Erik Bray <erik dot m dot bray at gmail dot com>
- To: cygwin at cygwin dot com
- Date: Thu, 11 Aug 2016 11:51:39 +0200
- Subject: 2.5.1: kill(pid, sig) before waitpid() returns -1 for sig != 0
- Authentication-results: sourceware.org; auth=none
Hi all,
This is a followup to a report back in 2011 about essentially the same issue:
https://cygwin.com/ml/cygwin/2011-04/msg00031.html
The same test program in that report demonstrates the issue, but with
kill sending any non-zero signal. To reiterate, the problem here is
POSIX compliance with respect to sending signals to zombie processes:
> Existing implementations vary on the result of a kill() with pid indicating an inactive process (a
> terminated process that has not been waited for by its parent). Some indicate success on such a
> call (subject to permission checking), while others give an error of [ESRCH]. Since the definition
> of process lifetime in this volume of POSIX.1-2008 covers inactive processes, the [ESRCH] error
> as described is inappropriate in this case. In particular, this means that an application cannot
> have a parent process check for termination of a particular child with kill(). (Usually this is done
> with the null signal; this can be done reliably with waitpid().)
In response to the originally issue, this was fixed *specifically* for
the case of kill(pid, 0). But my reading of the above is that kill()
should return 0 in this case regardless of the signal (modulo
permissions, etc.). On Linux, for example, when calling kill with pid
of a zombie process the kernel will happily deliver the signal to the
relevant task_struct; it will just never be acted on since the task
will never run again.
The below (untested) patch demonstrates the change I'm suggesting,
though I don't know what other code, if any, might be involved in
this.
Please CC me on any replies since I'm not subscribed to the list.
Thanks,
Erik
diff --git a/winsup/cygwin/signal.cc b/winsup/cygwin/signal.cc
index ff101e3..d819e77 100644
--- a/winsup/cygwin/signal.cc
+++ b/winsup/cygwin/signal.cc
@@ -260,7 +260,7 @@ _pinfo::kill (siginfo_t& si)
}
this_pid = pid;
}
- else if (si.si_signo == 0 && this && process_state == PID_EXITED)
+ else if (this && process_state == PID_EXITED)
{
this_process_state = process_state;
this_pid = pid;
--
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