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Re: [BUG] Bash not reacting to Ctrl-C


From: Linus Torvalds
Subject: Re: [BUG] Bash not reacting to Ctrl-C
Date: Mon, 7 Mar 2011 08:24:59 -0800

On Mon, Mar 7, 2011 at 7:52 AM, Chet Ramey <address@hidden> wrote:
>
> Let's take a step back and approach this a different way.  Instead of
> trying to intuit whether or not the child did anything with the SIGINT,
> let's try to make the race condition smaller.

I can still easily see the porblem. I think your patch is an
improvement over plain bash, but I can still see the problem. How much
harder it is to see is hard to say - it's pretty subjective. But I
think my patch (the second one, the one that had the "working" EINTR
logic) made it _harder_ to trigger than yours does.

The trace I just captured looks like this:

  08:12:54.424327 clone(child_stack=0,
flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0x7fdf2ad8e9f0) = 15397
  08:12:54.424394 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  08:12:54.424432 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
  08:12:54.424463 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  08:12:54.424486 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
  08:12:54.424510 rt_sigaction(SIGINT, {0x43c990, [], SA_RESTORER,
0x301f833140}, {SIG_DFL, [], SA_RESTORER, 0x301f833140}, 8) = 0
  08:12:54.424541 wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}],
0, NULL) = 15397
  08:12:54.424668 --- SIGINT (Interrupt) @ 0 (0) ---
  08:12:54.424680 rt_sigreturn(0x2)       = 15397
  08:12:54.424702 rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER,
0x301f833140}, {0x43c990, [], SA_RESTORER, 0x301f833140}, 8) = 0
  08:12:54.424739 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  08:12:54.424761 --- SIGCHLD (Child exited) @ 0 (0) ---
  08:12:54.424772 wait4(-1, 0x7fff0a9956dc, WNOHANG, NULL) = -1 ECHILD
(No child processes)
  08:12:54.424793 rt_sigreturn(0xffffffffffffffff) = 0
  08:12:54.424824 rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
  08:12:54.424850 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  08:12:54.424881 rt_sigprocmask(SIG_BLOCK, [INT CHLD], [], 8) = 0

and I think one reason why the race is hard to get rid of is simply
that system call return is _the_ common point of signal handling in
UNIX (technically, obviously any return to user space, but there are
no appreciable interrupts etc going on, and there _are_ a lot of
system calls). The above trace is one that my patch would have handled
correctly (it has no EINTR).

And there aren't a lot of different system calls in the tight loop
here: that's literally the whole loop of the "while : ; do /bin/true;
done" thing. So most of the time, SIGINT will happen at _one_ of those
system calls, and so even if you set that "waiting_for_child" flag
right around the call to "wait4()", the race is actually much easier
to hit than you'd think from "it's just a few CPU instructions".

So the above trace is one that my patch would have handled correctly
(since it has no EINTR). Maybe a combination of the two approaches
would work even better?

                             Linus



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