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Date: Fri, 14 Mar 2003 06:46:27 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Tim Robbins <tjr@FreeBSD.ORG>
Cc: current@FreeBSD.ORG
Subject: Re: failed to set signal flags properly for ast()
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On Wed, 12 Mar 2003, Tim Robbins wrote:

> Compile, run under gdb, then type "print test()" when the program receives
> SIGABRT. Seems to work incorrectly on 4.7 too.
>
> #include <stdio.h>
> #include <stdlib.h>
>
> void
> test(void)
> {
>
> 	puts("hello");
> }
>
> int
> main(int argc, char *argv[])
> {
>
> 	abort();
> 	exit(0);
> }

Here's a simpler example:

%%%
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
	sigset_t mask, omask;
	volatile int i;

	sigfillset(&mask);
	sigprocmask(SIG_SETMASK, &mask, &omask);
	exit(0);
}
%%%

Single stepping through this hangs at the end of the sigprocmask() and
triggers the INVARIANTS check.  This is because masking SIGTRAP fouls
up trap handling.  I first tried masking only SIGTRAP.  This caused a
SIGSEGV on return of sigprocmask().  Masking SIGSEGV as well stops gdb
seeing any signals.

abort() causes similar misbehaviour by masking almost all signals.
gdb gets control for SIGABRT because SIGABRT is not masked, but
"call test()" hangs because it generates a SIGTRAP but SIGTRAP is
masked.

The system states are approx. 10% user and 90% sys for the hang in
both cases, so it seems that the misbehaviour is just the usual one
for bogusly masking signals for restartable exceptions if such an
exception occurs, and the bug is mainly in the sanity check
(SIGPENDING()'s P_TRACED check is less than a pessimization).

Bruce

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