Re: Never mind. Re: Signal left blocked after signal handler.

[Linus Torvalds <torvalds@osdl.org>]

On Wed, 26 Nov 2003, Bruce Perens wrote:
>
> The test code works on 2.4, but the electric fence confidence test does
> not. Maybe something odd with SIGSEGV, which is
> what that confidence test is catching. I will go back and see why.

One difference in 2.4.x and 2.6.x is the signal blocking wrt blocked
signals that are _forced_ (ie anything that is thread-synchronous, like a
SIGSEGV/SIGTRAP/SIGBUS that happens as a result of a fault):

 - in 2.4.x they will just punch through the block
 - in 2.6.x they will refuse to punch through a blocked signal, but
   since they can't be delivered they will cause the process to be
   killed.

Trivial test program:

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

	void sigsegv(int sig)
	{
		*(int *)0=0;
	}

	int main(int argc, char **argv)
	{
		struct sigaction sa = { .sa_handler = sigsegv };

		sigaction(SIGSEGV, &sa, NULL);
		*(int *)0 = 0;
	}

and in 2.4.x this will cause infinte SIGSEGV's (well, they'll be caught by
the stack size eventually, but you see the problem: do a "strace" to see
what's going on). In 2.6.x the second SIGSEGV will just kill the program
immediately.

If you _want_ the recursive behaviour, you should add

	.sa_flags = SA_NODEFER

to the sigaction initializer.

I don't understand why your test-program works differently on 2.4.x,
though, since a "kill()" system call is _not_ thread-synchronous, and
should never punch through anything. Not even on 2.4.x.

		Linus