* Signal left blocked after signal handler. @ 2003-11-26 17:39 Bruce Perens 2003-11-26 17:55 ` Linus Torvalds 2003-11-27 9:20 ` Herbert Xu 0 siblings, 2 replies; 10+ messages in thread From: Bruce Perens @ 2003-11-26 17:39 UTC (permalink / raw) To: linux-kernel; +Cc: torvalds Hi, A signal should be blocked while its signal handler is executing, and then unblocked when the handler returns - unless SA_NOMASK is set. -test9 and -test10 leave the signal _blocked_forever_. This causes the build-time confidence test for Electric Fence to break, and no doubt lots of other code. If SA_NOMASK is set, the signal is not blocked. Test program attached below. Thanks Bruce #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <signal.h> #include <setjmp.h> static sigjmp_buf sjbuf; static int sig = SIGINT; static void handler(int i) { struct sigaction act; memset((void *)&act, 0, sizeof(act)); act.sa_handler = SIG_DFL; fprintf(stderr, "Signal handler hit!\n"); fflush(stderr); sigaction(sig, &act, 0); siglongjmp(sjbuf, 1); } static void invoke_signal() { struct sigaction act; memset((void *)&act, 0, sizeof(act)); act.sa_handler = handler; /* act.sa_flags = SA_NOMASK; */ if ( sigsetjmp(sjbuf, 0) == 0 ) { sigaction(sig, &act, 0); fprintf(stderr, "Sending signal... "); fflush(stderr); kill(getpid(), sig); fprintf(stderr, "Huh? Nothing happened. Signal was left blocked.\n"); } } int main(int argc, char * * argv) { sigset_t set; sigemptyset(&set); sigaddset(&set, sig); invoke_signal(); invoke_signal(); fprintf(stderr, "Unblocking signal... "); if ( sigsetjmp(sjbuf, 0) == 0 ) { sigprocmask(SIG_UNBLOCK, &set, 0); } return 0; } ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: Signal left blocked after signal handler. 2003-11-26 17:39 Signal left blocked after signal handler Bruce Perens @ 2003-11-26 17:55 ` Linus Torvalds [not found] ` <3FC4ED5F.4090901@perens.com> 2003-11-27 9:20 ` Herbert Xu 1 sibling, 1 reply; 10+ messages in thread From: Linus Torvalds @ 2003-11-26 17:55 UTC (permalink / raw) To: Bruce Perens, Ulrich Drepper; +Cc: Kernel Mailing List [ Uli added to participants ] On Wed, 26 Nov 2003, Bruce Perens wrote: > > A signal should be blocked while its signal handler is executing, and > then unblocked when the handler returns - unless SA_NOMASK is set. > > -test9 and -test10 leave the signal _blocked_forever_. >From what I can tell, this is a glibc bug. Do an "strace" on the program, and see how "siglongjmp()" doesn't appear to do any system calls at all. It's up to siglongjmp() to restore the signal mask that it saved on sigsetjmp(). What library version are you using (but yes, I see the same thing with "Fedora Core 1"). In fact, with strace I don't even see where the signal mask would be _saved_ in sigsetjmp. So as far as I can tell, this just cannot work. Linus --- rest of email saved for Uli --- > > This causes the build-time confidence test for Electric Fence to break, > and no doubt lots of other code. > > If SA_NOMASK is set, the signal is not blocked. > > Test program attached below. > > Thanks > > Bruce > > #include <stdio.h> > #include <stdlib.h> > #include <unistd.h> > #include <signal.h> > #include <setjmp.h> > > static sigjmp_buf sjbuf; > static int sig = SIGINT; > > static void > handler(int i) > { > struct sigaction act; > > memset((void *)&act, 0, sizeof(act)); > act.sa_handler = SIG_DFL; > > fprintf(stderr, "Signal handler hit!\n"); > fflush(stderr); > sigaction(sig, &act, 0); > siglongjmp(sjbuf, 1); > > } > > static void > invoke_signal() > { > struct sigaction act; > > memset((void *)&act, 0, sizeof(act)); > act.sa_handler = handler; > > /* act.sa_flags = SA_NOMASK; */ > > if ( sigsetjmp(sjbuf, 0) == 0 ) { > sigaction(sig, &act, 0); > fprintf(stderr, "Sending signal... "); > fflush(stderr); > kill(getpid(), sig); > fprintf(stderr, "Huh? Nothing happened. Signal was left blocked.\n"); > } > } > > int > main(int argc, char * * argv) > { > sigset_t set; > > sigemptyset(&set); > sigaddset(&set, sig); > > invoke_signal(); > invoke_signal(); > fprintf(stderr, "Unblocking signal... "); > if ( sigsetjmp(sjbuf, 0) == 0 ) { > sigprocmask(SIG_UNBLOCK, &set, 0); > } > > return 0; > } > ^ permalink raw reply [flat|nested] 10+ messages in thread
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* Re: Signal left blocked after signal handler. [not found] ` <3FC4ED5F.4090901@perens.com> @ 2003-11-26 18:21 ` Linus Torvalds [not found] ` <3FC4EF24.9040307@perens.com> 1 sibling, 0 replies; 10+ messages in thread From: Linus Torvalds @ 2003-11-26 18:21 UTC (permalink / raw) To: Bruce Perens; +Cc: Ulrich Drepper, Kernel Mailing List On Wed, 26 Nov 2003, Bruce Perens wrote: > > Sigsetjmp will save and restore the signal mask ONLY if its second > argument is nonzero. The libc code is correct. Oh, I didn't notice that part. > The test program works properly under 2.4 . What do you mean "properly"? If you're not saving/restoring the sigmasks, then the 2.6.x behaviour is the right one and your program is buggy. What's your point? Linus ^ permalink raw reply [flat|nested] 10+ messages in thread
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* Re: Signal left blocked after signal handler. [not found] ` <3FC4EF24.9040307@perens.com> @ 2003-11-26 18:34 ` Linus Torvalds [not found] ` <3FC4F248.8060307@perens.com> 1 sibling, 0 replies; 10+ messages in thread From: Linus Torvalds @ 2003-11-26 18:34 UTC (permalink / raw) To: Bruce Perens; +Cc: Ulrich Drepper, Kernel Mailing List On Wed, 26 Nov 2003, Bruce Perens wrote: > > OK, I see. The signal remains blocked forever if we jump out of the > handler. This is not the case on 2.4 . So, is this is semantic change? It _shouldn't_ be a semantic change. Quite frankly, 2.4.x shouldn't work the way you describe either. I wonder if it's the "sigaction()" call in the handler that unblocks the signal in 2.4.x. Linus ^ permalink raw reply [flat|nested] 10+ messages in thread
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* Re: Never mind. Re: Signal left blocked after signal handler. [not found] ` <3FC4F248.8060307@perens.com> @ 2003-11-26 18:45 ` Linus Torvalds 2003-11-26 19:04 ` Bruce Perens 0 siblings, 1 reply; 10+ messages in thread From: Linus Torvalds @ 2003-11-26 18:45 UTC (permalink / raw) To: Bruce Perens; +Cc: Ulrich Drepper, Kernel Mailing List 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 ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: Never mind. Re: Signal left blocked after signal handler. 2003-11-26 18:45 ` Never mind. " Linus Torvalds @ 2003-11-26 19:04 ` Bruce Perens 2003-11-26 19:14 ` Linus Torvalds 0 siblings, 1 reply; 10+ messages in thread From: Bruce Perens @ 2003-11-26 19:04 UTC (permalink / raw) To: Linus Torvalds; +Cc: Ulrich Drepper, Kernel Mailing List What happened is that I attempted to simplify the test code to send to you, and simplified out the problem by using kill() instead of causing a fault. :-) It's just what you describe here: >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 > > The behavior of 2.4 seems to be the same used by some dozens of Unix systems upon which my confidence test passed. I agree that we should not be wrong in the same way as everyone else, and wonder if POSIX says anything about this. I could have been the only one using this "feature". Thanks Bruce ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: Never mind. Re: Signal left blocked after signal handler. 2003-11-26 19:04 ` Bruce Perens @ 2003-11-26 19:14 ` Linus Torvalds 2003-11-26 19:34 ` Posix says "undefined". " Bruce Perens 2003-11-26 19:52 ` Never mind. " Jamie Lokier 0 siblings, 2 replies; 10+ messages in thread From: Linus Torvalds @ 2003-11-26 19:14 UTC (permalink / raw) To: Bruce Perens; +Cc: Ulrich Drepper, Kernel Mailing List On Wed, 26 Nov 2003, Bruce Perens wrote: > > The behavior of 2.4 seems to be the same used by some dozens of Unix > systems upon which my confidence test passed. Interesting. I know the 2.4.x behaviour wasn't arrived at due to any "compatibility testing" - it was purely a matter of "minimal code". The fact that other unixes did the same despite no other commonalities is interesting in itself ;) But we actually had another unrelated thread about this last week, where SIGTRAP on x86 worked differently under Linux and FreeBSD (both 2.4.x and 2.6.x behaviour differed from BSD behaviour), so clearly it's _not_ a 100% correlation. > I agree that we should not be wrong in the same way as everyone else, > and wonder if POSIX says anything about this. I could have been the only > one using this "feature". I can't say that I'd ever seen this documented anywhere. I personally think it is "good taste" to actually set the SA_NODEFER flag if you know you depend on the behaviour, but if there are lots of existing applications that actually depend on the "forced punch-through" behaviour, then I'll obviously have to change the 2.6.x behaviour (a stable user-level ABI is a lot more important than my personal preferences). But if ElectricFence is the only thing that cares, I'd rather just EF added a SA_NODEFER.. Linus ^ permalink raw reply [flat|nested] 10+ messages in thread
* Posix says "undefined". Re: Signal left blocked after signal handler. 2003-11-26 19:14 ` Linus Torvalds @ 2003-11-26 19:34 ` Bruce Perens 2003-11-26 19:52 ` Never mind. " Jamie Lokier 1 sibling, 0 replies; 10+ messages in thread From: Bruce Perens @ 2003-11-26 19:34 UTC (permalink / raw) To: Linus Torvalds; +Cc: Ulrich Drepper, Kernel Mailing List Linus, Posix says the behavior is undefined. See http://www.opengroup.org/onlinepubs/007904975/functions/sigprocmask.html . I think it makes sense to leave the 2.6 behavior as it is. Thanks Bruce Linus Torvalds wrote: > I personally think it is "good taste" to actually set the SA_NODEFER flag > if you know you depend on the behaviour, but if there are lots of existing > >applications that actually depend on the "forced punch-through" behaviour, >then I'll obviously have to change the 2.6.x behaviour (a stable >user-level ABI is a lot more important than my personal preferences). > >But if ElectricFence is the only thing that cares, I'd rather just EF >added a SA_NODEFER.. > > ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: Never mind. Re: Signal left blocked after signal handler. 2003-11-26 19:14 ` Linus Torvalds 2003-11-26 19:34 ` Posix says "undefined". " Bruce Perens @ 2003-11-26 19:52 ` Jamie Lokier 1 sibling, 0 replies; 10+ messages in thread From: Jamie Lokier @ 2003-11-26 19:52 UTC (permalink / raw) To: Linus Torvalds; +Cc: Bruce Perens, Ulrich Drepper, Kernel Mailing List Linus Torvalds wrote: > I personally think it is "good taste" to actually set the SA_NODEFER flag > if you know you depend on the behaviour, but if there are lots of existing > applications that actually depend on the "forced punch-through" behaviour, > then I'll obviously have to change the 2.6.x behaviour (a stable > user-level ABI is a lot more important than my personal preferences). I also have a program which depends on the behaviour of nesting SIGSEGVs, however luckily I already set the SA_NODEFER flag :) -- Jamie ^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: Signal left blocked after signal handler. 2003-11-26 17:39 Signal left blocked after signal handler Bruce Perens 2003-11-26 17:55 ` Linus Torvalds @ 2003-11-27 9:20 ` Herbert Xu 1 sibling, 0 replies; 10+ messages in thread From: Herbert Xu @ 2003-11-27 9:20 UTC (permalink / raw) To: Bruce Perens, linux-kernel Bruce Perens <bruce@perens.com> wrote: > > Test program attached below. I don't know about your other problems, but your code is buggy. > if ( sigsetjmp(sjbuf, 0) == 0 ) { For sigsetjmp to be useful, you need to call it with a nonzero value in the second argument. -- Debian GNU/Linux 3.0 is out! ( http://www.debian.org/ ) Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt ^ permalink raw reply [flat|nested] 10+ messages in thread
end of thread, other threads:[~2003-11-27 9:20 UTC | newest] Thread overview: 10+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2003-11-26 17:39 Signal left blocked after signal handler Bruce Perens 2003-11-26 17:55 ` Linus Torvalds [not found] ` <3FC4ED5F.4090901@perens.com> 2003-11-26 18:21 ` Linus Torvalds [not found] ` <3FC4EF24.9040307@perens.com> 2003-11-26 18:34 ` Linus Torvalds [not found] ` <3FC4F248.8060307@perens.com> 2003-11-26 18:45 ` Never mind. " Linus Torvalds 2003-11-26 19:04 ` Bruce Perens 2003-11-26 19:14 ` Linus Torvalds 2003-11-26 19:34 ` Posix says "undefined". " Bruce Perens 2003-11-26 19:52 ` Never mind. " Jamie Lokier 2003-11-27 9:20 ` Herbert Xu
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