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Date: Wed, 05 Dec 2007 16:14:46 +1000 From: David Holmes - Sun Microsystems <David.Holmes@....COM> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCH -v2] fix for futex_wait signal stack corruption Thanks for clarifying that Linus. Regards, David Holmes Linus Torvalds said the following on 5/12/07 04:06 PM: > > On Wed, 5 Dec 2007, David Holmes - Sun Microsystems wrote: >> While this was observed with process control signals, my concern was that >> other signals might cause pthread_cond_timedwait to return immediately in the >> same way. The test program allows for SIGUSR1 and SIGRTMIN testing as well, >> but these other signals did not cause the immediate return. But it would seem >> from Steven's analysis that this is just a fortuitous result. If I understand >> things correctly, any interruption of pthread_cond_timedwait by a signal, >> could result in waiting until an arbitrary time - depending on how the stack >> value was corrupted. Is that correct? > > No, very few things can actually cause the restart_block path to be taken. > An actual signal execution would turn that into an EINTR, the only case > that should ever trigger this is a signal that causes some kernel action > (ie the system call *is* interrupted), but does not actually result in any > user-visible state changes. > > The classic case is ^Z + bg, but iirc you can trigger it with ptrace too. > And I think two threads racing to pick up the same signal can cause it > too, for that matter (ie one thread takes the signal, the other one got > interrupted but there's nothing there, so it just causes a system call > restart). > > There's basically two different system call restart mechanisms in the > kernel: > > - returning -ERESTARTNOHAND will cause the system call to be restarted > with the *original* arguments if no signal handler was actually > invoked. This has been around for a long time, and is used by a lot of > system calls. It's fine for things that are idempotent, ie the argument > meaning doesn't change over time (things like a "read()" system call, > for example) > > - the "restart_block" model that returns -ERESTARTBLOCK, which will cause > the system call to be restarted with the arguments specified in the > system call restart block. This is for system calls that are *not* > idempotent, ie the argument might be a relative timeout or something > like that, where we need to actually behave *differently* when > restarting. > > The latter case is "new" (it's been around for a while, but relative to > the ERESTARTNOHAND one), and it relies on the system call itself setting > up its restart point and the argument save area. And each such system call > can obviously screw it up by saving/restoring the arguments with the > incorrect semantics. > > So this bug was really (a) specific to that particular futex restart > mechanism, and (b) only triggers for the (rather unusual) case where the > system call gets interrupted by a signal, but no signal handler actually > happens. In practice, ^Z is the most common case by far (other signals are > either ignored and don't even cause an interrupt event in the first place, > or they are "real" signals, and cause a signal handler to be invoked). > > Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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