Re: [PATCH -v2] fix for futex_wait signal stack corruption

[David Holmes - Sun Microsystems <David.Holmes@Sun.COM>]

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
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