Re: signalfd API issues (was Re: [PATCH/RFC] signal races/bugs, losing TIF_SIGPENDING and other woes)

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> a) Process-global signals can be read by any thread (inside or outside
> of the process receiving the signal).
> 
> Rationale:
> 	This should always work, so there's no reason to limit it.

I agree, with an appropriate fix to recalc_sigpending_tsk() to only
clear TIF_SIGPENDING if tsk == current (the patch Linus posted
basically) _along_ with a fix to avoid the notifier thingy if stealing
from another task, that would work.

> b) Thread-specific signals can only be read by their target thread.
> 
> Rationale:
> 	This behavior is required by POSIX, and if an application is using
> pthread_kill()/tkill()/tgkill()/etc. to specifically direct a signal, it
> damn well better get to where the app wants it to go.

I agree there too. I don't see the point of the 'feature' of allowing
those to be stolen and can only lead into all sort of new headaches
nobody needs.

> c) Synchronous signals ("Naturally" generated SIGILL, SIGFPE, SIGSEGV,
> SIGBUS, and SIGTRAP. Did I miss any?) are not delivered via signalfd()
> at all. (And by "naturally" generated, I mean signals that would have
> the SI_KERNEL flag set.)

Heh, well, as you say later, it can't be delivered anyway... I don't
think we need to do anything explicit to prevent them from being read()
in signalfd, it will just not happen.

> Rationale: 
> 	These are a subset of thread-specific signals, so they can only be read
> from a signalfd by their target thread.
> 
> However, there's no way for the target thread to get the signal because
> it is either:
> 
> a) not blocked in a syscall waiting for signal delivery and thus further
> execution beyond the instruction causing the signal is impossible
>  OR
> b) it is blocked in a syscall waiting for signal delivery and the error
> is caused by the signal delivery mechanism itself (i.e. a bad pointer
> passed to read/select/poll/epoll_wait/etc.) and thus the signal can't be
> delivered

Ben.