Re: [BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

[Oleg Nesterov <oleg@tv-sign.ru>]

On 05/29, Roland McGrath wrote:
>
> Your fix seems like the only way to go.  From skimming all the ERESTART*
> uses, I think that in all cases (except for n_tty.c:job_control before your
> patch), TIF_SIGPENDING is indeed set when a thread returns -ERESTART*.
> 
> But it makes me realize that there is a danger of leaking a -ERESTART*
> return code to userland when TIF_SIGPENDING gets cleared by another thread
> doing a recalc_sigpending_tsk.  Because of -ERESTART* I think we must make
> it a rule that no thread can clear another thread's TIF_SIGPENDING, only
> set it (unless it's known to be stopped in the signal code or something).
> >From our recent work on it, I think that do_sigaction is in fact the only
> place this can happen.  So that says we should err in the other direction
> from what I said before in do_sigaction, and not have it do recalc at all.

I think you are right.

But please note that cancel_freezing(p) is special. It is also called when
try_to_freeze_tasks() fails. So it should clear TIF_SIGPENDING if "p" is a
kernel thread, otherwise p may run with signal_pending() forever.

Unfortunately, it is not easy to detect the kernel thread, is_user_space()
is not reliable. Probably we should ignore this minor problem and do not
change cancel_freezing().

Oleg.