Re: [PATCH] signal: Restore the stop PTRACE_EVENT_EXIT

From: Oleg Nesterov
Date: Wed Feb 13 2019 - 09:38:58 EST

sorry for noise, but after I read the changelog I have a minor nit,
feel free to ignore...

On 02/12, Eric W. Biederman wrote:
> Skipping past dequeue_signal when we know a fatal signal has already
> been delivered resulted in SIGKILL remaining pending and
> TIF_SIGPENDING remaining set. This in turn caused the
> scheduler to not sleep in PTACE_EVENT_EXIT as it figured
> a fatal signal was pending.

Yes, but the status of TIF_SIGPENDING doesn't matter. However I agree
with recalc_sigpending() added by this patch, simply because this is what
the "normal" dequeue_signal() paths do.

> This also caused ptrace_freeze_traced
> in ptrace_check_attach to fail because it left a per thread
> SIGKILL pending which is what fatal_signal_pending tests for.

this is possible too, but in the likely case ptrace_check_attach() won't
be even called exactly because the tracee won't stop and thus waitpid()
won't report WIFSTOPPED. And even if waitpid() "wins" the race and debugger
calls ptrace(), most probably ptrace_freeze_traced() will fail because
task_is_traced() will be false.

I think this part of the changelog looks a bit confusing. It doesn't matter
why ptrace_check_attach() fails, it must fail if the tracee didn't stop.

PTACE_EVENT_EXIT won't stop and thus this event won't be reported, that is all.

again, feel free to ignore.