Re: [PATCH] signal: Restore the stop PTRACE_EVENT_EXIT

From: Eric W. Biederman
Date: Wed Feb 13 2019 - 09:58:40 EST

Oleg Nesterov <oleg@xxxxxxxxxx> writes:

> 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.

There might be a better way to say it. What I meant to convey is that
in my testing I could get PTRACE_EVENT_EXIT to stop by clearing
TIF_SIGPENDING (and leaving SIGKILL in pending). That was insufficient
to fix the bug. Without SIGKILL in pending ptrace_check_attach would
fail when the process was stopped in PTRACE_EVENT_EXIT.

Or in short it really bugs me that we have to have signal_group_exit
and fatal_signal_pending not in agreement after in do_exit to make the code
work. It bothers me because fatal_signal_pending and signal_group_exit
are in all other cases just different ways to ask the same question.