linux-kernel - Re: [PATCH] signal: Restore the stop PTRACE_EVENT

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Date:   Wed, 13 Feb 2019 08:58:28 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        jolsa@...hat.com, Namhyung Kim <namhyung@...nel.org>,
        luca abeni <luca.abeni@...tannapisa.it>,
        syzkaller <syzkaller@...glegroups.com>,
        Ivan Delalande <colona@...sta.com>
Subject: Re: [PATCH] signal: Restore the stop PTRACE_EVENT_EXIT

Oleg Nesterov <oleg@...hat.com> 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.

Eric