From: Florian Weimer <fweimer@redhat.com>
To: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Adhemerval Zanella Netto via Libc-help <libc-help@sourceware.org>,
Rain <glibc@sunshowers.io>
Subject: Re: posix_spawn: parent can get stuck in uninterruptible sleep if child receives SIGTSTP early enough
Date: Thu, 22 Sep 2022 19:38:00 +0200 [thread overview]
Message-ID: <87leqb1f9j.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <bfc7c457-bfe3-cd5d-789d-b7d697cd1290@linaro.org> (Adhemerval Zanella Netto's message of "Thu, 22 Sep 2022 13:56:39 -0300")
* Adhemerval Zanella Netto:
> On 22/09/22 09:18, Florian Weimer wrote:
>>> Is there anything that prevents to avoid using CLONE_VFORK? The code already
>>> uses a allocated stack and do synchronizes with waitpid.
>>
>> Assuming there is a way to create a thread which gets replaced by execve
>> only (instead the whole process), this won't work because we have to
>> block all signals for the new thread (it must not be visible to
>> application code, and signal handlers must not run on it), and we can't
>> unblock those signals prior to execve. With vfork, we can unblock them
>> after changing the signal handler disposition to SIG_DFL (preventing the
>> handler execution), but per-thread signal handlers have been removed
>> from Linux. So even if we somehow could prevent the termination signal
>> from beign sent to the whole process (and not just the fake thread), we
>> still have a gap.
>
> But we already block all internal signals with internal_signal_block_all
> prior clone call and it does not use CLONE_SIGHAND on the clone call.
> Also, independently of CLONE_SIGHAND, the calling process and child still
> have distinct signal masks. Recall for posix_spawn we do not use
> CLONE_THREAD, so per-thread signal handlers does not apply here.
This only works because we restore SIG_DFL before unblocking signals in
the new process. And that depends on a separate set of signal handlers.
> Doing some tests, the main problem is in fact how to synchronize
> the deallocation of the stack, since without CLONE_VFORK there is no way
> to advertise on a success call when execve has been called.
>
> But I agree that even without CLONE_VFORK we still have a small window,
> between the sigprocmask and execve, that the signal might act upon the
> child.
And that window shouldn't exist in the current implementation.
Thanks,
Florian
next prev parent reply other threads:[~2022-09-22 17:38 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-14 3:30 Rain
2022-08-14 3:38 ` Rain
2022-08-22 16:51 ` Adhemerval Zanella Netto
2022-08-22 17:00 ` Rain
2022-08-22 17:48 ` Adhemerval Zanella Netto
2022-08-22 18:21 ` Florian Weimer
2022-08-22 18:32 ` Adhemerval Zanella Netto
2022-08-22 22:28 ` Adhemerval Zanella Netto
2022-09-13 10:04 ` Florian Weimer
2022-09-21 15:24 ` Adhemerval Zanella Netto
2022-09-22 12:18 ` Florian Weimer
2022-09-22 16:56 ` Adhemerval Zanella Netto
2022-09-22 17:38 ` Florian Weimer [this message]
2022-09-22 19:14 ` Adhemerval Zanella Netto
2022-10-10 13:45 ` Florian Weimer
2022-10-18 20:04 ` Adhemerval Zanella Netto
2022-10-20 11:55 ` Florian Weimer
2022-10-21 1:40 ` Rain
2022-10-21 14:18 ` Szabolcs Nagy
2022-08-22 22:30 ` Adhemerval Zanella Netto
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