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From:   Jann Horn <jannh@google.com>
Date:   Thu, 30 Apr 2020 05:00:03 +0200
Message-ID: <CAG48ez25=6qrCU51dEAV_ciyU2jvPuRFWfFOAAT22kmeAZccuQ@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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On Thu, Apr 30, 2020 at 4:20 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Apr 29, 2020 at 6:08 PM Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
> >
> > I added the BIG FAT WARNNIG comments as a mitigation for that.
> > Did you like those comments?
>
> No.
>
> What's the point olf saying "THIS CODE IS GARBAGE" and then expecting
> that to make it ok?
>
> No,m that doesn't make it ok. It just means that it should have been
> done differently.
>
> > Yes, exactly, the point is the caller is expected to call wait in that
> > scenario, otherwise the -EAGAIN just repeats forever, that is an API
> > change, yes, but something unavoidable, and the patch tries hard to
> > limit it to cases where the live-lock or pseudo-dead-lock is unavoidable
> > anyway.
>
> I'm getting really fed up with your insistence on that KNOWN BROKEN
> garbage test-case.
>
> It's shit. The test-case is wrong. I've told you before.
>
> Your patch as-is breaks other cases that are *not* wrong in the kernel
> currently, and that don't have test-cases because they JustWork(tm).
>
> The livelock isn't interesting. The test-case that shows it is pure
> garbage, and is written wrong.
>
> IF that test-case hadn't been buggy in the first place, it would have
> had ignored its child (or had a handler for SIGCHLD), and not
> livelocked.

But if we go with Bernd's approach together with your restart
suggestion, then simply doing PTRACE_ATTACH on two threads A and B
would be enough to livelock, right?

tracer: PTRACE_ATTACHes to A
B: enters de_thread()
tracer: attempts to PTRACE_ATTACH to B

Now the tracer will loop on PTRACE_ATTACH, right?