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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 29 Apr 2020 20:50:15 -0700
Message-ID: <CAHk-=wh-sm0WHmfyz21w_BzFrDPqfXiaWZzE_8hqhxax-G2f8g@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Jann Horn <jannh@google.com>
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 Wed, Apr 29, 2020 at 8:41 PM Jann Horn <jannh@google.com> wrote:
>
> | So a ptrace() user (or [...] wouldn't even see the impossible EAGAIN error.
>
> So I assumed you explicitly wanted ptrace() to restart, too. I was
> just pointing out that that didn't make sense to me.

I'm actually ok with the restart option, simply because I continue to
maintain that the program is buggy. "Anything goes".

To not be buggy, the program needs to install a SIGCHLD handler so
that it can reap its (pseudo-)children.

At which point it doesn't actually make any difference whether we fix
the kernel or not, because then the non-buggy program will just work -
even with a non-modified kernel.

Honestly, the main argument for the kernel doing anything different at
all is that from a user-mode perspective, silently hanging in the
kernel waiting for something to happen is likely the least easy to
debug.

But if you do a return to user space - even if it's to just rinse and
repeat - it's at least not "silent" any more, even if the main noise
it makes is just to waste 100% CPU time. At least that's a big hint to
somebody to take a look.

But yes, we can make ptrace() - and _only_ ptrace() - then not repeat,
and return a new error code that it has never returned before. Like
EAGAIN. Mainly because in that case we're only breaking semantics of
something that was already broken - unlike "write()", which has
perfectly well-defined semantics and wasn't broken.

                 Linus