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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
From: Olivier Langlois <olivier@trillion01.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>,
Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
io-uring <io-uring@vger.kernel.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
Jens Axboe <axboe@kernel.dk>,
"Pavel Begunkov>" <asml.silence@gmail.com>
Date: Wed, 16 Jun 2021 15:23:14 -0400
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On Tue, 2021-06-15 at 17:08 -0500, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > > --- a/fs/coredump.c
> > > +++ b/fs/coredump.c
> > > @@ -519,7 +519,7 @@ static bool dump_interrupted(void)
> > > * but then we need to teach dump_write() to restart and
> > > clear
> > > * TIF_SIGPENDING.
> > > */
> > > - return signal_pending(current);
> > > + return fatal_signal_pending(current) ||
> > > freezing(current);
> > > }
> >
> >
> > Well yes, this is what the comment says.
> >
> > But note that there is another reason why dump_interrupted()
> > returns true
> > if signal_pending(), it assumes thagt __dump_emit()-
> > >__kernel_write() may
> > fail anyway if signal_pending() is true. Say, pipe_write(), or iirc
> > nfs,
> > perhaps something else...
> >
> > That is why zap_threads() clears TIF_SIGPENDING. Perhaps it should
> > clear
> > TIF_NOTIFY_SIGNAL as well and we should change io-uring to not
> > abuse the
> > dumping threads?
> >
> > Or perhaps we should change __dump_emit() to clear signal_pending()
> > and
> > restart __kernel_write() if it fails or returns a short write.
> >
> > Otherwise the change above doesn't look like a full fix to me.
>
> Agreed. The coredump to a pipe will still be short. That needs
> something additional.
>
> The problem Olivier Langlois <olivier@trillion01.com> reported was
> core dumps coming up short because TIF_NOTIFY_SIGNAL was being
> set during a core dump.
>
> We can see this with pipe_write returning -ERESTARTSYS
> on a full pipe if signal_pending which includes TIF_NOTIFY_SIGNAL
> is true.
>
Eric,
I redid my test but this time instead of dumping directly into a file,
I did let the coredump be piped to the systemd coredump module and the
coredump generation isn't working as expected when piping.
So your code review conclusions are correct.