linux-kernel - Re: [PATCH] coredump: Limit what can interrupt coredumps

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Date:   Wed, 16 Jun 2021 15:23:14 -0400
From:   Olivier Langlois <olivier@...llion01.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        "Pavel Begunkov>" <asml.silence@...il.com>
Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps

On Tue, 2021-06-15 at 17:08 -0500, Eric W. Biederman wrote:
> Oleg Nesterov <oleg@...hat.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@...llion01.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.