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

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Date:   Tue, 15 Jun 2021 17:08:23 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Olivier Langlois <olivier@...llion01.com>,
        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

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.

Looking further if the thread that is core dumping initiated
any io_uring work then io_ring_exit_work will use task_work_add
to request that thread clean up it's io_uring state.

Perhaps we can put a big comment in dump_emit and if we
get back -ERESTARTSYS run tracework_notify_signal.  I am not
seeing any locks held at that point in the coredump, so it
should be safe.  The coredump is run inside of file_start_write
which is the only potential complication.

The code flow is complicated but it looks like the entire
point of the exercise is to call io_uring_del_task_file
on the originating thread.  I suppose that keeps the
locking of the xarray in io_uring_task simple.

Hmm.   All of this comes from io_uring_release.
How do we get to io_uring_release?  The coredump should
be catching everything in exit_mm before exit_files?

Confused and hopeful someone can explain to me what is going on,
and perhaps simplify it.

Eric