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From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Olivier Langlois <olivier@trillion01.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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Date: Tue, 15 Jun 2021 17:08:23 -0500
In-Reply-To: <20210614141032.GA13677@redhat.com> (Oleg Nesterov's message of
"Mon, 14 Jun 2021 16:10:33 +0200")
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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
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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.
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