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Date:   Mon, 14 Jun 2021 16:10:33 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Olivier Langlois <olivier@trillion01.com>,
        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>
Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
Message-ID: <20210614141032.GA13677@redhat.com>
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Eric, et al, sorry for delay, I didn't read emails several days.

On 06/10, Eric W. Biederman wrote:
>
> v2: Don't remove the now unnecessary code in prepare_signal.

No, that code is still needed. Otherwise any fatal signal will be
turned into SIGKILL.

> --- 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.

Oleg.