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

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Date:   Mon, 14 Jun 2021 11:37:45 -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:

> 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...

The pipe_write case is a good case to consider.  In general filesystems
are only allowed to stop if fatal_signal_pending.  It is an ancient
unix/posix thing.

> 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?

I would very much like some clarity on TIF_NOTIFY_SIGNAL.  At the very
least it would be nice if it could get renamed TIF_NOTIFY_TASK_WORK.
I don't know what it has to do with signals.

> Or perhaps we should change __dump_emit() to clear signal_pending() and
> restart __kernel_write() if it fails or returns a short write.

Given that the code needs to handle pipes that seems a reasonable thing
to do.  Note.  All of the blocking of new signals in prepare_signal
is still in place.  So only a SIGKILL can come in set TIF_SIGPENDING.

So I would say that the current fix is correct (and safe to backport).
But still requires some magic in prepare_signal until we do more.

I don't understand the logic with well enough of adding work to
non-io_uring threads with task_work_add to understand why that happens
in the first place.

There are a lot of silly corners here.  Yes please let's keep on
digging.

Eric