linux-kernel - Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY

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Message-ID: <8b6f2ce48c4097a23f5d03d631ed9363ecd45ddf.camel@trillion01.com>
Date:   Wed, 05 Jan 2022 14:39:09 -0500
From:   Olivier Langlois <olivier@...llion01.com>
To:     Pavel Begunkov <asml.silence@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL

On Fri, 2021-12-24 at 10:37 +0000, Pavel Begunkov wrote:
> On 12/24/21 01:34, Olivier Langlois wrote:
> > On Fri, 2021-10-22 at 15:13 +0100, Pavel Begunkov wrote:
> > > On 6/9/21 21:17, Eric W. Biederman wrote:
> > > In short, a task creates an io_uring worker thread, then the
> > > worker
> > > submits a task_work item to the creator task and won't die until
> > > the item is executed/cancelled. And I found that the creator task
> > > is
> > > sleeping in do_coredump() -> wait_for_completion()
> > > 
> [...]
> > > A hack executing tws there helps (see diff below).
> > > Any chance anyone knows what this is and how to fix it?
> > > 
> [...]
> > Pavel,
> > 
> > I cannot comment on the merit of the proposed hack but my proposed
> > patch to fix the coredump truncation issue when a process using
> > io_uring core dumps that I submitted back in August is still
> > unreviewed!
> 
> That's unfortunate. Not like I can help in any case, but I assumed
> it was dealt with by
> 
> commit 06af8679449d4ed282df13191fc52d5ba28ec536
> Author: Eric W. Biederman <ebiederm@...ssion.com>
> Date:   Thu Jun 10 15:11:11 2021 -0500
> 
>      coredump: Limit what can interrupt coredumps
>      
>      Olivier Langlois has been struggling with coredumps being
> incompletely written in
>      processes using io_uring.
>      ...
> 
It was a partial fix only. Oleg Nesterov pointed out that the fix was
not good if if the core dump was written into a pipe.

https://lore.kernel.org/io-uring/20210614141032.GA13677@redhat.com/

> > https://lore.kernel.org/lkml/1625bc89782bf83d9d8c7c63e8ffcb651ccb15
> > fa.1629655338.git.olivier@...llion01.com/
> > 
> > I have been using it since then I must have generated many dozens
> > of
> > perfect core dump files with it and I have not seen a single
> > truncated
> > core dump files like I used to have prior to the patch.
> > 
> > I am bringing back my patch to your attention because one nice side
> > effect of it is that it would have avoided totally the problem that
> > you
> > have encountered in coredump_wait() since it does cancel io_uring
> > resources before calling coredump_wait()!
> 
> FWIW, I worked it around in io_uring back then by breaking the
> dependency.
> 
Yes I have seen your work to fix the problem.

It just seems to me that there is no good reason to keep io_uring
process requests once you are generating a core dump and simply
cancelling io_uring before generating the core dump would have avoided
the problem that you have encountered plus any other similar issues yet
to come...

Greetings,