linux-kernel - Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io

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Date:   Wed, 12 May 2021 10:44:40 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Olivier Langlois <olivier@...llion01.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Stefan Metzmacher <metze@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es'
 registers for io_threads

On Tue, May 11, 2021 at 9:24 PM Olivier Langlois <olivier@...llion01.com> wrote:
>
> I have reported an issue that I have with a user process using io_uring
> where when it core dumps, the dump fails to be generated.
> https://github.com/axboe/liburing/issues/346

I suspect most kernel developers don't have github notifications
enabled. I know I have them disabled because it would be *way* too
noisy not to.

But maybe Jens does for that libiouring part.

> Pavel did comment to my report and he did point out this thread as
> possibly a related issue.

I don't think this is related. The gdb confusion wouldn't affect core
dump generation.

I don't see why a core-dump shouldn't work from an IO thread these
days - the signal struct and synchronization should all be the same as
for a regular user thread.

That said, I do wonder if we should avoid generating core dumps from
the IO worker thread itself. The IO thread itself should never get a
SIGSEGV/SIGBUS anyway, it should have been turned into -EFAULT.

So maybe the

                if (current->flags & PF_IO_WORKER)
                        goto out;

in kernel/signal.c should be moved up above the do_coredump() logic regardless.

Jens, have you played with core-dumping when there are active io_uring
threads? There's a test-program in that github issue report..

              Linus