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

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Date:   Mon, 3 May 2021 15:08:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Thomas Gleixner <tglx@...utronix.de>,
        Stefan Metzmacher <metze@...ba.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 Mon, May 3, 2021 at 2:49 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> To be clear, I'm suggesting that we -EINVAL the PTRACE_GETREGS calls
> and such, not the ATTACH.  I have no idea what gdb will do if this
> happens, though.

I feel like the likelihood that it will make gdb work any better is
basically zero.

I think we should just do Stefan's patch - I assume it generates
something like four instructions (two loads, two stores) on x86-64,
and it "just works".

Yeah, yeah, it presumably generates 8 instructions on 32-bit x86, and
we could fix that by just using the constant __USER_CS/DS instead (no
loads necessary) since 32-bit doesn't have any compat issues.

But is it worth complicating the patch for a couple of instructions in
a non-critical path?

And I don't see anybody stepping up to say "yes, I will do the patch
for gdb", so I really think the least pain is to just take the very
straightforward and tested kernel patch.

Yes, yes, that also means admitting to ourselves that the gdb
situation isn't likely going to improve, but hey, if nobody in this
thread is willing to work on the gdb side to fix the known issues
there, isn't that the honest thing to do anyway?

            Linus