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

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Date:   Tue, 04 May 2021 00:56:20 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, 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 03 2021 at 15:08, Linus Torvalds wrote:
> 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?

GDB is one thing. But is this setup actually correct under all
circumstances?

It's all fine that we have lots of blurb about GDB, but there is no
reasoning why this does not affect regular kernel threads which take the
same code path.

Neither is there an answer what happens in case of a signal delivered to
this thread and what any other GDB/ptraced induced poking might cause.

This is a half setup user space thread which is assumed to behave like a
regular kernel thread, but is this assumption actually true?

Thanks,

        tglx