linux-kernel - Re: [PATCH] x86/uaccess: fix code generation in put

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Message-ID: <06B6997A-81AC-409D-A654-309FA8697F0C@zytor.com>
Date:   Fri, 23 Oct 2020 15:52:00 -0700
From:   hpa@...or.com
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/uaccess: fix code generation in put_user()

On October 23, 2020 2:11:19 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Fri, Oct 23, 2020 at 2:00 PM <hpa@...or.com> wrote:
>>
>> There is no same reason to mess around with hacks when we are talking
>about dx:ax, though.
>
>Sure there is.
>
>"A" doesn't actually mean %edx:%eax like you seem to think.
>
>It actually means %eax OR %edx, and then if given a 64-bit value, it
>will use the combination (with %edx being the high bits).
>
>So using "A" unconditionally doesn't work - it gives random behavior
>for 32-bit (or smaller) types.
>
>Or you'd have to cast the value to always be 64-bit, and have the
>extra code generation.
>
>IOW, an unconditional "A" is wrong.
>
>And the alternative is to just duplicate things, and go back to the
>explicit size testing, but honestly, I really think that's much worse
>than relying on a documented feature of "register asm()" that gcc
>_documents_ is for this kind of inline asm use.
>
>So the "don't do pointless conditional duplication" is certainly a
>very sane reason for the code.
>
>            Linus

Unconditional "A" is definitely wrong, no argument there.
-- 
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