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

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Date:   Fri, 23 Oct 2020 21:52:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Peter Anvin <hpa@...or.com>
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()

From: Linus Torvalds
> Sent: 23 October 2020 22:11
> 
> 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.

Could do_put_user() do an initial check for 64 bit
then expand a different #define that contains the actual
code passing either "a" or "A" for the constriant.

Apart from another level of indirection nothing is duplicated.

	David

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