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

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Message-ID: <3F1D3946-E7B8-4F77-9189-D49E1ECE0B0D@zytor.com>
Date:   Fri, 23 Oct 2020 16:39:46 -0700
From:   hpa@...or.com
To:     David Laight <David.Laight@...LAB.COM>,
        "'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 2:52:16 PM PDT, David Laight <David.Laight@...LAB.COM> wrote:
>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
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>MK1 1PT, UK
>Registration No: 1397386 (Wales)

Maybe #define ASM_AX64 or some such.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.