RE: [PATCH] x86/uaccess: fix code generation in put_user()

[hpa]

On October 23, 2020 2:52:16 PM PDT, David Laight <David.Laight@xxxxxxxxxx> wrote:
>From: Linus Torvalds
>> Sent: 23 October 2020 22:11
>> 
>> On Fri, Oct 23, 2020 at 2:00 PM <hpa@xxxxxxxxx> 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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Maybe #define ASM_AX64 or some such.
-- 
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