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

[David Laight <David.Laight@ACULAB.COM>]

From: Linus Torvalds
> Sent: 23 October 2020 22:11
> 
> On Fri, Oct 23, 2020 at 2:00 PM <hpa@zytor.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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