linux-kernel - RE: remove the last set_fs() in common code, and remove it for x86 and powerpc v3

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Date:   Thu, 10 Sep 2020 12:26:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     David Laight <David.Laight@...LAB.COM>,
        'Christophe Leroy' <christophe.leroy@...roup.eu>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>
CC:     linux-arch <linux-arch@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Luis Chamberlain" <mcgrof@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Christoph Hellwig <hch@....de>
Subject: RE: remove the last set_fs() in common code, and remove it for x86
 and powerpc v3

From: David Laight
> Sent: 10 September 2020 10:26
...
> > > I had an 'interesting' idea.
> > >
> > > Can you use a local asm register variable as an input and output to
> > > an 'asm volatile goto' statement?
> > >
> > > Well you can - but is it guaranteed to work :-)
> > >
> >
> > With gcc at least it should work according to
> > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html
> >
> > They even explicitely tell: "The only supported use for this feature is
> > to specify registers for input and output operands when calling Extended
> > asm "
> 
> A quick test isn't good....
> 
> int bar(char *z)
> {
>         __label__ label;
>         register int eax asm ("eax") = 6;
>         asm volatile goto (" mov $1, %%eax" ::: "eax" : label);
> label:
>         return eax;
> }
> 
> 0000000000000040 <bar>:
>   40:   b8 01 00 00 00          mov    $0x1,%eax
>   45:   b8 06 00 00 00          mov    $0x6,%eax
>   4a:   c3                      retq
> 
> although adding:
>         asm volatile ("" : "+r" (eax));
> either side of the 'asm volatile goto' does fix it.

Actually this is pretty sound:
	__label__ label;
	register int eax asm ("eax");
	// Ensure eax can't be reloaded from anywhere
	// In particular it can't be reloaded after the asm goto line
	asm volatile ("" : "=r" (eax));
	// Provided gcc doesn't save eax here...
	asm volatile goto ("xxxxx" ::: "eax" : label);
	// ... and reload the saved value here.
	// The input value here will be that modified by the 'asm goto'.
	// Since this modifies eax it can't be moved before the 'asm goto'.
	asm volatile ("" : "+r" (eax));
	// So here eax must contain the value set by the "xxxxx" instructions.

    David

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