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 10:20:30 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'Christophe Leroy'" <christophe.leroy@...roup.eu>,
        "'Linus Torvalds'" <torvalds@...ux-foundation.org>,
        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

On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote:
> 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));

This asm is fine.  It says it writes the "eax" variable, which lives in
the eax register *in that asm* (so *not* guaranteed after it!).

> 	// Provided gcc doesn't save eax here...
> 	asm volatile goto ("xxxxx" ::: "eax" : label);

So this is incorrect.

> 	// ... 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.

No, the register eax will contain the value of the eax variable.  In the
asm; it might well be there before or after the asm as well, but none of
that is guaranteed.


Segher