linux-kernel - Re: [RFC] LKMM: Add volatile

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20210604160955.GG18427@gate.crashing.org>
Date:   Fri, 4 Jun 2021 11:09:55 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>, will@...nel.org,
        paulmck@...nel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, linux-kernel@...r.kernel.org,
        linux-toolchains@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()

Hi!

On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> With optimizing compilers becoming more and more agressive and C so far
> refusing to acknowledge the concept of control-dependencies even while
> we keep growing the amount of reliance on them, things will eventually
> come apart.

Yes, C is still not a portable assembler.

> There have been talks with toolchain people on how to resolve this; one
> suggestion was allowing the volatile qualifier on branch statements like
> 'if', but so far no actual compiler has made any progress on this.

"if" is not a "branch statement".

> --- a/arch/powerpc/include/asm/barrier.h
> +++ b/arch/powerpc/include/asm/barrier.h
> @@ -80,6 +80,19 @@ do {									\
>  	___p1;								\
>  })
>  
> +#ifndef __ASSEMBLY__
> +/* Guarantee a conditional branch that depends on @cond. */
> +static __always_inline bool volatile_cond(bool cond)
> +{
> +	asm_volatile_goto("and. %0,%0,%0; bne %l[l_yes]"
> +			  : : "r" (cond) : "cc", "memory" : l_yes);
> +	return false;
> +l_yes:
> +	return true;
> +}
> +#define volatile_cond volatile_cond
> +#endif

"cmpwi" is ever so slightly better than "and.".  And you can write "cr0"
instead of "cc" more explicitely (it means the same thing though).


I didn't find a description of the expected precise semantics anywhere
in this patch.  This however is the most important thing required here!


Segher