linux-kernel - Re: [PATCH 20/20] objtool: Add Direction Flag validation

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Message-ID: <20190308215602.txm4bv2eg2tyubuj@treble>
Date:   Fri, 8 Mar 2019 15:56:02 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, luto@...nel.org,
        bp@...en8.de, dvlasenk@...hat.com, linux-kernel@...r.kernel.org,
        dvyukov@...gle.com, rostedt@...dmis.org
Subject: Re: [PATCH 20/20] objtool: Add Direction Flag validation

On Fri, Mar 08, 2019 at 10:33:43PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 08, 2019 at 03:16:03PM -0600, Josh Poimboeuf wrote:
> > On Thu, Mar 07, 2019 at 12:45:31PM +0100, Peter Zijlstra wrote:
> > > Having DF escape is BAD(tm).
> > > 
> > > Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > 
> > Can you elaborate on why (in the patch description)?  Did this actually
> > find any occurrences?
> 
> Nope, didn't find anything. Also, all DF users are in asm so I didn't
> really expect any. Having it escape would probably result in fairly
> instant wreckage though.
> 
> DF=1 results in things like rep mov going _backwards_.

Ok, I wonder if we really need to add this then.

-- 
Josh