linux-kernel - Re: [RFC][PATCH] objtool: STAC/CLAC validation

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Message-ID: <15DE0017-9AE5-4E7A-96CD-E5E9F5B72516@zytor.com>
Date:   Mon, 25 Feb 2019 00:47:00 -0800
From:   hpa@...or.com
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Julien Thierry <julien.thierry@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-arm-kernel@...ts.infradead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [RFC][PATCH] objtool: STAC/CLAC validation

On February 23, 2019 12:39:42 AM PST, Peter Zijlstra <peterz@...radead.org> wrote:
>On Fri, Feb 22, 2019 at 03:39:48PM -0800, hpa@...or.com wrote:
>> Objtool could also detect CLAC-STAC or STAC-CLAC sequences without
>> memory operations and remove them; don't know how often that happens,
>> but I know it *does* happen.
>
>Objtool doesn't know about memops; that'd be a lot of work. Also,
>objtool doesn't actually rewrite the text, at best it could warn about
>such occurences.

It doesn't have to understand the contents of the memop, but it seems that the presence of a modrm with mode ≠ 3 should be plenty. It needs to know that much in order to know the length of instructions anyway. For extra credit, ignore LEA or hinting instructions.
-- 
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