linux-kernel - Re: [PATCH] x86: Implement _

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Message-ID: <20170223160349.6f2245ie4bumrd7k@pd.tnic>
Date:   Thu, 23 Feb 2017 17:03:49 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     hpa@...or.com
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, arjan@...ux.intel.com,
        jpoimboe@...hat.com, richard.weinberger@...il.com
Subject: Re: [PATCH] x86: Implement __WARN using UD0

On Thu, Feb 23, 2017 at 07:32:07AM -0800, hpa@...or.com wrote:
> Well, once you are using invalid instructions, it depends not on
> what the CPU decodes but what your own handler expects. Consider
> Microsoft's use of C4 C4 /ib as a meta-instruction (called BOP, "BIOS
> operation")... that format has nothing to do with the CPU, but if you
> want to disassemble the resulting code you need to know about how they
> encode BOP.

How do they use that?

They rely on the fact that C4 C4 is going to #UD as it is an invalid VEX
insn? The second C4 selecting the 100b map which is reserved?

Or?

-- 
Regards/Gruss,
    Boris.

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