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Date:   Thu, 23 Feb 2017 17:03:49 +0100
From:   Borislav Petkov <>
Cc:     Peter Zijlstra <>,
        Thomas Gleixner <>,
        Ingo Molnar <>,,,,,
Subject: Re: [PATCH] x86: Implement __WARN using UD0

On Thu, Feb 23, 2017 at 07:32:07AM -0800, 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?



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