linux-kernel - Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f

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Date:   Wed, 7 Feb 2018 11:01:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     kbuild test robot <fengguang.wu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f

On Wed, Feb 7, 2018 at 10:38 AM, Randy Dunlap <rdunlap@...radead.org> wrote:
> On 02/07/2018 10:13 AM, Linus Torvalds wrote:
>>
>> That said, intel only _documents_ UD2 (0f 0b).
>
> Intel Order Number: 325383-064US, October 2017, documents UD0, UD1, and UD2.
> Section A.2.5, Table A-1, says:

Ahh, I had an older version.

Looking at the latest one I can find (325462-065US), it does specify
that it has a modrm byte:

  0F FF /r   UD0 1 r32, r/m32

so I think that our opcode maps are wrong, and it's a bit dangerous to
put random constants right after the UD0.

Maybe we should make our use of UD0 have a third byte: add a harmless
modrm byte before the warning constants?

But yes, at least my objdump just thinks it's a bad 2-byte sequence,
and doesn't look at any modrm bytes at all.

                Linus