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Date: Wed, 7 Feb 2018 19:35:43 +0100
From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kbuild test robot <fengguang.wu@intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [linus:master] BUILD REGRESSION
 a2e5790d841658485d642196dbb0927303d6c22f
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On Wed, Feb 07, 2018 at 10:13:35AM -0800, Linus Torvalds wrote:
> Adding more people for this funky warning from the kbuild robot.
> 
> Something is confused. UD0 is 0f ff, the bytes after that shouldn't
> matter. But I guess they can be interpreted as modrm bytes, and
> somebody started doing that.
> 
> That said, intel only _documents_ UD2 (0f 0b).

They documented UD0 and UD1 a year ago or so:

0F FF /r UD0¹  r32, r/m32 RM Valid Valid Raise invalid opcode exception
0F B9 /r UD1 r32, r/m32 RM Valid Valid Raise invalid opcode exception.

and the footnote says

"1. Some older processors decode the UD0 instruction without a ModR/M
byte. As a result, those processors would deliver an invalid- opcode
exception instead of a fault on instruction fetch when the instruction
with a ModR/M byte (and any implied bytes) would cross a page or segment
boundary."

So those two take a ModRM byte.

And we chose UD0 for WARN, see arch/x86/include/asm/bug.h for the
reasoning.

Except objdump can't handle that insn because it doesn't have it in its
insn tables. Thus it says:

  b3:   0f ff                   (bad)
  b5:   eb                      .byte 0xeb

> Maybe we should avoid using UD0/UD1 entirely.

Or that test should ignore UD0.

Or we should add UD0 only *decoding* support to binutils - not
generating.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.