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SubjectRe: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
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.

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