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 UD0ยน 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.
