[lkml]   [2018]   [Feb]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
On Wed, Feb 07, 2018 at 07:35:43PM +0100, Borislav Petkov wrote:
> 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.

Argh. So the SDM I'm looking at (March 2017) doesn't list UD0 as having
a ModR/M byte, it doesn't have that footnote.

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

Right, we picked UD0 because we _thought_ everybody agreed it being 2
bytes, just like UD2. This is now not true anymore?

 \ /
  Last update: 2018-02-07 19:50    [W:0.126 / U:5.308 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site