[v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0

Borislav Petkov bp at suse.de
Wed Apr 26 03:05:42 CDT 2017

On Tue, Apr 25, 2017 at 07:04:20PM -0700, Ricardo Neri wrote:
> For the specific case of ModRM.mod being 0, I feel I need to clarify
> that REX.B is not decoded and if SIB.base is %r13 the base is also 0.

Well, that all doesn't matter. The rule is this:

ModRM.mod == 00b and ModRM.r/m == 101b -> effective address: disp32

See Table 2-2. "32-Bit Addressing Forms with the ModR/M Byte" in the SDM.

So the base register is not used. How that base register is specified
then doesn't matter (undecoded REX bits or not).

> This comment adds clarity because REX.X is decoded when determining
> SIB.index.

Well, that's a different thing. The REX bits participating in the SIB
fields don't matter about this particular case. We only want to say that
we're returning a disp32 without a base register and the comment should
keep it simple without extraneous information.

I know, you want to mention what Table 2-5. "Special Cases of REX
Encodings" says but we should avoid unnecessary content in the comment.
People who want details can stare at the manuals - the comment should
only document what that particular case is.

Btw, you could write it even better:

	if (!X86_MODRM_MOD(insn->modrm.value) && X86_MODRM_RM(insn->modrm.value) == 5)

and then it is basically a 1:1 copy of the rule from Table 2-2.


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