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

Ricardo Neri
2017-04-27T22:49:58Z

On Wed, 2017-04-26 at 10:05 +0200, Borislav Petkov wrote:
> 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.

You are right. This summarizes the rule. Then I will shorten the
comment.
>
> 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.

It is!

Thanks and BR,
Ricardo