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 -- To unsubscribe from this list: send the line "unsubscribe linux-msdos" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html