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