Re: [v2 1/7] x86/mpx: Do not use SIB index if index points to R/ESP

[Andy Lutomirski]

On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual volume 2A states that when memory addressing is used
> (i.e., mod part of ModR/M is not 3), a SIB byte is used and the index of
> the SIB byte points to the R/ESP (i.e.,index = 4), the index should not be
> used in the computation of the memory address.
>
> An example of such instruction could be
>
>     insn -0x80(%rsp)
>
> This is represented as:
>
>      [opcode] 4c 24 80
>
>       ModR/M: mod: 1, reg: 1: r/m: 4 (R/ESP)
>       SIB 24: sc: 0, index: 100 (R/ESP), base(R/ESP): 100
>       Displacement -0x80
>
> The correct address is (base) + displacement; no index is used.
>
> Care is taken to allow R12 to be used as index, which is a valid scenario.

Since I have no idea what this patch has to do with the rest of the
series, I'll ask a question:

Why isn't this code in the standard x86 instruction decoder?  Is the
decoder similarly buggy?