Re: [PATCH 3/3] objtool/x86: Fix NOP decode

Peter Zijlstra
2025-09-25T12:43:36Z

On Thu, Sep 25, 2025 at 12:29:18PM +0100, Andrew Cooper wrote:
> > I've made that:
> >
> > case 0x90:
> > if (rex_b) /* XCHG %r8, %rax */
> > break;
> >
> > if (prefix == 0xf3) /* REP NOP := PAUSE */
> > break;
> >
> > insn->type = INSN_NOP;
> > break;
>
> Legacy prefixes can come in any order.  What is F3 66 90 ?
>
> Also, VEX/EVEX/REX2 want excluding too, all of which can encode rex_b
> differently.

So luckily objtool only really cares about instructions as found in the
kernel text. Neither f3 66 90 nor VEX/EVEX/REX2 prefixes are of much
concern.

But yes.. I happen to have an insn_is_nop() function that can be used on
userspace, and that certainly wants to be taught about these... x86 is
such a pain :/

> Is it really only rex_b which prevents NOP becoming a pause, or is it
> any REX prefix?  I would have thought it was any REX prefix.

SDM opcode table and instruction reference seems consistent with f3
only.