linux-kernel - Re: [PATCH 5/9] x86/alternative: Handle Jcc __x86_indirect_thunk

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Date:   Wed, 13 Oct 2021 23:08:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     x86@...nel.org, jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 5/9] x86/alternative: Handle Jcc __x86_indirect_thunk_\reg

On Wed, Oct 13, 2021 at 01:11:45PM -0700, Nick Desaulniers wrote:

> > +       /*
> > +        * Convert:
> > +        *
> > +        *   Jcc.d32 __x86_indirect_thunk_\reg
> > +        *
> > +        * into:
> > +        *
> > +        *   Jncc.d8 1f
> > +        *   jmp *%\reg
> > +        *   nop
> > +        * 1:
> > +        */
> > +       if (op == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80) {
> > +               cc = insn->opcode.bytes[1] & 0xf;
> > +               cc ^= 1; /* invert condition */
> > +
> > +               bytes[i++] = 0x70 + cc; /* Jcc.d8 */
> > +               bytes[i++] = insn->length - 2;
> 
> Isn't `insn->length - 2` always 4 (in this case)? We could avoid
> computing that at runtime I suspect if we just hardcoded it.

Yeah, but I found this to be more expressive. The purpose is getting to
the next instruction.

Also, if clang ever does instruction stuffing to hit alignment targets
without extra nops (ISTR reading about such passes) this logic still
works.

That is, I think you can prefix this with REX.W just to make a longer
instruction.

> Either way, I've looked at the disassembly enough that this LGTM.
> Thanks for the patch.
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks!