linux-kernel - Re: [PATCH] objtool: Fix SLS checks

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Date:   Thu, 5 May 2022 14:03:30 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: Fix SLS checks

On Wed, May 04, 2022 at 09:26:47AM +0200, Peter Zijlstra wrote:
> > I don't quite follow this 2nd sentence and how it's related here, since
> > this function doesn't actually deal with direct jumps.
> 
> Ah, my bad. Also, this wrong.
> 
> I suppose this wants to be something like:
> 
> 	if (i < insn->length && op == JMP32_INSN_OPCODE)
> 		bytes[i++] = INT3_INSN_OPCODE;
> 
> So this *can* be a jump, but typically won't be I suppose.

Yep.

> > Speaking of, I guess we'll eventually need to hack this SLS mess into
> > jump labels :-/
> 
> Urgh... can't we reason that the straight line case is actually expected
> to run with the given register state anyway and ignore this?

Yeah, that makes sense.

So for jump labels the SLS path would probably not be worse than a
typical v1-style conditional branch misspeculation into the 'else' path,
and we've already given up on worrying about those anyway.

-- 
Josh