linux-kernel - Re: [PATCH v3 18/26] objtool: Fix !CFI insn

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Date:   Wed, 25 Mar 2020 00:00:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz, brgerst@...il.com
Subject: Re: [PATCH v3 18/26] objtool: Fix !CFI insn_state propagation

On Tue, Mar 24, 2020 at 11:11:09PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 24, 2020 at 04:40:06PM -0500, Josh Poimboeuf wrote:
> > On Tue, Mar 24, 2020 at 04:31:31PM +0100, Peter Zijlstra wrote:
> 
> > > +		if (!save_insn->visited) {
> > > +			/*
> > > +			 * Oops, no state to copy yet.
> > > +			 * Hopefully we can reach this
> > > +			 * instruction from another branch
> > > +			 * after the save insn has been
> > > +			 * visited.
> > > +			 */
> > > +			if (insn == first)
> > > +				return 0; // XXX
> > 
> > Yeah, moving this code out to apply_insn_hint() seems like a nice idea,
> > but it wouldn't be worth it if it breaks this case.  TBH I don't
> > remember if this check was for a real-world case.  Might be worth
> > looking at...  If this case doesn't exist in reality then we could just
> > remove this check altogether.
> 
> I'll go run a bunch of builds with a print on it, that should tell us I
> suppose.

I can a bunch of builds, including an allmodconfig with the below on top
and it 'works'.

So I suppose we can remove this special case.

---
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -2134,11 +2134,13 @@ static int apply_insn_hint(struct objtoo
 			 * after the save insn has been
 			 * visited.
 			 */
-			if (insn == first)
-				return 0; // XXX

 			WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
 					sec, insn->offset);
+
+			if (insn == first)
+				return -1;
+
 			return 1;
 		}