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SubjectRe: [PATCH v3 18/26] objtool: Fix !CFI insn_state propagation
On Wed, Mar 25, 2020 at 12:00:10AM +0100, Peter Zijlstra wrote:
> 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;
> +

I think all the validate_branch() callers aren't prepared to handle a -1
return code.

We can just be lazy and remove this 'insn == first' check and consider
it a non-fatal warning.

--
Josh

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