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

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Date:   Tue, 24 Mar 2020 23:11:09 +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 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.

> > +
> > +			WARN_FUNC("objtool isn't smart enough to handle this CFI save/restore combo",
> > +					sec, insn->offset);
> > +			return 1;
> > +		}
> > +
> > +		insn->state = save_insn->state;
> > +	}
> > +
> > +	*state = insn->state;
> 
> This would have been easier to review if apply_insn_hint() were added in
> a separate patch.

27 it will be!

> > +
> > +	/* restore !CFI state */
> > +	state->df = old.df;
> > +	state->uaccess = old.uaccess;
> > +	state->uaccess_stack = old.uaccess_stack;
> 
> Maybe we should just move the CFI stuff into a state->cfi substruct.
> That would remove the need for these bits and probably also the comment
> above the insn_state declaration.

Indeed!