linux-kernel - Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

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Date:   Thu, 26 Mar 2020 23:19:41 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, x86@...nel.org,
        mhiramat@...nel.org, mbenes@...e.cz
Subject: Re: [PATCH v4 01/13] objtool: Remove CFI save/restore special case

On Thu, Mar 26, 2020 at 04:38:41PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 26, 2020 at 08:44:48AM -0500, Josh Poimboeuf wrote:
> > On Thu, Mar 26, 2020 at 01:58:44PM +0100, Peter Zijlstra wrote:
> > > So instr_begin() / instr_end() have this exact problem, but worse. Those
> > > actually do nest and I've ran into the following situation:
> > > 
> > > 	if (cond1) {
> > > 		instr_begin();
> > > 		// code1
> > > 		instr_end();
> > > 	}
> > > 	// code
> > > 
> > > 	if (cond2) {
> > > 		instr_begin();
> > > 		// code2
> > > 		instr_end();
> > > 	}
> > > 	// tail
> > > 
> > > Where objtool then finds the path: !cond1, cond2, which ends up at code2
> > > with 0, instead of 1.
> > 
> > Hm, I don't see the nesting in this example, can you clarify?
> 
> Indeed no nesting here, but because they can nest we have that begin as
> +1, end as -1 and then we sum it over the code flow.
> 
> Then given that, the above, ends up as -1 + 1 in the !cond1,cond2 case,
> because that -1 escapes the cond1 block.

Ok, I see now.  I wonder if you can take a similar approach to the
save/restore patch I sent out.  Though I guess the nesting adds an
additional wrinkle I'm too tired to think about.

-- 
Josh