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

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Message-ID: <20200326134448.5zci3ikdlf5ar2w5@treble>
Date:   Thu, 26 Mar 2020 08:44:48 -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 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?

> I've also seen:
> 
> 	if (cond) {
> 		instr_begin();
> 		// code1
> 		instr_end();
> 	}
> 	instr_begin();
> 	// code2
> 	instr_end();
> 
> Where instr_end() and instr_begin() merge onto the same instruction of
> code2 as a 0, and again code2 will issue a false warning.
> 
> You can also not make objtool lift the end marker to the previous
> instruction, because then:
> 
> 	if (cond1) {
> 		instr_begin();
> 		if (cond2) {
> 			// code2
> 		}
> 		instr_end();
> 	}
> 
> Suffers the reverse problem, instr_end() becomes part of the @cond2
> block and cond1 grows a path that misses it entirely.
> 
> So far I've not had any actual solution except adding a NOP to anchor
> the annotation on.

Are you adding the NOP to the instr_end() annotation itself?  Seems like
that would be the cleanest/easiest.

Though it is sad that we have to change the code to make objtool happy
-- would be nice if we could come up with something less intrusive.

-- 
Josh