linux-kernel - Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET

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Message-ID: <20200401133303.6773c574@gandalf.local.home>
Date:   Wed, 1 Apr 2020 13:33:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Julien Thierry <jthierry@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org, mhiramat@...nel.org,
        mbenes@...e.cz
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

On Wed, 1 Apr 2020 19:09:10 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > > +	if (state->cfa.offset != initial_func_cfi.cfa.offset &&
> > > +	    !(ret_offset && state->cfa.offset == initial_func_cfi.cfa.offset + ret_offset))  
> > 
> > Isn't that the same thing as "state->cfa.offset !=
> > initial_func_cfi.cfa.offset + ret_offset" ?  
> 
> I'm confused on what cfa.offset is, sometimes it increase with
> stack_size, sometimes it doesn't.

I believe what Julien is saying is the above logic is equivalent:

	if (x != y &&
	    !(z && x == y + z))

is the same as:

	if (x != y + z)

-- Steve