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

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Date:   Wed, 1 Apr 2020 19:45:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.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, Apr 01, 2020 at 01:33:03PM -0400, Steven Rostedt wrote:
> 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)

It is not, the former will accept either x==y || x==y+z, while the
latter will only accept x==y+z.

For stack_size, I'm confident in just x==y+z, for offset I saw
conflicting things.

Since the annotation is now used in only a single place, maybe x==y+z
will just work, I'll go try once I've managed to unconfuse myself on the
whole fp-unwind situation.