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Date:   Wed, 1 Apr 2020 12:37:09 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Julien Thierry <jthierry@redhat.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, x86@kernel.org, mhiramat@kernel.org,
        mbenes@suse.cz, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
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On Wed, Apr 01, 2020 at 07:09:10PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
> 
> > > +static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
> > >   {
> > > +	u8 ret_offset = insn->ret_offset;
> > >   	int i;
> > > 
> > > -	if (state->cfa.base != initial_func_cfi.cfa.base ||
> > > -	    state->cfa.offset != initial_func_cfi.cfa.offset ||
> > > -	    state->stack_size != initial_func_cfi.cfa.offset ||
> > > -	    state->drap)
> > > +	if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
> > > +		return true;
> > > +
> > > +	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.
>
> ISTR that for the ftrace case it was indeed cfa.offset + 8, but for the
> IRET case below (where it is now not used anymore) it was cfa.offset
> (not cfa.offset + 40, which I was expecting).

It depends on the value of cfa.base.  If cfa.base is CFI_SP, then
cfa.offset changes with stack_size.  If cfa.base is CFI_BP (i.e. if the
function uses a frame pointer), then cfa.offset is constant (the
distance between RBP on the stack and the previous frame).

-- 
Josh