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Date:   Wed, 1 Apr 2020 22:20:53 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Julien Thierry <jthierry@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, x86@kernel.org, mhiramat@kernel.org,
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Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
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On Wed, Apr 01, 2020 at 02:20:15PM -0400, Steven Rostedt wrote:
> On Wed, 1 Apr 2020 19:45:44 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > 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.
> 
> No, the former accepts:
> 
> 	x==y || (z && x == y + z)
> 
> Which is the same as: x == y + z
> 
> As the second condition is only tested if z != 0, and x ==  y is the same
> as x == y + 0

Right, so it accepts both +0 and +z, while the latter will only accept
+z.

( in the iret case I had offset at +0 and stack_size at +40, while with
  the ftrace case I had both at +8; which is why I wrote the form that
  accepts +0 and +z )

Anyway, I tested it, and for the ftrace case (the only current user of
the hint) +z is correct for both offset and stack_size. I build both FP
and ORC variants.