linux-kernel - Re: [PATCH 4/7] objtool: Add support for return trampoline call

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Message-ID: <20200403152214.3frjrlhxvodffghg@treble>
Date:   Fri, 3 Apr 2020 10:22:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Julien Thierry <jthierry@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call

On Fri, Apr 03, 2020 at 10:17:57AM -0500, Josh Poimboeuf wrote:
> On Fri, Apr 03, 2020 at 09:11:55AM +0100, Julien Thierry wrote:
> > 
> > 
> > On 4/2/20 4:40 PM, Peter Zijlstra wrote:
> > > On Thu, Apr 02, 2020 at 04:31:05PM +0100, Julien Thierry wrote:
> > > > My understanding is that once you find an intra-function call, either you
> > > > hit a return, ending the branch, so the return should undo the modification
> > > > the intra-function call did (whether is it a retpoline return or not).
> > > > Otherwise, the intra-function call branch will need to reach an end in some
> > > > way (e.g. hitting a CONTEXT_SWITCH instruction, calling a
> > > > dead_end_function).
> > > > 
> > > > Am I missing something?
> > > 
> > > The thing is basically doing:
> > > 
> > > 	mov  $n, cx
> > > 1:	call 2f
> > > 2:	dec  cx
> > > 	jnz  1b
> > > 	add  8*n, sp
> > > 
> > > So it does N calls to self, then subtracts N words from the stack.
> > > 
> > > The reason being that the CPU has a return-stack-buffer for predicting
> > > returns, and call/ret being naturally paired, that works. The above
> > > is a software flush of the RSB.
> > > 
> > 
> > Ah, lovely... Maybe that's where SAVE/RESTORE unwind hints could be nice ;)
> > .
> > 
> > Otherwise, I don't really have a good suggestion for this...
> 
> Peter, I think my previous idea for UNWIND_HINT_ADJUST stack_add=8 would
> work here?

And if we're going to need that hint anyway, maybe we could get rid of
the nasty arch_exception_frame_size for the IRET thing and just use a
hint there after all ;-)

-- 
Josh