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

[Julien Thierry]

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...

--
Julien Thierry