Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET

[Julien Thierry]

On 4/2/20 8:50 AM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2020 at 07:41:46AM +0100, Julien Thierry wrote:
> > On 4/1/20 6:09 PM, Peter Zijlstra wrote:
>
> > > The code in question (x86's sync_core()), is an exception return to
> > > self. It pushes an exception frame that points to right after the
> > > exception return instruction.
> > >
> > > This is the only usage of IRET in STT_FUNC symbols.
> > >
> > > So rather than teaching objtool how to interpret the whole
> > > push;push;push;push;push;iret sequence, teach it how big the frame is
> > > (arch_exception_frame_size) and let it continue.
> > >
> > > All the other (real) IRETs are in STT_NOTYPE in the entry assembly.
> >
> > Right, I see.. However I'm not completely convinced by this. I must admit I
> > haven't followed the whole conversation, but what was the issue with the
> > HINT_IRET_SELF? It seemed more elegant, but I might be missing some context.
>
> https://lkml.kernel.org/r/20200331211755.pb7f3wa6oxzjnswc@treble
>
> Josh didn't think it was worth it, I think.
>
> > Otherwise, it might be worth having a comment in the code to point that this
> > only handles the sync_core() case.
>
> I can certainly do that. Does ARM have any ERETs sprinkled around in
> places it should not have? That is, is this going to be a problem for
> you?

I had a quick look and I don't think there are ERETS in function 
symbols. And, worst case scenario, I could also just keep the arm64 
decoder making ERETS as INSN_CONTEXT_SWITCH as I didn't need more 
semantics so far (and arm64 ERET don't affect the stack anyway).

After you pointed out this only affect this very specific pattern, I 
admit that my concerns are more about "not having weird stuff in the 
generic part".
If it's too much of a hassle I can understand if you prefer to just put 
a comment. But if most of this can be kept to the arch specific decoder 
I think it'd be nicer :) .

> > Also, instead of adding a special "arch_exception_frame_size", I could
> > suggest:
> > - Picking this patch [1] from a completely arbitrary source
> > - Getting rid of INSN_STACK type, any instruction could then include stack
> > ops on top of their existing semantics, they can just have an empty list if
> > they don't touch SP/BP
> > - x86 decoder adds a stack_op to the iret to modify the stack pointer by the
> > right amount
>
> That's not the worst idea, lemme try that.

Thanks, keep me in Cc if you post a new version using that!

Cheers,

--
Julien Thierry