Re: [PATCH v2 9/9] objtool: Abstract unwind hint reading

[Julien Thierry]

On 7/31/20 3:04 PM, Josh Poimboeuf wrote:
> On Fri, Jul 31, 2020 at 08:00:58AM +0100, Julien Thierry wrote:
> > > > +	cfa->offset = hint->sp_offset;
> > > > +	insn->cfi.hint_type = hint->type;
> > > > +	insn->cfi.end = hint->end;
> > > > +
> > > > +	insn->cfi.sp_only = hint->type == ORC_TYPE_REGS || hint->type == ORC_TYPE_REGS_IRET;
> > >
> > > What does "sp" mean here in sp_only?
> >
> > Stack pointer, like in CFI_SP. When objtool encounters one of these hints,
> > it starts to only track the stack frame with the stack pointer (no BP, no
> > drap register, no move to temporary registers). Just trying to make some
> > sense of this corner case.
>
> I think that's not quite right, because ORC_TYPE_CALL could also be
> "sp_only" in some cases, by that definition.

But in that case the code will still track when/if the CFI becomes 
pointed to by BP.

> The call to update_cfi_state_regs() is really regs-specific, not
> sp-specific.

I must admit I don't really understand what "regs" is and why exactly 
such an exception in stack state tracking is made where only operations 
to SP are taken into account.

--
Julien Thierry