On Mon, Aug 03, 2020 at 01:13:14PM +0100, Julien Thierry wrote:
>
>
> 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.
"regs" is a special type of stack frame, usually for asm entry code,
where the frame is actually an instance of 'struct pt_regs'. So if
there's a variable associated it with it, maybe it should have "regs" in
the name.
Though I think non-x86 arches will also have regs frames, so would it
make sense to just make the unwind hint types a global multiarch thing?
They could be renamed to UNWIND_HINT_TYPE_REGS{_PARTIAL}. Then there
wouldn't really be a need for the "sp_only" thing.
--
Josh
