Re: [RFC 3/6] objtool: arm64: Adapt the stack frame checks and the section analysis for the arm architecture

Josh Poimboeuf
2019-04-25T16:25:37Z

On Thu, Apr 25, 2019 at 08:12:24AM +0000, Raphael Gault wrote:
> Hi Josh,
>
> On 4/24/19 5:56 PM, Josh Poimboeuf wrote:
> > On Wed, Apr 24, 2019 at 04:32:44PM +0000, Raphael Gault wrote:
> >>>> diff --git a/tools/objtool/arch/arm64/decode.c b/tools/objtool/arch/arm64/decode.c
> >>>> index 0feb3ae3af5d..8b293eae2b38 100644
> >>>> --- a/tools/objtool/arch/arm64/decode.c
> >>>> +++ b/tools/objtool/arch/arm64/decode.c
> >>>> @@ -105,6 +105,33 @@ unsigned long arch_compute_rela_sym_offset(int addend)
> >>>> return addend;
> >>>> }
> >>>>
> >>>> +/*
> >>>> + * In order to know if we are in presence of a sibling
> >>>> + * call and not in presence of a switch table we look
> >>>> + * back at the previous instructions and see if we are
> >>>> + * jumping inside the same function that we are already
> >>>> + * in.
> >>>> + */
> >>>> +bool arch_is_insn_sibling_call(struct instruction *insn)
> >>>> +{
> >>>> +struct instruction *prev;
> >>>> +struct list_head *l;
> >>>> +struct symbol *sym;
> >>>> +list_for_each_prev(l, &insn->list) {
> >>>> +prev = (void *)l;
> >>>> +if (!prev->func
> >>>> +|| prev->func->pfunc != insn->func->pfunc)
> >>>> +return false;
> >>>> +if (prev->stack_op.src.reg != ADR_SOURCE)
> >>>> +continue;
> >>>> +sym = find_symbol_containing(insn->sec, insn->immediate);
> >>>> +if (!sym || sym->type != STT_FUNC
> >>>> +|| sym->pfunc != insn->func->pfunc)
> >>>> +return true;
> >>>> +break;
> >>>> +}
> >>>> +return true;
> >>>> +}
> >>>
> >>> I get the feeling there might be a better way to do this, but I can't
> >>> figure out what this function is actually doing. It looks like it
> >>> searches backwards in the function for an instruction which has
> >>> stack_op.src.reg != ADR_SOURCE -- what does that mean? And why doesn't
> >>> it do anything with the instruction after it finds it?
> >>>
> >>
> >> I will indeed try to make it better.
> >
> > I still don't quite get what it's trying to accomplish, but I wonder if
> > there's some kind of tracking you can add in validate_branch() to keep
> > track of whatever you're looking for, leading up to the indirect jump.
> >
>
> The motivation behind this is that the `br <Xn>` instruction is a
> dynamic jump (jump to the address contained in the provided register).
> This instruction is used for sibling calls but can also be used for
> switch table. I use this to differentiate these two cases from one another:
>
> Generally the `adr/adrp` instruction is used prior to `br` in order to
> load the address into the register. What I do here is go back throught
> the instructions and try to identify if the address loaded.
>
> I also thought of implementing some sort of tracking in validate branch
> because it could be useful for identifying the switch tables as well.
> But it seemed to me like a major change in the sementic of this tool:
> indeed, from my perspective I would have to track the state of the
> registers and I don't know if we want to do that.

I don't have much time to look at this today (and I'll be out next
week), but we had a similar problem in x86. See the comments above
find_switch_table(), particularly #3. Does that function not work for
the arm64 case?

> >>>> -hash_add(file->insn_hash, &insn->hash, insn->offset);
> >>>> +/*
> >>>> + * For arm64 architecture, we sometime split instructions so that
> >>>> + * we can track the state evolution (i.e. load/store of pairs of registers).
> >>>> + * We thus need to take both into account and not erase the previous ones.
> >>>> + */
> >>>
> >>> Ew... Is this an architectural thing, or just a quirk of the arm64
> >>> decoder?
> >>>
> >>
> >> The motivation for this is to simulate the two consecutive operations
> >> that would be executed on x86 but are done in one on arm64. This is
> >> strictly a decoder related quirk. I don't know if there is a better way
> >> to do it without modifying the struct op_src and struct instruction.
> >
> > Ah. Which ops are those? Hopefully we can find a better way to
> > represent that with a single instruction. Adding fake instructions is
> > fragile.
> >
>
> Those are the load/store of pairs of registers, mainly stp/ldp. Those
> are often use in the function prologues/epilogues to save/restore the
> stack pointers and frame pointers however it can be used with any
> register pair.
>
> The idea to add a new instruction could work but I would need to extend
> the `struct op_src` as well I think.

Again I don't have much time to look at it, but I do think that changing
op_src/dest to allow for the stp/ldp instructions would work better than
inserting a fake instruction to emulate x86.

Or another idea would be to associate multiple stack_ops with a single
instruction.

--
Josh