Messages in this thread Patch in this message | | | Date | Thu, 25 Apr 2019 11:25:28 -0500 | From | Josh Poimboeuf <> | Subject | Re: [RFC 3/6] objtool: arm64: Adapt the stack frame checks and the section analysis for the arm architecture |
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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
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