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

>>>> -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.

Thanks,

--
Raphael Gault
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