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Date:   Tue, 30 Apr 2019 12:20:56 +0000
From:   Raphael Gault <Raphael.Gault@....com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        Julien Thierry <Julien.Thierry@....com>
Subject: Re: [RFC 3/6] objtool: arm64: Adapt the stack frame checks and the
 section analysis for the arm architecture

Hi Josh,

On 4/25/19 5:25 PM, Josh Poimboeuf wrote:
> 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?
>

Honestly, I don't have a full understanding of how the switch tables are
handled on arm64. All I know is that I've identified a case in which it
doesn't work (and I get an unreachable instruction warning).
When trying to figure out how the switch tables work on arm64 and how
objtool is retrieving them (on x86 at least) I realised that you look
for 2 relocations :
- One from (.rela).text which refers to the .rodata section
- One from (.rela).rodata which refers somewhere else.
On the case I identified the second relocation doesn't exist thus the
function doesn't find the switch table.

Again since I do not have a good understanding about this I am not able
to say if it is a corner case or not.

>>>>>> -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.
>

I haven't looked at it in depth yet but I will try to figure out a good
way to represent those instructions on a more proper manner.

Thanks,

--
Raphael Gault
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