Date:   Wed, 8 Apr 2020 19:06:50 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Julien Thierry <jthierry@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH V2 5/9] objtool: Add support for intra-function calls



On 4/8/20 6:04 PM, Julien Thierry wrote:
> 
> 
> On 4/8/20 5:03 PM, Alexandre Chartre wrote:
>>
>>
>> On 4/8/20 4:19 PM, Julien Thierry wrote:
>>>
>>>
>>> On 4/8/20 3:06 PM, Alexandre Chartre wrote:
>>>>
>>>>
>>>> On 4/7/20 3:28 PM, Alexandre Chartre wrote:
>>>>>
>>>>> On 4/7/20 3:07 PM, Peter Zijlstra wrote:
>>>>>> On Tue, Apr 07, 2020 at 09:31:38AM +0200, Alexandre Chartre wrote:
>>>>>>
>>>>>>> index a62e032863a8..7ee1561bf7ad 100644
>>>>>>> --- a/tools/objtool/arch/x86/decode.c
>>>>>>> +++ b/tools/objtool/arch/x86/decode.c
>>>>>>> @@ -497,3 +497,15 @@ void arch_initial_func_cfi_state(struct cfi_state *state)
>>>>>>>       state->regs[16].base = CFI_CFA;
>>>>>>>       state->regs[16].offset = -8;
>>>>>>>   }
>>>>>>> +
>>>>>>> +
>>>>>>> +void arch_configure_intra_function_call(struct stack_op *op)
>>>>>>> +{
>>>>>>> +    /*
>>>>>>> +     * For the impact on the stack, make an intra-function
>>>>>>> +     * call behaves like a push of an immediate value (the
>>>>>>> +     * return address).
>>>>>>> +     */
>>>>>>> +    op->src.type = OP_SRC_CONST;
>>>>>>> +    op->dest.type = OP_DEST_PUSH;
>>>>>>> +}
>>>>>>
>>>>>> An alternative is to always set up stack ops for CALL/RET on decode, but
>>>>>> conditionally run update_insn_state() for them.
>>>>>>
>>>>>> Not sure that makes more logical sense, but the patch would be simpler I
>>>>>> think.
>>>>>
>>>>> Right, this would avoid adding a new arch dependent function and the patch
>>>>> will be simpler. This probably makes sense as the stack impact is the same
>>>>> for all calls (but objtool will use it only for intra-function calls).
>>>>>
>>>>
>>>> Actually the processing of the ret instruction is more complicated than I
>>>> anticipated with intra-function calls, and so my implementation is not
>>>> complete at the moment.
>>>>
>>>> The issue is to correctly handle how the ret is going to behave depending how
>>>> the stack (or register on arm) is modified before the ret. Adjusting the stack
>>>> offset makes the stack state correct, but objtool still needs to correctly
>>>> figure out where the ret is going to return and where the code flow continues.
>>>>
>>>
>>> A hint indicating the target "jump" address could be useful. It could
>>> be used to add the information on some call/jump dynamic that aren't
>>> associated with jump tables. Currently when objtool finds a jump
>>> dynamic, if no branches were added to it, it will just return.
>>>
>>> Having such a hint could help make additional links (at least on
>>> arm64). I don't know what Peter and Josh would think of that (if that
>>> helps in your case of course).
>>>
>>
>> Yes, I am thinking about tracking intra-function call return address,
>> and having hints to specify a return address changes. For example,
>> on x86, when we push the branch address on the stack we overwrite the
>> last return address (the return address of the last intra-function call).
>> Then the return instruction can figure out where to branch.
> 
> I see, I was thinking about a more generic hint, that would just
> indicate "this instruction actually jumps here". So in your case it
> would just point that a certain return instruction causes to branch
> somewhere.

I thought about doing that but the problem is that on x86 the same
retpoline code can branch differently depending on how it is used.
Basically we have a return instruction that will branch differently
based on what's on the stack. So we can just tell that this ret
instruction will branch/return there.

alex.

> This way the hint could also be used for other instructions (e.g.
> INSN_JUMP_DYNAMIC).
> 
> 
> 

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