linux-kernel - Re: [PATCH V2 5/9] objtool: Add support for intra-function calls

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Message-ID: <adf5a21b-2b34-9b5d-f832-b1b6575609da@redhat.com>
Date:   Wed, 8 Apr 2020 18:07:00 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.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 6:06 PM, Alexandre Chartre wrote:
> 
> 
> 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.
> 

Oh, I see. Nevermind then!

-- 
Julien Thierry