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

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Message-ID: <b8d4777e-1c33-7165-33f6-f32007ef7305@redhat.com>
Date:   Wed, 8 Apr 2020 15:19:58 +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 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).

-- 
Julien Thierry