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

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Date:   Wed, 8 Apr 2020 16:06:18 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V2 5/9] objtool: Add support for intra-function calls



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.

alex.