linux-kernel - Re: [PATCH v2 9/9] objtool: Abstract unwind hint reading

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Date:   Mon, 3 Aug 2020 13:13:14 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org, mbenes@...e.cz,
        raphael.gault@....com, benh@...nel.crashing.org
Subject: Re: [PATCH v2 9/9] objtool: Abstract unwind hint reading



On 7/31/20 3:04 PM, Josh Poimboeuf wrote:
> On Fri, Jul 31, 2020 at 08:00:58AM +0100, Julien Thierry wrote:
>>>> +	cfa->offset = hint->sp_offset;
>>>> +	insn->cfi.hint_type = hint->type;
>>>> +	insn->cfi.end = hint->end;
>>>> +
>>>> +	insn->cfi.sp_only = hint->type == ORC_TYPE_REGS || hint->type == ORC_TYPE_REGS_IRET;
>>>
>>> What does "sp" mean here in sp_only?
>>>
>>
>> Stack pointer, like in CFI_SP. When objtool encounters one of these hints,
>> it starts to only track the stack frame with the stack pointer (no BP, no
>> drap register, no move to temporary registers). Just trying to make some
>> sense of this corner case.
> 
> I think that's not quite right, because ORC_TYPE_CALL could also be
> "sp_only" in some cases, by that definition.
> 

But in that case the code will still track when/if the CFI becomes 
pointed to by BP.

> The call to update_cfi_state_regs() is really regs-specific, not
> sp-specific.
> 

I must admit I don't really understand what "regs" is and why exactly 
such an exception in stack state tracking is made where only operations 
to SP are taken into account.

-- 
Julien Thierry