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Subject: Re: [PATCH v2 9/9] objtool: Abstract unwind hint reading
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, peterz@infradead.org, mbenes@suse.cz,
        raphael.gault@arm.com, benh@kernel.crashing.org
References: <20200730094652.28297-1-jthierry@redhat.com>
 <20200730094652.28297-10-jthierry@redhat.com>
 <20200730150341.udqnykbw7yfsjvin@treble>
 <1a078563-001d-c666-d2f5-9291f0efd35a@redhat.com>
 <20200731140441.cpzr4lrlkcrmoz2c@treble>
From:   Julien Thierry <jthierry@redhat.com>
Message-ID: <6a314dba-4086-717b-d226-6c292240a3e9@redhat.com>
Date:   Mon, 3 Aug 2020 13:13:14 +0100
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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