linux-kernel - Re: [PATCH 4/7] objtool: Add support for return trampoline call

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Date:   Fri, 3 Apr 2020 09:19:37 +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 4/7] objtool: Add support for return trampoline call


On 4/2/20 5:27 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2020 at 10:22:17AM +0200, Alexandre Chartre wrote:
>> With retpoline, the return instruction is used to branch to an address
>> stored on the stack. So, unlike a regular return instruction, when a
>> retpoline return instruction is reached the stack has been modified
>> compared to what we have when the function was entered.
>>
>> Provide the mechanism to explicitly call-out such return instruction
>> so that objtool can correctly handle them.
> 
> https://lkml.kernel.org/r/20200331222703.GH2452@worktop.programming.kicks-ass.net
> 
> And also, the split out version:
> 
>    https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=core/objtool&id=ec9d9549901dfd2ff411676dfc624e50219e4d5a
> 

Okay, so I can get rid of my retpoline_ret annotation and use UNWIND_HINT_RET_OFFSET
instead (which is more generic). Basically I can change my RETPOLINE_RET macro to:

.macro RETPOLINE_RET
	UNWIND_HINT_RET_OFFSET
	ret
.endm

Thanks,

alex.