linux-kernel - Re: [PATCH 5/7] x86/speculation: Annotate intra-function calls

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Date:   Fri, 3 Apr 2020 10:14:49 -0700 (PDT)
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, peterz@...radead.org,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH 5/7] x86/speculation: Annotate intra-function calls


On 4/3/20 6:16 PM, Josh Poimboeuf wrote:
> On Fri, Apr 03, 2020 at 11:05:38AM -0500, Josh Poimboeuf wrote:
>> On Thu, Apr 02, 2020 at 10:22:18AM +0200, Alexandre Chartre wrote:
>>>   .macro RETPOLINE_JMP reg:req
>>> -	call	.Ldo_rop_\@
>>> +	INTRA_FUNCTION_CALL .Ldo_rop_\@
>>>   .Lspec_trap_\@:
>>>   	pause
>>>   	lfence
>>> @@ -102,7 +116,7 @@
>>>   .Ldo_retpoline_jmp_\@:
>>>   	RETPOLINE_JMP \reg
>>>   .Ldo_call_\@:
>>> -	call	.Ldo_retpoline_jmp_\@
>>> +	INTRA_FUNCTION_CALL .Ldo_retpoline_jmp_\@
>>>   .endm
>>
>> There's a catch: this is part of an alternative.  Which means if
>> X86_FEATURE_RETPOLINE isn't set at runtime, then the retpoline won't be
>> there and the ORC data will be wrong.
>>
>> In fact objtool should probably be made smart enough to warn about this
>> situation, when an alternative changes the stack state.
>>
>> The only way I can think of to fix this is to have ORC alternatives :-/

So that means that any alternative that does a stack manipulation isn't
currently supported?

alex.

> Or they could be converted to use static branches instead of
> alternatives.
>