linux-kernel - Re: [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC

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Message-ID: <06605b1c-c9ae-fd18-bcb1-e4a48f2cb60e@oracle.com>
Date:   Tue, 7 Apr 2020 19:01:43 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, jthierry@...hat.com,
        tglx@...utronix.de
Subject: Re: [PATCH V2 9/9] x86/speculation: Remove all
 ANNOTATE_NOSPEC_ALTERNATIVE directives



On 4/7/20 6:28 PM, Josh Poimboeuf wrote:
> On Tue, Apr 07, 2020 at 06:18:51PM +0200, Alexandre Chartre wrote:
>>
>> On 4/7/20 4:32 PM, Alexandre Chartre wrote:
>>>
>>> On 4/7/20 3:34 PM, Josh Poimboeuf wrote:
>>>> On Tue, Apr 07, 2020 at 03:28:37PM +0200, Peter Zijlstra wrote:
>>>>> Josh, we should probably have objtool verify it doesn't emit ORC entries
>>>>> in alternative ranges.
>>>>
>>>> Agreed, it might be as simple as checking for insn->alt_group in the
>>>> INSN_STACK check or in update_insn_state().
>>>>
>>>
>>> We could do that only for the "objtool orc generate" command. That way
>>> "objtool check" would still check the alternative, but "objtool orc generate"
>>> will just use the first half of the alternative (like it does today with
>>> ANNOTATE_NOSPEC_ALTERNATIVE). We can even keep all ANNOTATE_NOSPEC_ALTERNATIVE
>>> but only use them for "objtool orc generate".
>>>
>>
>> I have checked and objtool doesn't emit ORC entries for alternative:
>> decode_instructions() doesn't mark such section with sec->text = true
>> so create_orc_sections() doesn't emit corresponding ORC entries.
>>
>> So I think we can remove the ANNOTATE_NOSPEC_ALTERNATIVE directives,
>> this will allow objtool to check the instructions but it still won't
>> emit ORC entries (same behavior as today). In the future, if ORC
>> eventually supports alternative we will be ready to have objtool emit
>> ORC entries.
> 
> What's the benefit of removing ANNOTATE_NOSPEC_ALTERNATIVE if there's no
> ORC support to go along with it?

To have the code validated by objtool like any other alternative code
(which is not tagged with ANNOTATE_NOSPEC_ALTERNATIVE).

> Also I want to avoid adding "ORC alternatives".  ORC is nice and simple
> and we should keep it that way as much as possible.
> 
> Again, we should warn on stack changes inside alternatives, and then
> look at converting RSB and retpolines to use static branches so they
> have deterministic stacks.
> 
objtool doesn't currently warn on stack changes inside alternatives.
The RSB/retpoline alternatives have warning because objtool doesn't
support retpoline ret and intra-function calls. If you have an alternative
doing stack changes that objtool understand (like push/pop, add/remove
to sp) then you won't have a warning.

I think that's the case with smap_save:

static __always_inline unsigned long smap_save(void)
{
         unsigned long flags;

         asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
                                   X86_FEATURE_SMAP)
                       : "=rm" (flags) : : "memory", "cc");

         return flags;
}

The alternative does change the stack but objtool won't complain
because it handles the pushf and pop instruction.

alex.