Re: [PATCH] x86/alternative: fix undefined reference to __ibt_endbr_seal[_end]

[Miaohe Lin]

On 2022/10/9 21:11, Peter Zijlstra wrote:
> On Sun, Oct 09, 2022 at 08:44:26PM +0800, Miaohe Lin wrote:
>> On 2022/10/9 20:28, Peter Zijlstra wrote:
>>> On Sun, Oct 09, 2022 at 11:45:32PM +0800, Miaohe Lin wrote:
>>>> When I compile the kernel bzImage, there are several compile errors:
>>>>
>>>> ld: arch/x86/kernel/alternative.o: in function
>>>> `alternative_instructions':
>>>> alternative.c:(.init.text+0x15d): undefined reference to
>>>> `__ibt_endbr_seal_end'
>>>> ld: alternative.c:(.init.text+0x164): undefined reference to
>>>> `__ibt_endbr_seal'
>>>>
>>>> It's because __ibt_endbr_seal and __ibt_endbr_seal_end are not optimized
>>>> away when CONFIG_X86_KERNEL_IBT isn't enabled. Remove noinline attribute
>>>> to help gcc optimize them away.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>> ---
>>>>  arch/x86/kernel/alternative.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>>>> index 5cadcea035e0..beaf9fc44e2f 100644
>>>> --- a/arch/x86/kernel/alternative.c
>>>> +++ b/arch/x86/kernel/alternative.c
>>>> @@ -624,7 +624,7 @@ void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end)
>>>>  
>>>>  #else
>>>>  
>>>> -void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { }
>>>> +void __init_or_module apply_ibt_endbr(s32 *start, s32 *end) { }
>>>>  
>>>>  #endif /* CONFIG_X86_KERNEL_IBT */
>>>>  
>>>
>>> None of this makes any sense...
>>
>> This compile error occurs in my env when CONFIG_X86_KERNEL_IBT isn't
>> enabled with gcc version 7.3.0 (GCC).
> 
> *sigh*... geriatric GCC :/

Yes, a geriatric GCC problem. :(

> 
> Anyway; perhaps clarify the changelog to state something like:
> 
> Due to the explicit 'noinline' GCC-7.3 is not able to optimize away the
> argument setup of:
> 
> 	apply_ibt_endbr(__ibt_endbr_seal, __ibt_enbr_seal_end);
> 
> even when X86_KERNEL_IBT=n and the function is an empty stub -- leading
> to link errors due to missing __ibt_endbr_seal* symbols.

Will do it in v2. Thanks for your suggestion and comment.

Thanks,
Miaohe Lin

> 
> 
> .
>