linux-kernel - Re: [PATCH] x86/alternative: fix undefined reference to __ibt_endbr_seal[

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Date:   Sun, 9 Oct 2022 15:11:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/alternative: fix undefined reference to
 __ibt_endbr_seal[_end]

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@...wei.com>
> >> ---
> >>  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 :/

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.