linux-kernel - Re: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE

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Date:   Mon, 26 Sep 2022 11:01:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     YingChi Long <me@...lyc.cn>
Cc:     tglx@...utronix.de, ndesaulniers@...gle.com,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE_ALIGN

On Sun, Sep 25, 2022 at 11:31:50PM +0800, YingChi Long wrote:
> WG14 N2350 made very clear that it is an UB having type definitions with
> in "offsetof". This patch change the implementation of macro
> "TYPE_ALIGN" to builtin "__alignof__" to avoid undefined behavior.
> 
> I've grepped all source files to find any type definitions within
> "offsetof".
> 
>     offsetof\(struct .*\{ .*,
> 
> This implementation of macro "TYPE_ALIGN" seemes to be the only case of
> type definitions within offsetof in the kernel codebase.
> 
> Signed-off-by: YingChi Long <me@...lyc.cn>
> Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
> ---
>  arch/x86/kernel/fpu/init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
> index 621f4b6cac4a..41425ba0b6b1 100644
> --- a/arch/x86/kernel/fpu/init.c
> +++ b/arch/x86/kernel/fpu/init.c
> @@ -134,7 +134,7 @@ static void __init fpu__init_system_generic(void)
>  }
>  
>  /* Get alignment of the TYPE. */
> -#define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test)
> +#define TYPE_ALIGN(TYPE) __alignof__(TYPE)

IIRC there's a problem with alignof() in that it will return the ABI
alignment instead of that preferred or natural alignment for some types.

Notably I think 'long long' has 4 byte alignment on i386 and some other
32bit archs.

That said; please just replace the *one* instance of TYPE_ALIGN entirely
and get rid of the thing.