Messages in this thread | | | Date | Mon, 26 Sep 2022 11:01:17 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE_ALIGN |
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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@inclyc.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.
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