Re: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE_ALIGN

[YingChi Long]

Seems GCC __alignof__ is not evaluated to the minimum alignment of some 
TYPE,
and depends on fields of the struct.

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

C11 _Alignof matches in the case (see godbolt link below). How about 
switch to
_Alignof?


Link: https://godbolt.org/z/T749MfM9o
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10360
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69560

On 2022/9/26 17:01, Peter Zijlstra wrote:
> 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@xxxxxxxxx>
> > 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.