Messages in this thread | | | Date | Mon, 26 Sep 2022 21:18:18 +0800 | Subject | Re: [PATCH] x86/fpu: use __alignof__ to avoid UB in TYPE_ALIGN | From | YingChi Long <> |
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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@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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