Messages in this thread Patch in this message | | | From | YingChi Long <> | Subject | [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN | Date | Tue, 27 Sep 2022 23:33:38 +0800 |
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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.
I've made a clang patch that rejects any definitions within __builtin_offsetof (usually #defined with "offsetof"), and tested compiling with this patch, there is no error if this patch is applied.
In PATCH v1 "TYPE_ALIGN" was substituted with "__alignof__" which is a GCC extension, which returns the *preferred alignment*, that is different from C11 "_Alignof" returning *ABI alignment*. For example, on i386 __alignof__(long long) evaluates to 8 but _Alignof(long long) evaluates to 4. See godbolt links below.
In this patch, I'd like to use "__alignof__" to "_Alignof" to preserve the behavior here.
Signed-off-by: YingChi Long <me@inclyc.cn> Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm Link: https://godbolt.org/z/13xTYYd11 Link: https://godbolt.org/z/T749MfM9o Link: https://gcc.gnu.org/onlinedocs/gcc/Alignment.html 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 Link: https://reviews.llvm.org/D133574 --- arch/x86/kernel/fpu/init.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c index 621f4b6cac4a..de96c11e1fe9 100644 --- a/arch/x86/kernel/fpu/init.c +++ b/arch/x86/kernel/fpu/init.c @@ -133,9 +133,6 @@ static void __init fpu__init_system_generic(void) fpu__init_system_mxcsr(); }
-/* Get alignment of the TYPE. */ -#define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test) - /* * Enforce that 'MEMBER' is the last field of 'TYPE'. * @@ -143,8 +140,8 @@ static void __init fpu__init_system_generic(void) * because that's how C aligns structs. */ #define CHECK_MEMBER_AT_END_OF(TYPE, MEMBER) \ - BUILD_BUG_ON(sizeof(TYPE) != ALIGN(offsetofend(TYPE, MEMBER), \ - TYPE_ALIGN(TYPE))) + BUILD_BUG_ON(sizeof(TYPE) != \ + ALIGN(offsetofend(TYPE, MEMBER), _Alignof(TYPE)))
/* * We append the 'struct fpu' to the task_struct: -- 2.35.1
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