[PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN

[YingChi Long]

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@xxxxxxxxx>
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