[PATCH 5.4 50/55] x86/fpu: Use _Alignof to avoid undefined behavior in TYPE_ALIGN

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: YingChi Long <me@inclyc.cn>

commit 55228db2697c09abddcb9487c3d9fa5854a932cd upstream.

WG14 N2350 specifies that it is an undefined behavior to have type
definitions within offsetof", see

  https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm

This specification is also part of C23.

Therefore, replace the TYPE_ALIGN macro with the _Alignof builtin to
avoid undefined behavior. (_Alignof itself is C11 and the kernel is
built with -gnu11).

ISO C11 _Alignof is subtly different from the GNU C extension
__alignof__. Latter is the preferred alignment and _Alignof the
minimal alignment. For long long on x86 these are 8 and 4
respectively.

The macro TYPE_ALIGN's behavior matches _Alignof rather than
__alignof__.

  [ bp: Massage commit message. ]

Signed-off-by: YingChi Long <me@inclyc.cn>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220925153151.2467884-1-me@inclyc.cn
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/fpu/init.c |    7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -139,9 +139,6 @@ static void __init fpu__init_system_gene
 unsigned int fpu_kernel_xstate_size;
 EXPORT_SYMBOL_GPL(fpu_kernel_xstate_size);
 
-/* 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'.
  *
@@ -149,8 +146,8 @@ EXPORT_SYMBOL_GPL(fpu_kernel_xstate_size
  * 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: