Subject: [PATCH 4.19] x86/fpu: Use _Alignof to avoid undefined behavior in TYPE_ALIGN

From: YingChi Long <[email-masked]>
Date: Fri, 18 Nov 2022 08:55:35 +0800

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 <[email-masked]>
Signed-off-by: Borislav Petkov <[email-masked]>
Reviewed-by: Nick Desaulniers <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Nathan Chancellor <[email-masked]>
---
 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 9692ccc583bb..a3a570df6be1 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -138,9 +138,6 @@ static void __init fpu__init_system_generic(void)
 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'.
  *
@@ -148,8 +145,8 @@ 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:

base-commit: c652c812211c7a427d16be1d3f904eb02eb4265f
-- 
2.39.0