linux-kernel - [tip: x86/fpu] x86/fpu: Use _Alignof to avoid undefined behavior in TYPE

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Message-ID: <166913440179.4906.11745141572917292511.tip-bot2@tip-bot2>
Date:   Tue, 22 Nov 2022 16:26:41 -0000
From:   "tip-bot2 for YingChi Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     YingChi Long <me@...lyc.cn>, Borislav Petkov <bp@...e.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/fpu] x86/fpu: Use _Alignof to avoid undefined behavior in
 TYPE_ALIGN

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     55228db2697c09abddcb9487c3d9fa5854a932cd
Gitweb:        https://git.kernel.org/tip/55228db2697c09abddcb9487c3d9fa5854a932cd
Author:        YingChi Long <me@...lyc.cn>
AuthorDate:    Fri, 18 Nov 2022 08:55:35 +08:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Tue, 22 Nov 2022 17:13:03 +01:00

x86/fpu: Use _Alignof to avoid undefined behavior in TYPE_ALIGN

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@...lyc.cn>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Link: https://lore.kernel.org/r/20220925153151.2467884-1-me@inclyc.cn
---
 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 8946f89..851eb13 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: