linux-kernel - [PATCH v4] x86/fpu: use _Alignof to avoid UB in TYPE

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Message-Id: <20221102181425.2122504-1-me@inclyc.cn>
Date:   Thu,  3 Nov 2022 02:14:25 +0800
From:   YingChi Long <me@...lyc.cn>
To:     bp@...en8.de
Cc:     chang.seok.bae@...el.com, dave.hansen@...ux.intel.com,
        david.laight@...lab.com, hpa@...or.com,
        linux-kernel@...r.kernel.org, me@...lyc.cn, mingo@...hat.com,
        ndesaulniers@...gle.com, pbonzini@...hat.com, tglx@...utronix.de,
        x86@...nel.org
Subject: [PATCH v4] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN

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

WG14 N2350 made very clear that it is an undefined behavior having type
definitions with in "offsetof". Replace the 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.

Link: https://reviews.llvm.org/D133574

A clang patch has been made that rejects any definitions within
__builtin_offsetof (usually #defined with "offsetof"), and tested
compiling the kernel using clang, there are no error if this patch
applied.

Link: https://gcc.gnu.org/onlinedocs/gcc/Alignment.html
Link: https://godbolt.org/z/sPs1GEhbT

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

The macro TYPE_ALIGN has behavior that matches _Alignof rather than
__alignof__.

Signed-off-by: YingChi Long <me@...lyc.cn>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
v4:
- commit message changes suggested by Borislav Petkov

v3:
- commit message changes suggested by Nick and David

v3: https://lore.kernel.org/all/20221006141442.2475978-1-me@inclyc.cn/

v2: https://lore.kernel.org/all/20220927153338.4177854-1-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 8946f89761cc..851eb13edc01 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:
--
2.37.4