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

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Message-Id: <20220926135125.891584-1-me@inclyc.cn>
Date:   Mon, 26 Sep 2022 21:51:24 +0800
From:   YingChi Long <me@...lyc.cn>
To:     peterz@...radead.org
Cc:     me@...lyc.cn, ndesaulniers@...gle.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN

WG14 N2350 made very clear that it is an UB having type definitions
within "offsetof". This patch changes 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.

GCC builtin __alignof__ has different semantics here. __alignof__() will
in fact return the 'sane' result while structure alignment and
_Alignof() return the ABI required minimum alignment.


Signed-off-by: YingChi Long <me@...lyc.cn>
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
Link: https://godbolt.org/z/T749MfM9o
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