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

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Date:   Sun, 25 Sep 2022 23:31:50 +0800
From:   YingChi Long <me@...lyc.cn>
To:     tglx@...utronix.de
Cc:     me@...lyc.cn, ndesaulniers@...gle.com,
        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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        linux-kernel@...r.kernel.org
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 with
in "offsetof". This patch change 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.

Signed-off-by: YingChi Long <me@...lyc.cn>
Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
---
 arch/x86/kernel/fpu/init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 621f4b6cac4a..41425ba0b6b1 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -134,7 +134,7 @@ static void __init fpu__init_system_generic(void)
 }
 
 /* Get alignment of the TYPE. */
-#define TYPE_ALIGN(TYPE) offsetof(struct { char x; TYPE test; }, test)
+#define TYPE_ALIGN(TYPE) __alignof__(TYPE)
 
 /*
  * Enforce that 'MEMBER' is the last field of 'TYPE'.
-- 
2.35.1