linux-kernel - [PATCH v1 1/3] vdso: Switch get/put unaligned from packed struct to memcpy

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Message-ID: <20250617005800.1410112-2-irogers@google.com>
Date: Mon, 16 Jun 2025 17:57:58 -0700
From: Ian Rogers <irogers@...gle.com>
To: Eric Biggers <ebiggers@...gle.com>, Yuzhuo Jing <yuzhuo@...gle.com>, 
	Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Arnaldo Carvalho de Melo <acme@...hat.com>, 
	Al Viro <viro@...iv.linux.org.uk>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	"Jason A. Donenfeld" <Jason@...c4.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Cc: Ian Rogers <irogers@...gle.com>
Subject: [PATCH v1 1/3] vdso: Switch get/put unaligned from packed struct to memcpy

Type punning is necessary for get/put unaligned but the use of a
packed struct violates strict aliasing rules, requiring
-fno-strict-aliasing to be passed to the C compiler. Switch to using
memcpy so that -fno-strict-aliasing isn't necessary.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 include/vdso/unaligned.h | 48 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b6513e..81f632e3c5eb 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -2,14 +2,50 @@
 #ifndef __VDSO_UNALIGNED_H
 #define __VDSO_UNALIGNED_H
 
-#define __get_unaligned_t(type, ptr) ({							\
-	const struct { type x; } __packed * __get_pptr = (typeof(__get_pptr))(ptr);	\
-	__get_pptr->x;									\
+#include <linux/string.h> // For memcpy.
+
+#define ____get_unaligned_type(type) type: (type)0
+/**
+ * __get_unaligned_t - read an unaligned value from memory.
+ * @ptr:	the pointer to load from.
+ * @type:	the type to load from the pointer.
+ *
+ * Use memcpy to affect an unaligned type sized load avoiding undefined behavior
+ * from approaches like type punning that require -fno-strict-aliasing in order
+ * to be correct. As type may be const, use _Generic to map to a non-const type
+ * - you can't memcpy into a const type. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type __get_unaligned_map_ctrl = 0;				\
+	typeof(_Generic(__get_unaligned_map_ctrl,			\
+		____get_unaligned_type(short int),			\
+		____get_unaligned_type(unsigned short int),		\
+		____get_unaligned_type(int),				\
+		____get_unaligned_type(unsigned int),			\
+		____get_unaligned_type(long),				\
+		____get_unaligned_type(unsigned long),			\
+		____get_unaligned_type(long long),			\
+		____get_unaligned_type(unsigned long long),		\
+		default: (type)0					\
+		)) __get_unaligned_val;					\
+	(void)__get_unaligned_map_ctrl;					\
+	memcpy(&__get_unaligned_val, (void *)(ptr), sizeof(__get_unaligned_val)); \
+	__get_unaligned_val;						\
 })
 
-#define __put_unaligned_t(type, val, ptr) do {						\
-	struct { type x; } __packed * __put_pptr = (typeof(__put_pptr))(ptr);		\
-	__put_pptr->x = (val);								\
+/**
+ * __put_unaligned_t - write an unaligned value to memory.
+ * @type:	the type of the value to store.
+ * @val:	the value to store.
+ * @ptr:	the pointer to store to.
+ *
+ * Use memcpy to affect an unaligned type sized store avoiding undefined
+ * behavior from approaches like type punning that require -fno-strict-aliasing
+ * in order to be correct. The void* cast silences ubsan warnings.
+ */
+#define __put_unaligned_t(type, val, ptr) do {				\
+	type __put_unaligned_val = (val);				\
+	memcpy((void *)(ptr), &__put_unaligned_val, sizeof(__put_unaligned_val)); \
 } while (0)
 
 #endif /* __VDSO_UNALIGNED_H */
-- 
2.50.0.rc2.692.g299adb8693-goog