linux-kernel - [PATCH v3 1/3] vdso: Switch get/put unaligned from packed struct to memcpy

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Message-ID: <20250626054826.433453-2-irogers@google.com>
Date: Wed, 25 Jun 2025 22:48:24 -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 v3 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 | 39 +++++++++++++++++++++++++++++++++------
 1 file changed, 33 insertions(+), 6 deletions(-)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b6513e..0ccbcc9aa493 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -2,14 +2,41 @@
 #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/compiler_types.h>
+
+/**
+ * __get_unaligned_t - read an unaligned value from memory.
+ * @type:	the type to load from the pointer.
+ * @ptr:	the pointer to load from.
+ *
+ * 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 __unqual_scalar_typeof 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_ctrl_type __always_unused;			\
+	__unqual_scalar_typeof(__get_unaligned_ctrl_type) __get_unaligned_val; \
+	__builtin_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);				\
+	__builtin_memcpy((void *)(ptr), &__put_unaligned_val,		\
+			 sizeof(__put_unaligned_val));			\
 } while (0)
 
 #endif /* __VDSO_UNALIGNED_H */
-- 
2.50.0.727.gbf7dc18ff4-goog