[PATCH v5 0/4] Switch get/put unaligned to use memcpy

[PATCH v5 0/4] Switch get/put unaligned to use memcpy

[Ian Rogers]

The existing type punning approach with packed structs requires
 -fno-strict-aliasing to be passed to the compiler for
correctness. This is true in the kernel tree but was not true in the
tools directory until this patch from Eric Biggers <ebiggers@google.com>:
https://lore.kernel.org/lkml/20250625202311.23244-2-ebiggers@kernel.org/

Requiring -fno-strict-aliasing seems unfortunate and so this patch
makes the unaligned code work via memcpy rather than type punning with
the packed attribute.

v5: add a patch to make parisc still use a punned version of
    get_unaligned_le32 for an unusual boot case they have. This is
    untested but suggested as necessary by:
    https://lore.kernel.org/lkml/202509051042.7KOze0fZ-lkp@intel.com/
    I wasn't clear if this work was picked up, but I don't see it in
    v6.18-rc1 and so I'm resending rebased as v5.

v4: switch the type/expression variable __get_unaligned_ctrl_type that
    is used by _Generic to be a pointer to avoid 0 vs NULL usage
    warnings - always use NULL and dereference the type. This should
    also hopefully address analysis bots complaints.

v3: switch to __unqual_scalar_typeof, reducing the code, and use an
    uninitialized variable rather than a cast of 0 to try to avoid a
    sparse warning about not using NULL. The code is trying to
    navigate a minefield of uninitialized and casting warnings,
    hopefully the best balance has been struck, but the code will fail
    for cases like:
    const void *val = get_unaligned((const void * const *)ptr);
    due to __unqual_scalar_typeof leaving the 2nd const of the cast in
    place. Thankfully no code does this - tested with an
    allyesconfig. Support would be achievable by using void* as a
    default case in __unqual_scalar_typeof, it just doesn't seem worth
    it for a fairly unusual const case.

v2: switch memcpy to __builtin_memcpy to avoid potential/disallowed
    memcpy calls in vdso caused by -fno-builtin. Reported by
    Christophe Leroy <christophe.leroy@csgroup.eu>:
    https://lore.kernel.org/lkml/c57de5bf-d55c-48c5-9dfa-e2fb844dafe9@csgroup.eu/

Ian Rogers (4):
  parisc: Inline a type punning version of get_unaligned_le32
  vdso: Switch get/put unaligned from packed struct to memcpy
  tools headers: Update the linux/unaligned.h copy with the kernel
    sources
  tools headers: Remove unneeded ignoring of warnings in unaligned.h

 arch/parisc/boot/compressed/misc.c   | 15 +++++++++-
 include/vdso/unaligned.h             | 41 ++++++++++++++++++++++++----
 tools/include/linux/compiler_types.h | 22 +++++++++++++++
 tools/include/linux/unaligned.h      |  4 ---
 tools/include/vdso/unaligned.h       | 41 ++++++++++++++++++++++++----
 5 files changed, 106 insertions(+), 17 deletions(-)

-- 
2.51.0.858.gf9c4a03a3a-goog

[PATCH v5 1/4] parisc: Inline a type punning version of get_unaligned_le32

[Ian Rogers]

Reading the byte/char output_len with get_unaligned_le32 can trigger
compiler warnings due to the size read. Avoid these warnings by using
type punning. This avoids issues when switching get_unaligned_t to
__builtin_memcpy.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 arch/parisc/boot/compressed/misc.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/parisc/boot/compressed/misc.c b/arch/parisc/boot/compressed/misc.c
index 9c83bd06ef15..111f267230a1 100644
--- a/arch/parisc/boot/compressed/misc.c
+++ b/arch/parisc/boot/compressed/misc.c
@@ -278,6 +278,19 @@ static void parse_elf(void *output)
 	free(phdrs);
 }
 
+/*
+ * The regular get_unaligned_le32 uses __builtin_memcpy which can trigger
+ * warnings when reading a byte/char output_len as an integer, as the size of a
+ * char is less than that of an integer. Use type punning and the packed
+ * attribute, which requires -fno-strict-aliasing, to work around the problem.
+ */
+static u32 punned_get_unaligned_le32(const void *p)
+{
+	const struct { __le32 x; } __packed * __get_pptr = p;
+
+	return le32_to_cpu(__get_pptr->x);
+}
+
 asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 		unsigned int command_line,
 		const unsigned int rd_start,
@@ -309,7 +322,7 @@ asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 	 * leave 2 MB for the stack.
 	 */
 	vmlinux_addr = (unsigned long) &_ebss + 2*1024*1024;
-	vmlinux_len = get_unaligned_le32(&output_len);
+	vmlinux_len = punned_get_unaligned_le32(&output_len);
 	output = (char *) vmlinux_addr;
 
 	/*
-- 
2.51.0.858.gf9c4a03a3a-goog

[PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy

[Ian Rogers]

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@google.com>
---
 include/vdso/unaligned.h | 41 ++++++++++++++++++++++++++++++++++------
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b6513e..9076483c9fbb 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */
-- 
2.51.0.858.gf9c4a03a3a-goog

[PATCH v5 3/4] tools headers: Update the linux/unaligned.h copy with the kernel sources

[Ian Rogers]

To pick up the changes in:

  vdso: Switch get/put unaligned from packed struct to memcpy

As the code is dependent on __unqual_scalar_typeof, update the tools
version of compiler_types.h to include this.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/include/linux/compiler_types.h | 22 +++++++++++++++
 tools/include/vdso/unaligned.h       | 41 ++++++++++++++++++++++++----
 2 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/tools/include/linux/compiler_types.h b/tools/include/linux/compiler_types.h
index d09f9dc172a4..890982283a5e 100644
--- a/tools/include/linux/compiler_types.h
+++ b/tools/include/linux/compiler_types.h
@@ -40,4 +40,26 @@
 #define asm_goto_output(x...) asm goto(x)
 #endif
 
+/*
+ * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
+ *			       non-scalar types unchanged.
+ */
+/*
+ * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
+ * is not type-compatible with 'signed char', and we define a separate case.
+ */
+#define __scalar_type_to_expr_cases(type)				\
+		unsigned type:	(unsigned type)0,			\
+		signed type:	(signed type)0
+
+#define __unqual_scalar_typeof(x) typeof(				\
+		_Generic((x),						\
+			 char:	(char)0,				\
+			 __scalar_type_to_expr_cases(char),		\
+			 __scalar_type_to_expr_cases(short),		\
+			 __scalar_type_to_expr_cases(int),		\
+			 __scalar_type_to_expr_cases(long),		\
+			 __scalar_type_to_expr_cases(long long),	\
+			 default: (x)))
+
 #endif /* __LINUX_COMPILER_TYPES_H */
diff --git a/tools/include/vdso/unaligned.h b/tools/include/vdso/unaligned.h
index ff0c06b6513e..9076483c9fbb 100644
--- a/tools/include/vdso/unaligned.h
+++ b/tools/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */
-- 
2.51.0.858.gf9c4a03a3a-goog

[PATCH v5 4/4] tools headers: Remove unneeded ignoring of warnings in unaligned.h

[Ian Rogers]

Now the get/put unaligned use memcpy the -Wpacked and -Wattributes
warnings don't need disabling.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/include/linux/unaligned.h | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/tools/include/linux/unaligned.h b/tools/include/linux/unaligned.h
index 395a4464fe73..d51ddafed138 100644
--- a/tools/include/linux/unaligned.h
+++ b/tools/include/linux/unaligned.h
@@ -6,9 +6,6 @@
  * This is the most generic implementation of unaligned accesses
  * and should work almost anywhere.
  */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wpacked"
-#pragma GCC diagnostic ignored "-Wattributes"
 #include <vdso/unaligned.h>
 
 #define get_unaligned(ptr)	__get_unaligned_t(typeof(*(ptr)), (ptr))
@@ -143,6 +140,5 @@ static inline u64 get_unaligned_be48(const void *p)
 {
 	return __get_unaligned_be48(p);
 }
-#pragma GCC diagnostic pop
 
 #endif /* __LINUX_UNALIGNED_H */
-- 
2.51.0.858.gf9c4a03a3a-goog

Re: [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy

[David Laight]

On Thu, 16 Oct 2025 13:51:24 -0700
Ian Rogers <irogers@google.com> wrote:

> 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.

Does the compiler always manage to optimise everything away?
You really do need it to generate the code for a misaligned
memory access.

You might be better off removing the 'strict-aliasing' warning
by 'laundering' the pointer through an integer type (probably long).

	David

> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  include/vdso/unaligned.h | 41 ++++++++++++++++++++++++++++++++++------
>  1 file changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
> index ff0c06b6513e..9076483c9fbb 100644
> --- a/include/vdso/unaligned.h
> +++ b/include/vdso/unaligned.h
> @@ -2,14 +2,43 @@
>  #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
> + * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
> + * expression rather than type, a pointer is used to avoid warnings about mixing
> + * the use of 0 and NULL. The void* cast silences ubsan warnings.
> + */
> +#define __get_unaligned_t(type, ptr) ({					\
> +	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
> +	__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 */

[tip: timers/vdso] tools headers: Remove unneeded ignoring of warnings in unaligned.h

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     576d8a7a985dde4f51a4561dddc0d8734494d3de
Gitweb:        https://git.kernel.org/tip/576d8a7a985dde4f51a4561dddc0d8734494d3de
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:26 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 13 Jan 2026 14:46:01 +01:00

tools headers: Remove unneeded ignoring of warnings in unaligned.h

Now that get/put_unaligned() use memcpy() the -Wpacked and -Wattributes
warnings don't need disabling anymore.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-5-irogers@google.com
---
 tools/include/linux/unaligned.h | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/tools/include/linux/unaligned.h b/tools/include/linux/unaligned.h
index 395a446..d51ddaf 100644
--- a/tools/include/linux/unaligned.h
+++ b/tools/include/linux/unaligned.h
@@ -6,9 +6,6 @@
  * This is the most generic implementation of unaligned accesses
  * and should work almost anywhere.
  */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wpacked"
-#pragma GCC diagnostic ignored "-Wattributes"
 #include <vdso/unaligned.h>
 
 #define get_unaligned(ptr)	__get_unaligned_t(typeof(*(ptr)), (ptr))
@@ -143,6 +140,5 @@ static inline u64 get_unaligned_be48(const void *p)
 {
 	return __get_unaligned_be48(p);
 }
-#pragma GCC diagnostic pop
 
 #endif /* __LINUX_UNALIGNED_H */

[tip: timers/vdso] tools headers: Update the linux/unaligned.h copy with the kernel sources

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     029a9504d871ce72adf9a13e5f1ce66f1db48be3
Gitweb:        https://git.kernel.org/tip/029a9504d871ce72adf9a13e5f1ce66f1db48be3
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:25 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 13 Jan 2026 14:46:00 +01:00

tools headers: Update the linux/unaligned.h copy with the kernel sources

To pick up the changes in:

  vdso: Switch get/put_unaligned() from packed struct to memcpy

As the code is dependent on __unqual_scalar_typeof, update also the tools
version of compiler_types.h to include this.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-4-irogers@google.com
---
 tools/include/linux/compiler_types.h | 22 ++++++++++++++-
 tools/include/vdso/unaligned.h       | 41 +++++++++++++++++++++++----
 2 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/tools/include/linux/compiler_types.h b/tools/include/linux/compiler_types.h
index d09f9dc..8909822 100644
--- a/tools/include/linux/compiler_types.h
+++ b/tools/include/linux/compiler_types.h
@@ -40,4 +40,26 @@
 #define asm_goto_output(x...) asm goto(x)
 #endif
 
+/*
+ * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
+ *			       non-scalar types unchanged.
+ */
+/*
+ * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
+ * is not type-compatible with 'signed char', and we define a separate case.
+ */
+#define __scalar_type_to_expr_cases(type)				\
+		unsigned type:	(unsigned type)0,			\
+		signed type:	(signed type)0
+
+#define __unqual_scalar_typeof(x) typeof(				\
+		_Generic((x),						\
+			 char:	(char)0,				\
+			 __scalar_type_to_expr_cases(char),		\
+			 __scalar_type_to_expr_cases(short),		\
+			 __scalar_type_to_expr_cases(int),		\
+			 __scalar_type_to_expr_cases(long),		\
+			 __scalar_type_to_expr_cases(long long),	\
+			 default: (x)))
+
 #endif /* __LINUX_COMPILER_TYPES_H */
diff --git a/tools/include/vdso/unaligned.h b/tools/include/vdso/unaligned.h
index ff0c06b..9076483 100644
--- a/tools/include/vdso/unaligned.h
+++ b/tools/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */

[tip: timers/vdso] vdso: Switch get/put_unaligned() from packed struct to memcpy()

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     e04a494143bab7ea804fe1ebe286701ee8288e4a
Gitweb:        https://git.kernel.org/tip/e04a494143bab7ea804fe1ebe286701ee8288e4a
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:24 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 13 Jan 2026 14:46:00 +01:00

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@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-3-irogers@google.com
---
 include/vdso/unaligned.h | 41 +++++++++++++++++++++++++++++++++------
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b..9076483 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */

[tip: timers/vdso] parisc: Inline a type punning version of get_unaligned_le32()

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     feffe3a9d4b0d1672db4c16457f029f1c22b35da
Gitweb:        https://git.kernel.org/tip/feffe3a9d4b0d1672db4c16457f029f1c22b35da
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:23 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 13 Jan 2026 14:46:00 +01:00

parisc: Inline a type punning version of get_unaligned_le32()

Reading the byte/char output_len with get_unaligned_le32() can trigger
compiler warnings due to the size read. Avoid these warnings by using
type punning. This avoids issues when switching get_unaligned_t() to
__builtin_memcpy().

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-2-irogers@google.com
---
 arch/parisc/boot/compressed/misc.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/parisc/boot/compressed/misc.c b/arch/parisc/boot/compressed/misc.c
index 9c83bd0..111f267 100644
--- a/arch/parisc/boot/compressed/misc.c
+++ b/arch/parisc/boot/compressed/misc.c
@@ -278,6 +278,19 @@ static void parse_elf(void *output)
 	free(phdrs);
 }
 
+/*
+ * The regular get_unaligned_le32 uses __builtin_memcpy which can trigger
+ * warnings when reading a byte/char output_len as an integer, as the size of a
+ * char is less than that of an integer. Use type punning and the packed
+ * attribute, which requires -fno-strict-aliasing, to work around the problem.
+ */
+static u32 punned_get_unaligned_le32(const void *p)
+{
+	const struct { __le32 x; } __packed * __get_pptr = p;
+
+	return le32_to_cpu(__get_pptr->x);
+}
+
 asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 		unsigned int command_line,
 		const unsigned int rd_start,
@@ -309,7 +322,7 @@ asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 	 * leave 2 MB for the stack.
 	 */
 	vmlinux_addr = (unsigned long) &_ebss + 2*1024*1024;
-	vmlinux_len = get_unaligned_le32(&output_len);
+	vmlinux_len = punned_get_unaligned_le32(&output_len);
 	output = (char *) vmlinux_addr;
 
 	/*

[tip: timers/vdso] tools headers: Remove unneeded ignoring of warnings in unaligned.h

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     10a62a0611f5544d209446acfde5beb7b27773c7
Gitweb:        https://git.kernel.org/tip/10a62a0611f5544d209446acfde5beb7b27773c7
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:26 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Wed, 14 Jan 2026 08:56:41 +01:00

tools headers: Remove unneeded ignoring of warnings in unaligned.h

Now that get/put_unaligned() use memcpy() the -Wpacked and -Wattributes
warnings don't need disabling anymore.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-5-irogers@google.com
---
 tools/include/linux/unaligned.h | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/tools/include/linux/unaligned.h b/tools/include/linux/unaligned.h
index 395a446..d51ddaf 100644
--- a/tools/include/linux/unaligned.h
+++ b/tools/include/linux/unaligned.h
@@ -6,9 +6,6 @@
  * This is the most generic implementation of unaligned accesses
  * and should work almost anywhere.
  */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wpacked"
-#pragma GCC diagnostic ignored "-Wattributes"
 #include <vdso/unaligned.h>
 
 #define get_unaligned(ptr)	__get_unaligned_t(typeof(*(ptr)), (ptr))
@@ -143,6 +140,5 @@ static inline u64 get_unaligned_be48(const void *p)
 {
 	return __get_unaligned_be48(p);
 }
-#pragma GCC diagnostic pop
 
 #endif /* __LINUX_UNALIGNED_H */

[tip: timers/vdso] tools headers: Update the linux/unaligned.h copy with the kernel sources

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     1d7cf255eefbb479d0eea9aa3b6372a1e52f8c62
Gitweb:        https://git.kernel.org/tip/1d7cf255eefbb479d0eea9aa3b6372a1e52f8c62
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:25 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Wed, 14 Jan 2026 08:56:41 +01:00

tools headers: Update the linux/unaligned.h copy with the kernel sources

To pick up the changes in:

  vdso: Switch get/put_unaligned() from packed struct to memcpy

As the code is dependent on __unqual_scalar_typeof, update also the tools
version of compiler_types.h to include this.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-4-irogers@google.com
---
 tools/include/linux/compiler_types.h | 22 ++++++++++++++-
 tools/include/vdso/unaligned.h       | 41 +++++++++++++++++++++++----
 2 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/tools/include/linux/compiler_types.h b/tools/include/linux/compiler_types.h
index d09f9dc..8909822 100644
--- a/tools/include/linux/compiler_types.h
+++ b/tools/include/linux/compiler_types.h
@@ -40,4 +40,26 @@
 #define asm_goto_output(x...) asm goto(x)
 #endif
 
+/*
+ * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
+ *			       non-scalar types unchanged.
+ */
+/*
+ * Prefer C11 _Generic for better compile-times and simpler code. Note: 'char'
+ * is not type-compatible with 'signed char', and we define a separate case.
+ */
+#define __scalar_type_to_expr_cases(type)				\
+		unsigned type:	(unsigned type)0,			\
+		signed type:	(signed type)0
+
+#define __unqual_scalar_typeof(x) typeof(				\
+		_Generic((x),						\
+			 char:	(char)0,				\
+			 __scalar_type_to_expr_cases(char),		\
+			 __scalar_type_to_expr_cases(short),		\
+			 __scalar_type_to_expr_cases(int),		\
+			 __scalar_type_to_expr_cases(long),		\
+			 __scalar_type_to_expr_cases(long long),	\
+			 default: (x)))
+
 #endif /* __LINUX_COMPILER_TYPES_H */
diff --git a/tools/include/vdso/unaligned.h b/tools/include/vdso/unaligned.h
index ff0c06b..9076483 100644
--- a/tools/include/vdso/unaligned.h
+++ b/tools/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */

[tip: timers/vdso] vdso: Switch get/put_unaligned() from packed struct to memcpy()

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     a339671db64b12bb02492557d2b0658811286277
Gitweb:        https://git.kernel.org/tip/a339671db64b12bb02492557d2b0658811286277
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:24 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Wed, 14 Jan 2026 08:56:41 +01:00

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@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-3-irogers@google.com
---
 include/vdso/unaligned.h | 41 +++++++++++++++++++++++++++++++++------
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b..9076483 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -2,14 +2,43 @@
 #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
+ * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
+ * expression rather than type, a pointer is used to avoid warnings about mixing
+ * the use of 0 and NULL. The void* cast silences ubsan warnings.
+ */
+#define __get_unaligned_t(type, ptr) ({					\
+	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
+	__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 */

[tip: timers/vdso] parisc: Inline a type punning version of get_unaligned_le32()

[tip-bot2 for Ian Rogers]

The following commit has been merged into the timers/vdso branch of tip:

Commit-ID:     df0f9a664be55a8529362a1ada847a19a91e4807
Gitweb:        https://git.kernel.org/tip/df0f9a664be55a8529362a1ada847a19a91e4807
Author:        Ian Rogers <irogers@google.com>
AuthorDate:    Thu, 16 Oct 2025 13:51:23 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Wed, 14 Jan 2026 08:56:41 +01:00

parisc: Inline a type punning version of get_unaligned_le32()

Reading the byte/char output_len with get_unaligned_le32() can trigger
compiler warnings due to the size read. Avoid these warnings by using
type punning. This avoids issues when switching get_unaligned_t() to
__builtin_memcpy().

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20251016205126.2882625-2-irogers@google.com
---
 arch/parisc/boot/compressed/misc.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/parisc/boot/compressed/misc.c b/arch/parisc/boot/compressed/misc.c
index 9c83bd0..111f267 100644
--- a/arch/parisc/boot/compressed/misc.c
+++ b/arch/parisc/boot/compressed/misc.c
@@ -278,6 +278,19 @@ static void parse_elf(void *output)
 	free(phdrs);
 }
 
+/*
+ * The regular get_unaligned_le32 uses __builtin_memcpy which can trigger
+ * warnings when reading a byte/char output_len as an integer, as the size of a
+ * char is less than that of an integer. Use type punning and the packed
+ * attribute, which requires -fno-strict-aliasing, to work around the problem.
+ */
+static u32 punned_get_unaligned_le32(const void *p)
+{
+	const struct { __le32 x; } __packed * __get_pptr = p;
+
+	return le32_to_cpu(__get_pptr->x);
+}
+
 asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 		unsigned int command_line,
 		const unsigned int rd_start,
@@ -309,7 +322,7 @@ asmlinkage unsigned long __visible decompress_kernel(unsigned int started_wide,
 	 * leave 2 MB for the stack.
 	 */
 	vmlinux_addr = (unsigned long) &_ebss + 2*1024*1024;
-	vmlinux_len = get_unaligned_le32(&output_len);
+	vmlinux_len = punned_get_unaligned_le32(&output_len);
 	output = (char *) vmlinux_addr;
 
 	/*

Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy

[Stefan Kerkmann]

Hi Ian,

On 10/16/25 22:51, Ian Rogers wrote:
> 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@google.com>
> ---
>  include/vdso/unaligned.h | 41 ++++++++++++++++++++++++++++++++++------
>  1 file changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
> index ff0c06b6513e..9076483c9fbb 100644
> --- a/include/vdso/unaligned.h
> +++ b/include/vdso/unaligned.h
> @@ -2,14 +2,43 @@
>  #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
> + * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
> + * expression rather than type, a pointer is used to avoid warnings about mixing
> + * the use of 0 and NULL. The void* cast silences ubsan warnings.
> + */
> +#define __get_unaligned_t(type, ptr) ({					\
> +	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
> +	__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 */

commit a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to
memcpy()"), which landed in 7.0, causes a performance regression on an NXP
i.MX25 (ARMv5TE) SoC.

I found it while updating a client's board from 6.12 to 7.0. A fio 4k randwrite
benchmark on a NAND storage with UBI and UBIFS filesystem was the only workload
that showed a clear regression between those two versions, so I bisected with
it:

perf stat -e irq:irq_handler_entry --filter 'irq == 49' -a \
    -- \
    fio --name=rw \
    --filename=/var/stat/testfile \
    --size=8M \
    --rw=randwrite \
    --bs=4k \
    --direct=0 \
    --fsync=1 \
    --numjobs=4 \
    --group_reporting

| kernel | irq_handler_entry | fio bw   |
| ------ | ----------------- | -------- |
| 6.12   | 155783            | 401KiB/s |
| 6.13   | 162317            | 395KiB/s |
| 6.14   | 168741            | 392KiB/s |
| 6.15   | 168556            | 401KiB/s |
| 6.16   | 166090            | 403KiB/s |
| 6.17   | 162541            | 385KiB/s |
| 6.18   | 157527            | 386KiB/s |
| 6.19   | 183675            | 381KiB/s |
| 7.0    | 190372            | 297KiB/s |

The bisect targeted the large drop between 6.19 and 7.0; the smaller 6.17
regression predates this commit and is unrelated. Reverting a339671db64b
restores throughput to the 6.17 level (~385 KiB/s). The commit is still
present in 7.3-rc5, and the same codegen problem reproduces there.

Digging deeper, I built 7.3-rc5 with my config and GCC 16.2, with and without
the commit, and compared the object files: 114 of them differ. As
<vdso/unaligned.h> is included by <linux/unaligned.h>, every
get/put_unaligned() call site depends on it transitively. GCC did not inline
__builtin_memcpy() and turned it into a function call, e.g. in crypto/crc32c.c
(__chksum_finup(), inlined into chksum_digest()):

Without the commit:

<chksum_digest>:
                   str    lr, [sp, #-0x4]!
                   sub    sp, sp, #12
                   str    lr, [sp, #-0x4]!
                   bl    0xc0 <chksum_digest+0xc> @ imm = #-0x8
        R_ARM_CALL    __gnu_mcount_nc
                   ldr    r0, [r0]
                   str    r3, [sp, #0x4]
                   ldr    r0, [r0, #0x20]
                   bl    0xd0 <chksum_digest+0x1c> @ imm = #-0x8
        R_ARM_CALL    crc32c
                   mvn    r2, r0
                   mov    r0, #0
                   ldr    r3, [sp, #0x4]
                   lsr    r12, r2, #8
                   lsr    r1, r2, #16
                   strb    r2, [r3]
                   lsr    r2, r2, #24
                   strb    r12, [r3, #0x1]
                   strb    r1, [r3, #0x2]
                   strb    r2, [r3, #0x3]
                   add    sp, sp, #12
                   ldr    pc, [sp], #4

With the commit:

<chksum_digest>:
                   push    {r4, lr}
                   sub    sp, sp, #8
                   str    lr, [sp, #-0x4]!
                   bl    0x124 <chksum_digest+0xc> @ imm = #-0x8
        R_ARM_CALL    __gnu_mcount_nc
                   ldr    r0, [r0]
                   ldr    r12, [pc, #0x54]        @ 0x188 <chksum_digest+0x70>
                   ldr    r0, [r0, #0x20]
                   mov    r4, r3
                   ldr    r12, [r12]
                   str    r12, [sp, #0x4]
                   mov    r12, #0
                   bl    0x144 <chksum_digest+0x2c> @ imm = #-0x8
        R_ARM_CALL    crc32c
                   mvn    r3, r0
                   mov    r2, #4
                   mov    r0, r4
                   mov    r1, sp
                   str    r3, [sp]
                   bl    0x15c <chksum_digest+0x44> @ imm = #-0x8
        R_ARM_CALL    memcpy
                   ldr    r3, [pc, #0x20]         @ 0x188 <chksum_digest+0x70>
                   ldr    r2, [r3]
                   ldr    r3, [sp, #0x4]
                   eors    r2, r3, r2
                   mov    r3, #0
                   bne    0x184 <chksum_digest+0x6c> @ imm = #0x8
                   mov    r0, #0
                   add    sp, sp, #8
                   pop    {r4, pc}
                   bl    0x184 <chksum_digest+0x6c> @ imm = #-0x8
        R_ARM_CALL    __stack_chk_fail
     188: 00 00 00 00      .word    0x00000000
        R_ARM_ABS32    __stack_chk_guard

Is this an accepted trade-off? My understanding is that the kernel is always
built with -fno-strict-aliasing, so the packed-struct type punning was well
defined there, and the __packed annotation is what lets GCC generate valid
code for the unaligned access.

Best regards,
Stefan

-- 
Pengutronix e.K.                       | Stefan Kerkmann             |
Steuerwalder Str. 21                   | https://www.pengutronix.de/ |
31137 Hildesheim, Germany              | Phone: +49-5121-206917-128  |
Amtsgericht Hildesheim, HRA 2686       | Fax:   +49-5121-206917-9    |

Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy

[Ian Rogers]

On Mon, Sep 28, 2026 at 8:39 AM Stefan Kerkmann
<s.kerkmann@pengutronix.de> wrote:
>
> Hi Ian,
>
> On 10/16/25 22:51, Ian Rogers wrote:
> > 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@google.com>
> > ---
> >  include/vdso/unaligned.h | 41 ++++++++++++++++++++++++++++++++++------
> >  1 file changed, 35 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
> > index ff0c06b6513e..9076483c9fbb 100644
> > --- a/include/vdso/unaligned.h
> > +++ b/include/vdso/unaligned.h
> > @@ -2,14 +2,43 @@
> >  #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
> > + * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
> > + * expression rather than type, a pointer is used to avoid warnings about mixing
> > + * the use of 0 and NULL. The void* cast silences ubsan warnings.
> > + */
> > +#define __get_unaligned_t(type, ptr) ({                                      \
> > +     type *__get_unaligned_ctrl_type __always_unused = NULL;         \
> > +     __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 */
>
> commit a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to
> memcpy()"), which landed in 7.0, causes a performance regression on an NXP
> i.MX25 (ARMv5TE) SoC.
>
> I found it while updating a client's board from 6.12 to 7.0. A fio 4k randwrite
> benchmark on a NAND storage with UBI and UBIFS filesystem was the only workload
> that showed a clear regression between those two versions, so I bisected with
> it:
>
> perf stat -e irq:irq_handler_entry --filter 'irq == 49' -a \
>     -- \
>     fio --name=rw \
>     --filename=/var/stat/testfile \
>     --size=8M \
>     --rw=randwrite \
>     --bs=4k \
>     --direct=0 \
>     --fsync=1 \
>     --numjobs=4 \
>     --group_reporting
>
> | kernel | irq_handler_entry | fio bw   |
> | ------ | ----------------- | -------- |
> | 6.12   | 155783            | 401KiB/s |
> | 6.13   | 162317            | 395KiB/s |
> | 6.14   | 168741            | 392KiB/s |
> | 6.15   | 168556            | 401KiB/s |
> | 6.16   | 166090            | 403KiB/s |
> | 6.17   | 162541            | 385KiB/s |
> | 6.18   | 157527            | 386KiB/s |
> | 6.19   | 183675            | 381KiB/s |
> | 7.0    | 190372            | 297KiB/s |
>
> The bisect targeted the large drop between 6.19 and 7.0; the smaller 6.17
> regression predates this commit and is unrelated. Reverting a339671db64b
> restores throughput to the 6.17 level (~385 KiB/s). The commit is still
> present in 7.3-rc5, and the same codegen problem reproduces there.
>
> Digging deeper, I built 7.3-rc5 with my config and GCC 16.2, with and without
> the commit, and compared the object files: 114 of them differ. As
> <vdso/unaligned.h> is included by <linux/unaligned.h>, every
> get/put_unaligned() call site depends on it transitively. GCC did not inline
> __builtin_memcpy() and turned it into a function call, e.g. in crypto/crc32c.c
> (__chksum_finup(), inlined into chksum_digest()):
>
> Without the commit:
>
> <chksum_digest>:
>                    str    lr, [sp, #-0x4]!
>                    sub    sp, sp, #12
>                    str    lr, [sp, #-0x4]!
>                    bl    0xc0 <chksum_digest+0xc> @ imm = #-0x8
>         R_ARM_CALL    __gnu_mcount_nc
>                    ldr    r0, [r0]
>                    str    r3, [sp, #0x4]
>                    ldr    r0, [r0, #0x20]
>                    bl    0xd0 <chksum_digest+0x1c> @ imm = #-0x8
>         R_ARM_CALL    crc32c
>                    mvn    r2, r0
>                    mov    r0, #0
>                    ldr    r3, [sp, #0x4]
>                    lsr    r12, r2, #8
>                    lsr    r1, r2, #16
>                    strb    r2, [r3]
>                    lsr    r2, r2, #24
>                    strb    r12, [r3, #0x1]
>                    strb    r1, [r3, #0x2]
>                    strb    r2, [r3, #0x3]
>                    add    sp, sp, #12
>                    ldr    pc, [sp], #4
>
> With the commit:
>
> <chksum_digest>:
>                    push    {r4, lr}
>                    sub    sp, sp, #8
>                    str    lr, [sp, #-0x4]!
>                    bl    0x124 <chksum_digest+0xc> @ imm = #-0x8
>         R_ARM_CALL    __gnu_mcount_nc
>                    ldr    r0, [r0]
>                    ldr    r12, [pc, #0x54]        @ 0x188 <chksum_digest+0x70>
>                    ldr    r0, [r0, #0x20]
>                    mov    r4, r3
>                    ldr    r12, [r12]
>                    str    r12, [sp, #0x4]
>                    mov    r12, #0
>                    bl    0x144 <chksum_digest+0x2c> @ imm = #-0x8
>         R_ARM_CALL    crc32c
>                    mvn    r3, r0
>                    mov    r2, #4
>                    mov    r0, r4
>                    mov    r1, sp
>                    str    r3, [sp]
>                    bl    0x15c <chksum_digest+0x44> @ imm = #-0x8
>         R_ARM_CALL    memcpy
>                    ldr    r3, [pc, #0x20]         @ 0x188 <chksum_digest+0x70>
>                    ldr    r2, [r3]
>                    ldr    r3, [sp, #0x4]
>                    eors    r2, r3, r2
>                    mov    r3, #0
>                    bne    0x184 <chksum_digest+0x6c> @ imm = #0x8
>                    mov    r0, #0
>                    add    sp, sp, #8
>                    pop    {r4, pc}
>                    bl    0x184 <chksum_digest+0x6c> @ imm = #-0x8
>         R_ARM_CALL    __stack_chk_fail
>      188: 00 00 00 00      .word    0x00000000
>         R_ARM_ABS32    __stack_chk_guard
>
> Is this an accepted trade-off? My understanding is that the kernel is always
> built with -fno-strict-aliasing, so the packed-struct type punning was well
> defined there, and the __packed annotation is what lets GCC generate valid
> code for the unaligned access.

Hi Stefan,

and sorry for the performance regression! I largely work on the perf
tool which is in the tools/ directory but uses kernel header files,
such as for unaligned accesses. My work was motivated by trying to
avoid using -fno-strict-aliasing in the perf tool.

As the compiler is given __builtin_memcpy of a fixed memory size then
the lowering should match that of using the packed struct. Are there
compiler flags you are using that disable compiler optimizations? This
feels like a compiler bug, a workaround is to use the packed struct
helpers in linux/unaligned/packed_struct.h

Thanks,
Ian

> Best regards,
> Stefan
>
> --
> Pengutronix e.K.                       | Stefan Kerkmann             |
> Steuerwalder Str. 21                   | https://www.pengutronix.de/ |
> 31137 Hildesheim, Germany              | Phone: +49-5121-206917-128  |
> Amtsgericht Hildesheim, HRA 2686       | Fax:   +49-5121-206917-9    |
>

Re: [Regression] [PATCH v5 2/4] vdso: Switch get/put unaligned from packed struct to memcpy

[Marc Kleine-Budde]

[-- Attachment #1: Type: text/plain, Size: 8561 bytes --]

Cc+ Arnd

On 28.09.2026 17:39:15, Stefan Kerkmann wrote:
> Hi Ian,
>
> On 10/16/25 22:51, Ian Rogers wrote:
> > 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@google.com>
> > ---
> >  include/vdso/unaligned.h | 41 ++++++++++++++++++++++++++++++++++------
> >  1 file changed, 35 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
> > index ff0c06b6513e..9076483c9fbb 100644
> > --- a/include/vdso/unaligned.h
> > +++ b/include/vdso/unaligned.h
> > @@ -2,14 +2,43 @@
> >  #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
> > + * __get_unaligned_ctrl_type gives __unqual_scalar_typeof its required
> > + * expression rather than type, a pointer is used to avoid warnings about mixing
> > + * the use of 0 and NULL. The void* cast silences ubsan warnings.
> > + */
> > +#define __get_unaligned_t(type, ptr) ({					\
> > +	type *__get_unaligned_ctrl_type __always_unused = NULL;		\
> > +	__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 */
>
> commit a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to
> memcpy()"), which landed in 7.0, causes a performance regression on an NXP
> i.MX25 (ARMv5TE) SoC.
>
> I found it while updating a client's board from 6.12 to 7.0. A fio 4k randwrite
> benchmark on a NAND storage with UBI and UBIFS filesystem was the only workload
> that showed a clear regression between those two versions, so I bisected with
> it:
>
> perf stat -e irq:irq_handler_entry --filter 'irq == 49' -a \
>     -- \
>     fio --name=rw \
>     --filename=/var/stat/testfile \
>     --size=8M \
>     --rw=randwrite \
>     --bs=4k \
>     --direct=0 \
>     --fsync=1 \
>     --numjobs=4 \
>     --group_reporting
>
> | kernel | irq_handler_entry | fio bw   |
> | ------ | ----------------- | -------- |
> | 6.12   | 155783            | 401KiB/s |
> | 6.13   | 162317            | 395KiB/s |
> | 6.14   | 168741            | 392KiB/s |
> | 6.15   | 168556            | 401KiB/s |
> | 6.16   | 166090            | 403KiB/s |
> | 6.17   | 162541            | 385KiB/s |
> | 6.18   | 157527            | 386KiB/s |
> | 6.19   | 183675            | 381KiB/s |
> | 7.0    | 190372            | 297KiB/s |
>
> The bisect targeted the large drop between 6.19 and 7.0; the smaller 6.17
> regression predates this commit and is unrelated. Reverting a339671db64b
> restores throughput to the 6.17 level (~385 KiB/s). The commit is still
> present in 7.3-rc5, and the same codegen problem reproduces there.
>
> Digging deeper, I built 7.3-rc5 with my config and GCC 16.2, with and without
> the commit, and compared the object files: 114 of them differ. As
> <vdso/unaligned.h> is included by <linux/unaligned.h>, every
> get/put_unaligned() call site depends on it transitively. GCC did not inline
> __builtin_memcpy() and turned it into a function call, e.g. in crypto/crc32c.c
> (__chksum_finup(), inlined into chksum_digest()):
>
> Without the commit:
>
> <chksum_digest>:
>                    str    lr, [sp, #-0x4]!
>                    sub    sp, sp, #12
>                    str    lr, [sp, #-0x4]!
>                    bl    0xc0 <chksum_digest+0xc> @ imm = #-0x8
>         R_ARM_CALL    __gnu_mcount_nc
>                    ldr    r0, [r0]
>                    str    r3, [sp, #0x4]
>                    ldr    r0, [r0, #0x20]
>                    bl    0xd0 <chksum_digest+0x1c> @ imm = #-0x8
>         R_ARM_CALL    crc32c
>                    mvn    r2, r0
>                    mov    r0, #0
>                    ldr    r3, [sp, #0x4]
>                    lsr    r12, r2, #8
>                    lsr    r1, r2, #16
>                    strb    r2, [r3]
>                    lsr    r2, r2, #24
>                    strb    r12, [r3, #0x1]
>                    strb    r1, [r3, #0x2]
>                    strb    r2, [r3, #0x3]
>                    add    sp, sp, #12
>                    ldr    pc, [sp], #4
>
> With the commit:
>
> <chksum_digest>:
>                    push    {r4, lr}
>                    sub    sp, sp, #8
>                    str    lr, [sp, #-0x4]!
>                    bl    0x124 <chksum_digest+0xc> @ imm = #-0x8
>         R_ARM_CALL    __gnu_mcount_nc
>                    ldr    r0, [r0]
>                    ldr    r12, [pc, #0x54]        @ 0x188 <chksum_digest+0x70>
>                    ldr    r0, [r0, #0x20]
>                    mov    r4, r3
>                    ldr    r12, [r12]
>                    str    r12, [sp, #0x4]
>                    mov    r12, #0
>                    bl    0x144 <chksum_digest+0x2c> @ imm = #-0x8
>         R_ARM_CALL    crc32c
>                    mvn    r3, r0
>                    mov    r2, #4
>                    mov    r0, r4
>                    mov    r1, sp
>                    str    r3, [sp]
>                    bl    0x15c <chksum_digest+0x44> @ imm = #-0x8
>         R_ARM_CALL    memcpy
>                    ldr    r3, [pc, #0x20]         @ 0x188 <chksum_digest+0x70>
>                    ldr    r2, [r3]
>                    ldr    r3, [sp, #0x4]
>                    eors    r2, r3, r2
>                    mov    r3, #0
>                    bne    0x184 <chksum_digest+0x6c> @ imm = #0x8
>                    mov    r0, #0
>                    add    sp, sp, #8
>                    pop    {r4, pc}
>                    bl    0x184 <chksum_digest+0x6c> @ imm = #-0x8
>         R_ARM_CALL    __stack_chk_fail
>      188: 00 00 00 00      .word    0x00000000
>         R_ARM_ABS32    __stack_chk_guard
>
> Is this an accepted trade-off? My understanding is that the kernel is always
> built with -fno-strict-aliasing, so the packed-struct type punning was well
> defined there, and the __packed annotation is what lets GCC generate valid
> code for the unaligned access.
>
> Best regards,
> Stefan
>
> --
> Pengutronix e.K.                       | Stefan Kerkmann             |
> Steuerwalder Str. 21                   | https://www.pengutronix.de/ |
> 31137 Hildesheim, Germany              | Phone: +49-5121-206917-128  |
> Amtsgericht Hildesheim, HRA 2686       | Fax:   +49-5121-206917-9    |
>
>

--
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
Vertretung Nürnberg              | Phone: +49-5121-206917-129 |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-9   |

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