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 | 48 +++++++++++++++++++++++++++++++++++-----
1 file changed, 42 insertions(+), 6 deletions(-)
diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index ff0c06b6513e..f33c1eefbf68 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; \
+#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; \
+ __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.rc2.701.gf1e915cc24-googOn Tue, 17 Jun 2025 13:53:18 -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.
>.... \
> +/**
> + * __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)
Does that actually work?
If 'ptr' has type 'long *' gcc will (validly according to C) assume
it is aligned and will generate a misaligned memory write.
The (void *) cast make no difference.
Using (void *)(long)(ptr) might lose the alignment.
Otherwise you may need to use OPTIMISER_HIDE_VAR() and live
with the extra register-register move it tends to generate.
David
Hi Ian,
kernel test robot noticed the following build warnings:
[auto build test WARNING on linus/master]
[also build test WARNING on v6.16-rc2 next-20250618]
[cannot apply to tip/timers/vdso acme/perf/core]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Ian-Rogers/vdso-Switch-get-put-unaligned-from-packed-struct-to-memcpy/20250618-045610
base: linus/master
patch link: https://lore.kernel.org/r/20250617205320.1580946-2-irogers%40google.com
patch subject: [PATCH v2 1/3] vdso: Switch get/put unaligned from packed struct to memcpy
config: parisc-allnoconfig (https://download.01.org/0day-ci/archive/20250618/202506182044.OhyWGCSm-lkp@intel.com/config)
compiler: hppa-linux-gcc (GCC) 15.1.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250618/202506182044.OhyWGCSm-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202506182044.OhyWGCSm-lkp@intel.com/
All warnings (new ones prefixed by >>):
In file included from include/linux/swab.h:5,
from include/uapi/linux/byteorder/big_endian.h:14,
from include/linux/byteorder/big_endian.h:5,
from arch/parisc/include/uapi/asm/byteorder.h:5,
from arch/parisc/include/asm/bitops.h:11,
from include/linux/bitops.h:67,
from include/linux/kernel.h:23,
from arch/parisc/include/asm/bug.h:5,
from include/linux/bug.h:5,
from include/linux/thread_info.h:13,
from include/linux/sched.h:14,
from include/linux/uaccess.h:9,
from arch/parisc/boot/compressed/misc.c:7:
In function 'get_unaligned_le32',
inlined from 'decompress_kernel' at arch/parisc/boot/compressed/misc.c:312:16:
>> include/vdso/unaligned.h:30:9: warning: '__builtin_memcpy' reading 4 bytes from a region of size 1 [-Wstringop-overread]
30 | __builtin_memcpy(&__get_unaligned_val, (void *)(ptr), \
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
31 | sizeof(__get_unaligned_val)); \
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/uapi/linux/swab.h:120:19: note: in definition of macro '__swab32'
120 | __fswab32(x))
| ^
include/linux/byteorder/generic.h:89:21: note: in expansion of macro '__le32_to_cpu'
89 | #define le32_to_cpu __le32_to_cpu
| ^~~~~~~~~~~~~
include/linux/unaligned.h:23:28: note: in expansion of macro '__get_unaligned_t'
23 | return le32_to_cpu(__get_unaligned_t(__le32, p));
| ^~~~~~~~~~~~~~~~~
arch/parisc/boot/compressed/misc.c: In function 'decompress_kernel':
arch/parisc/boot/compressed/misc.c:29:13: note: source object 'output_len' of size 1
29 | extern char output_len;
| ^~~~~~~~~~
vim +/__builtin_memcpy +30 include/vdso/unaligned.h
4
5 #define ____get_unaligned_type(type) type: (type)0
6 /**
7 * __get_unaligned_t - read an unaligned value from memory.
8 * @ptr: the pointer to load from.
9 * @type: the type to load from the pointer.
10 *
11 * Use memcpy to affect an unaligned type sized load avoiding undefined behavior
12 * from approaches like type punning that require -fno-strict-aliasing in order
13 * to be correct. As type may be const, use _Generic to map to a non-const type
14 * - you can't memcpy into a const type. The void* cast silences ubsan warnings.
15 */
16 #define __get_unaligned_t(type, ptr) ({ \
17 type __get_unaligned_map_ctrl = 0; \
18 typeof(_Generic(__get_unaligned_map_ctrl, \
19 ____get_unaligned_type(short int), \
20 ____get_unaligned_type(unsigned short int), \
21 ____get_unaligned_type(int), \
22 ____get_unaligned_type(unsigned int), \
23 ____get_unaligned_type(long), \
24 ____get_unaligned_type(unsigned long), \
25 ____get_unaligned_type(long long), \
26 ____get_unaligned_type(unsigned long long), \
27 default: (type)0 \
28 )) __get_unaligned_val; \
29 (void)__get_unaligned_map_ctrl; \
> 30 __builtin_memcpy(&__get_unaligned_val, (void *)(ptr), \
31 sizeof(__get_unaligned_val)); \
32 __get_unaligned_val; \
33 })
34
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
Le 17/06/2025 à 22:53, Ian Rogers a écrit :
> 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>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> 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..f33c1eefbf68 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; \
> +#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; \
> + __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 */
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