[PATCH v2 1/2] vdso: Add VDSO_UNALIGNED_PACKED config option

[Ian Rogers]

Commit a339671db64b ("vdso: Switch get/put_unaligned() from packed struct
to memcpy()") switched __get_unaligned_t() and __put_unaligned_t() from
using a __packed struct to __builtin_memcpy() to avoid strict aliasing
undefined behavior.

On targets without efficient unaligned memory accesses (such as ARMv5),
GCC may not inline __builtin_memcpy() into byte shifts and instead emit
stack spills or much slower out-of-line memcpy() calls. Add
CONFIG_VDSO_UNALIGNED_PACKED, defaulting to
!HAVE_EFFICIENT_UNALIGNED_ACCESS, to use the __packed struct
implementation on strict-alignment targets while keeping
__builtin_memcpy() elsewhere and in tools/.

Fixes: a339671db64b ("vdso: Switch get/put_unaligned() from packed struct to memcpy()")
Reported-by: Stefan Kerkmann <s.kerkmann@xxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/lkml/47f8c30b-e326-4ff0-a2bc-5683209a9e2d@xxxxxxxxxxxxxx/
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
v2: Default VDSO_UNALIGNED_PACKED to !HAVE_EFFICIENT_UNALIGNED_ACCESS
    instead of CPU_32v5 to cover all strict-alignment targets (Sashiko,
    Arnd Bergmann).

 include/vdso/unaligned.h | 16 ++++++++++++++++
 lib/vdso/Kconfig         | 10 ++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/vdso/unaligned.h b/include/vdso/unaligned.h
index 9076483c9fbb..726fd15efc82 100644
--- a/include/vdso/unaligned.h
+++ b/include/vdso/unaligned.h
@@ -4,6 +4,20 @@
 
 #include <linux/compiler_types.h>
 
+#ifdef CONFIG_VDSO_UNALIGNED_PACKED
+
+#define __get_unaligned_t(type, ptr) ({							\
+	const struct { type x; } __packed * __get_pptr = (typeof(__get_pptr))(ptr);	\
+	__get_pptr->x;									\
+})
+
+#define __put_unaligned_t(type, val, ptr) do {						\
+	struct { type x; } __packed * __put_pptr = (typeof(__put_pptr))(ptr);		\
+	__put_pptr->x = (val);								\
+} while (0)
+
+#else /* !CONFIG_VDSO_UNALIGNED_PACKED */
+
 /**
  * __get_unaligned_t - read an unaligned value from memory.
  * @type:	the type to load from the pointer.
@@ -41,4 +55,6 @@
 			 sizeof(__put_unaligned_val));			\
 } while (0)
 
+#endif /* CONFIG_VDSO_UNALIGNED_PACKED */
+
 #endif /* __VDSO_UNALIGNED_H */
diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
index 597f5f0f9681..2436c20ee0ef 100644
--- a/lib/vdso/Kconfig
+++ b/lib/vdso/Kconfig
@@ -23,3 +23,13 @@ config VDSO_GETRANDOM
 	select VDSO_DATASTORE
 	help
 	  Selected by architectures that support vDSO getrandom().
+
+config VDSO_UNALIGNED_PACKED
+	bool
+	default !HAVE_EFFICIENT_UNALIGNED_ACCESS
+	help
+	  Use packed structs rather than memcpy() in include/vdso/unaligned.h
+	  for unaligned memory accesses. Enabled by default when the
+	  architecture does not have efficient unaligned accesses, as GCC may
+	  emit stack spills or out-of-line memcpy() calls instead of inlining
+	  byte shifts.
-- 
2.56.0.212.g129ede20d9-goog