[PATCH v17 1/2] eal: add __rte_may_alias and __rte_aligned to unaligned typedefs

[scott.k.mitch1@gmail.com]

From: Scott Mitchell <scott.k.mitch1@gmail.com>

Add __rte_may_alias attribute to unaligned_uint{16,32,64}_t typedefs
to prevent GCC strict-aliasing optimization bugs. GCC has a bug where
it incorrectly elides struct initialization when strict aliasing is
enabled, causing reads from uninitialized memory.

Add __rte_aligned(1) attribute to unaligned_uint{16,32,64}_t typedefs
which allows for safe access at any alignment. Without this, accessing
a uint16_t at an odd address is undefined behavior. Without this
UBSan detects `UndefinedBehaviorSanitizer: undefined-behavior`.

Fixes: 7621d6a8d0bd ("eal: add and use unaligned integer types")
Cc: Cyril Chemparathy <cchemparathy@ezchip.com>
Cc: stable@dpdk.org
Signed-off-by: Scott Mitchell <scott.k.mitch1@gmail.com>
---
 lib/eal/include/rte_common.h | 39 +++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
index 573bf4f2ce..15d379619a 100644
--- a/lib/eal/include/rte_common.h
+++ b/lib/eal/include/rte_common.h
@@ -121,16 +121,32 @@ extern "C" {
 #define __rte_aligned(a) __attribute__((__aligned__(a)))
 #endif
 
-#ifdef RTE_ARCH_STRICT_ALIGN
-typedef uint64_t unaligned_uint64_t __rte_aligned(1);
-typedef uint32_t unaligned_uint32_t __rte_aligned(1);
-typedef uint16_t unaligned_uint16_t __rte_aligned(1);
+/**
+ * Macro to mark a type that is not subject to type-based aliasing rules
+ */
+#ifdef RTE_TOOLCHAIN_MSVC
+#define __rte_may_alias
 #else
-typedef uint64_t unaligned_uint64_t;
-typedef uint32_t unaligned_uint32_t;
-typedef uint16_t unaligned_uint16_t;
+#define __rte_may_alias __attribute__((__may_alias__))
 #endif
 
+/**
+ * Types for potentially unaligned access.
+ *
+ * __rte_aligned(1) - Reduces alignment requirement to 1 byte, allowing
+ *                    these types to safely access memory at any address.
+ *                    Without this, accessing a uint16_t at an odd address
+ *                    is undefined behavior (even on x86 where hardware
+ *                    handles it).
+ *
+ * __rte_may_alias  - Prevents strict-aliasing optimization bugs where
+ *                    compilers may incorrectly elide memory operations
+ *                    when casting between pointer types.
+ */
+typedef uint64_t unaligned_uint64_t __rte_may_alias __rte_aligned(1);
+typedef uint32_t unaligned_uint32_t __rte_may_alias __rte_aligned(1);
+typedef uint16_t unaligned_uint16_t __rte_may_alias __rte_aligned(1);
+
 /**
  * @deprecated
  * @see __rte_packed_begin
@@ -159,15 +175,6 @@ typedef uint16_t unaligned_uint16_t;
 #define __rte_packed_end __attribute__((__packed__))
 #endif
 
-/**
- * Macro to mark a type that is not subject to type-based aliasing rules
- */
-#ifdef RTE_TOOLCHAIN_MSVC
-#define __rte_may_alias
-#else
-#define __rte_may_alias __attribute__((__may_alias__))
-#endif
-
 /******* Macro to mark functions and fields scheduled for removal *****/
 #ifdef RTE_TOOLCHAIN_MSVC
 #define __rte_deprecated
-- 
2.39.5 (Apple Git-154)