[PATCH v6] eal: fix rte_memcpy strict aliasing/alignment bugs

[Luc Pelletier <lucp.at.work@gmail.com>]

Calls to rte_memcpy for 1 < n < 16 could result in unaligned
loads/stores, which is undefined behaviour according to the C
standard, and strict aliasing violations.

The code was changed to use a packed structure that allows aliasing
(using the __may_alias__ attribute) to perform the load/store
operations. This results in code that has the same performance as the
original code and that is also C standard-compliant.

Fixes: d35cc1fe6a7a ("eal/x86: revert select optimized memcpy at run-time")
Cc: Xiaoyun Li <xiaoyun.li@intel.com>
Cc: stable@dpdk.org

Signed-off-by: Luc Pelletier <lucp.at.work@gmail.com>
---
v6:
* Refocus to fix strict aliasing problems discovered following
discussions in this thread.
* Modified the code to use __may_alias__ and packed structure. This fixes
both the undefined behaviour of unaligned access (which is not the main
concern), and also fixes the strict aliasing violations (which can cause
major bugs, as demonstrated in a previous message in this thread).
* Renamed new function from rte_mov15_or_less_unaligned to
rte_mov15_or_less.
* Modified code that copies <= 15 bytes to call rte_mov15_or_less.

v5:
* Replaced assembly with pure C code that uses a packed structure to make
unaligned loads conform to C standard.

v4:
* Added volatile to asm statements, which is required under gcc.

v3:
* Removed for loop and went back to using assembly.

v2:
* Replaced assembly with a regular for loop that copies bytes one by
one.

v1:
* Fix undefined behaviour of unaligned stores/loads by using assembly
to perform stores/loads.

 lib/eal/x86/include/rte_memcpy.h | 133 ++++++++++++-------------------
 1 file changed, 51 insertions(+), 82 deletions(-)

diff --git a/lib/eal/x86/include/rte_memcpy.h b/lib/eal/x86/include/rte_memcpy.h
index 1b6c6e585f..360b7e069e 100644
--- a/lib/eal/x86/include/rte_memcpy.h
+++ b/lib/eal/x86/include/rte_memcpy.h
@@ -45,6 +45,52 @@ extern "C" {
 static __rte_always_inline void *
 rte_memcpy(void *dst, const void *src, size_t n);
 
+/**
+ * Copy bytes from one location to another,
+ * locations should not overlap.
+ * Use with n <= 15.
+ */
+static __rte_always_inline void *
+rte_mov15_or_less(void *dst, const void *src, size_t n)
+{
+	/**
+	 * Use the following structs to avoid violating C standard
+	 * alignment requirements and to avoid strict aliasing bugs
+	 */
+	struct rte_uint64_alias {
+		uint64_t val;
+	} __attribute__((__may_alias__, __packed__));
+	struct rte_uint32_alias {
+		uint32_t val;
+	} __attribute__((__may_alias__, __packed__));
+	struct rte_uint16_alias {
+		uint16_t val;
+	} __attribute__((__may_alias__, __packed__));
+
+	void *ret = dst;
+	if (n & 8) {
+		((struct rte_uint64_alias *)dst)->val =
+			((const struct rte_uint64_alias *)src)->val;
+		src = (const uint64_t *)src + 1;
+		dst = (uint64_t *)dst + 1;
+	}
+	if (n & 4) {
+		((struct rte_uint32_alias *)dst)->val =
+			((const struct rte_uint32_alias *)src)->val;
+		src = (const uint32_t *)src + 1;
+		dst = (uint32_t *)dst + 1;
+	}
+	if (n & 2) {
+		((struct rte_uint16_alias *)dst)->val =
+			((const struct rte_uint16_alias *)src)->val;
+		src = (const uint16_t *)src + 1;
+		dst = (uint16_t *)dst + 1;
+	}
+	if (n & 1)
+		*(uint8_t *)dst = *(const uint8_t *)src;
+	return ret;
+}
+
 #if defined __AVX512F__ && defined RTE_MEMCPY_AVX512
 
 #define ALIGNMENT_MASK 0x3F
@@ -171,8 +217,6 @@ rte_mov512blocks(uint8_t *dst, const uint8_t *src, size_t n)
 static __rte_always_inline void *
 rte_memcpy_generic(void *dst, const void *src, size_t n)
 {
-	uintptr_t dstu = (uintptr_t)dst;
-	uintptr_t srcu = (uintptr_t)src;
 	void *ret = dst;
 	size_t dstofss;
 	size_t bits;
@@ -181,24 +225,7 @@ rte_memcpy_generic(void *dst, const void *src, size_t n)
 	 * Copy less than 16 bytes
 	 */
 	if (n < 16) {
-		if (n & 0x01) {
-			*(uint8_t *)dstu = *(const uint8_t *)srcu;
-			srcu = (uintptr_t)((const uint8_t *)srcu + 1);
-			dstu = (uintptr_t)((uint8_t *)dstu + 1);
-		}
-		if (n & 0x02) {
-			*(uint16_t *)dstu = *(const uint16_t *)srcu;
-			srcu = (uintptr_t)((const uint16_t *)srcu + 1);
-			dstu = (uintptr_t)((uint16_t *)dstu + 1);
-		}
-		if (n & 0x04) {
-			*(uint32_t *)dstu = *(const uint32_t *)srcu;
-			srcu = (uintptr_t)((const uint32_t *)srcu + 1);
-			dstu = (uintptr_t)((uint32_t *)dstu + 1);
-		}
-		if (n & 0x08)
-			*(uint64_t *)dstu = *(const uint64_t *)srcu;
-		return ret;
+		return rte_mov15_or_less(dst, src, n);
 	}
 
 	/**
@@ -379,8 +406,6 @@ rte_mov128blocks(uint8_t *dst, const uint8_t *src, size_t n)
 static __rte_always_inline void *
 rte_memcpy_generic(void *dst, const void *src, size_t n)
 {
-	uintptr_t dstu = (uintptr_t)dst;
-	uintptr_t srcu = (uintptr_t)src;
 	void *ret = dst;
 	size_t dstofss;
 	size_t bits;
@@ -389,25 +414,7 @@ rte_memcpy_generic(void *dst, const void *src, size_t n)
 	 * Copy less than 16 bytes
 	 */
 	if (n < 16) {
-		if (n & 0x01) {
-			*(uint8_t *)dstu = *(const uint8_t *)srcu;
-			srcu = (uintptr_t)((const uint8_t *)srcu + 1);
-			dstu = (uintptr_t)((uint8_t *)dstu + 1);
-		}
-		if (n & 0x02) {
-			*(uint16_t *)dstu = *(const uint16_t *)srcu;
-			srcu = (uintptr_t)((const uint16_t *)srcu + 1);
-			dstu = (uintptr_t)((uint16_t *)dstu + 1);
-		}
-		if (n & 0x04) {
-			*(uint32_t *)dstu = *(const uint32_t *)srcu;
-			srcu = (uintptr_t)((const uint32_t *)srcu + 1);
-			dstu = (uintptr_t)((uint32_t *)dstu + 1);
-		}
-		if (n & 0x08) {
-			*(uint64_t *)dstu = *(const uint64_t *)srcu;
-		}
-		return ret;
+		return rte_mov15_or_less(dst, src, n);
 	}
 
 	/**
@@ -672,8 +679,6 @@ static __rte_always_inline void *
 rte_memcpy_generic(void *dst, const void *src, size_t n)
 {
 	__m128i xmm0, xmm1, xmm2, xmm3, xmm4, xmm5, xmm6, xmm7, xmm8;
-	uintptr_t dstu = (uintptr_t)dst;
-	uintptr_t srcu = (uintptr_t)src;
 	void *ret = dst;
 	size_t dstofss;
 	size_t srcofs;
@@ -682,25 +687,7 @@ rte_memcpy_generic(void *dst, const void *src, size_t n)
 	 * Copy less than 16 bytes
 	 */
 	if (n < 16) {
-		if (n & 0x01) {
-			*(uint8_t *)dstu = *(const uint8_t *)srcu;
-			srcu = (uintptr_t)((const uint8_t *)srcu + 1);
-			dstu = (uintptr_t)((uint8_t *)dstu + 1);
-		}
-		if (n & 0x02) {
-			*(uint16_t *)dstu = *(const uint16_t *)srcu;
-			srcu = (uintptr_t)((const uint16_t *)srcu + 1);
-			dstu = (uintptr_t)((uint16_t *)dstu + 1);
-		}
-		if (n & 0x04) {
-			*(uint32_t *)dstu = *(const uint32_t *)srcu;
-			srcu = (uintptr_t)((const uint32_t *)srcu + 1);
-			dstu = (uintptr_t)((uint32_t *)dstu + 1);
-		}
-		if (n & 0x08) {
-			*(uint64_t *)dstu = *(const uint64_t *)srcu;
-		}
-		return ret;
+		return rte_mov15_or_less(dst, src, n);
 	}
 
 	/**
@@ -818,27 +805,9 @@ rte_memcpy_aligned(void *dst, const void *src, size_t n)
 {
 	void *ret = dst;
 
-	/* Copy size <= 16 bytes */
+	/* Copy size < 16 bytes */
 	if (n < 16) {
-		if (n & 0x01) {
-			*(uint8_t *)dst = *(const uint8_t *)src;
-			src = (const uint8_t *)src + 1;
-			dst = (uint8_t *)dst + 1;
-		}
-		if (n & 0x02) {
-			*(uint16_t *)dst = *(const uint16_t *)src;
-			src = (const uint16_t *)src + 1;
-			dst = (uint16_t *)dst + 1;
-		}
-		if (n & 0x04) {
-			*(uint32_t *)dst = *(const uint32_t *)src;
-			src = (const uint32_t *)src + 1;
-			dst = (uint32_t *)dst + 1;
-		}
-		if (n & 0x08)
-			*(uint64_t *)dst = *(const uint64_t *)src;
-
-		return ret;
+		return rte_mov15_or_less(dst, src, n);
 	}
 
 	/* Copy 16 <= size <= 32 bytes */
-- 
2.35.1