From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Thu, 2 Feb 2023 15:25:57 +0000 (-0500)
Subject: Fix: use unaligned pointer accesses for lttng_inline_memcpy
X-Git-Url: http://git.liburcu.org/?p=lttng-ust.git;a=commitdiff_plain;h=7fdb690ce0a7187b23f439d569ddd629a702871f

Fix: use unaligned pointer accesses for lttng_inline_memcpy

lttng_inline_memcpy receives pointers which can be unaligned. This
causes issues (traps) specifically on arm 32-bit with 8-byte strings
(including \0).

Use unaligned pointer accesses for loads/stores within
lttng_inline_memcpy instead.

There is an impact on code generation on some architectures.  Using the
following test code on godbolt.org:

void copy16_aligned(void *dest, void *src) {
    *(uint16_t *)dest = *(uint16_t *) src;
}

void copy16_unaligned(void *dest, void *src) {
    STORE_UNALIGNED_INT(uint16_t, dest, LOAD_UNALIGNED_INT(uint16_t, src));
}

void copy32_aligned(void *dest, void *src) {
    *(uint32_t *)dest = *(uint32_t *) src;
}

void copy32_unaligned(void *dest, void *src) {
    STORE_UNALIGNED_INT(uint32_t, dest, LOAD_UNALIGNED_INT(uint32_t, src));
}

void copy64_aligned(void *dest, void *src) {
    *(uint64_t *)dest = *(uint64_t *) src;
}

void copy64_unaligned(void *dest, void *src) {
    STORE_UNALIGNED_INT(uint64_t, dest, LOAD_UNALIGNED_INT(uint64_t, src));
}

The resulting assembler (gcc 12.2.0 in -O2) between aligned and
unaligned:

- x86-32: unchanged.
- x86-64: unchanged.
- powerpc32: unchanged.
- powerpc64: unchanged.
- arm32: 16 and 32-bit copy: unchanged. Added code for 64-bit unaligned copy.
- aarch64: unchanged.
- mips32: added code for unaligned.
- mips64: added code for unaligned.
- riscv: added code for unaligned.

If we want to improve the situation on mips and riscv, this would
require introducing a new "lttng_inline_integer_copy" and expose
additional ring buffer client APIs in addition to event_write() which
take integers as inputs. Let's not introduce that complexity yet until
it is justified.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I1e6471d4607ac6aff89f16ef24d5370e804b7612
---

diff --git a/src/common/ringbuffer/backend_internal.h b/src/common/ringbuffer/backend_internal.h
index bd422177..a3303b72 100644
--- a/src/common/ringbuffer/backend_internal.h
+++ b/src/common/ringbuffer/backend_internal.h
@@ -595,6 +595,21 @@ int update_read_sb_index(const struct lttng_ust_ring_buffer_config *config,
 #define inline_memcpy(dest, src, n)	memcpy(dest, src, n)
 #endif
 
+#define LOAD_UNALIGNED_INT(type, p) \
+	({ \
+		struct packed_struct { type __v; } __attribute__((packed)); \
+		(((const struct packed_struct *) (p))->__v); \
+	})
+
+#define STORE_UNALIGNED_INT(type, p, v)	\
+	do { \
+		struct packed_struct { type __v; } __attribute__((packed)); \
+		((struct packed_struct *) (p))->__v = (v); \
+	} while (0)
+
+/*
+ * Copy from src into dest, assuming unaligned src and dest.
+ */
 static inline
 void lttng_inline_memcpy(void *dest, const void *src,
 		unsigned long len)
@@ -608,13 +623,13 @@ void lttng_inline_memcpy(void *dest, const void *src,
 		*(uint8_t *) dest = *(const uint8_t *) src;
 		break;
 	case 2:
-		*(uint16_t *) dest = *(const uint16_t *) src;
+		STORE_UNALIGNED_INT(uint16_t, dest, LOAD_UNALIGNED_INT(uint16_t, src));
 		break;
 	case 4:
-		*(uint32_t *) dest = *(const uint32_t *) src;
+		STORE_UNALIGNED_INT(uint32_t, dest, LOAD_UNALIGNED_INT(uint32_t, src));
 		break;
 	case 8:
-		*(uint64_t *) dest = *(const uint64_t *) src;
+		STORE_UNALIGNED_INT(uint64_t, dest, LOAD_UNALIGNED_INT(uint64_t, src));
 		break;
 	default:
 		inline_memcpy(dest, src, len);