tcg: Avoid undefined behaviour patching code at unaligned addresses

To avoid C undefined behaviour when patching generated code,
provide wrappers tcg_patch8/16/32/64 which use the usual memcpy
trick, and use them in the i386 backend.
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

tcg/i386/tcg-target.c

@@ -151,14 +151,14 @@ static void patch_reloc(uint8_t *code_ptr, int type,
         if (value != (int32_t)value) {
             tcg_abort();
         }
-        *(uint32_t *)code_ptr = value;
+        tcg_patch32(code_ptr, value);
         break;
     case R_386_PC8:
         value -= (uintptr_t)code_ptr;
         if (value != (int8_t)value) {
             tcg_abort();
         }
-        *(uint8_t *)code_ptr = value;
+        tcg_patch8(code_ptr, value);
         break;
     default:
         tcg_abort();
@@ -1276,9 +1276,9 @@ static void tcg_out_qemu_ld_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
     uint8_t **label_ptr = &l->label_ptr[0];
 
     /* resolve label address */
-    *(uint32_t *)label_ptr[0] = (uint32_t)(s->code_ptr - label_ptr[0] - 4);
+    tcg_patch32(label_ptr[0], s->code_ptr - label_ptr[0] - 4);
     if (TARGET_LONG_BITS > TCG_TARGET_REG_BITS) {
-        *(uint32_t *)label_ptr[1] = (uint32_t)(s->code_ptr - label_ptr[1] - 4);
+        tcg_patch32(label_ptr[1], s->code_ptr - label_ptr[1] - 4);
     }
 
     if (TCG_TARGET_REG_BITS == 32) {
@@ -1360,9 +1360,9 @@ static void tcg_out_qemu_st_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
     TCGReg retaddr;
 
     /* resolve label address */
-    *(uint32_t *)label_ptr[0] = (uint32_t)(s->code_ptr - label_ptr[0] - 4);
+    tcg_patch32(label_ptr[0], s->code_ptr - label_ptr[0] - 4);
     if (TARGET_LONG_BITS > TCG_TARGET_REG_BITS) {
-        *(uint32_t *)label_ptr[1] = (uint32_t)(s->code_ptr - label_ptr[1] - 4);
+        tcg_patch32(label_ptr[1], s->code_ptr - label_ptr[1] - 4);
     }
 
     if (TCG_TARGET_REG_BITS == 32) {

tcg/tcg.c

@@ -122,6 +122,11 @@ static inline void tcg_out8(TCGContext *s, uint8_t v)
     *s->code_ptr++ = v;
 }
 
+static inline void tcg_patch8(uint8_t *p, uint8_t v)
+{
+    memcpy(p, &v, sizeof(v));
+}
+
 static inline void tcg_out16(TCGContext *s, uint16_t v)
 {
     uint8_t *p = s->code_ptr;
@@ -129,6 +134,11 @@ static inline void tcg_out16(TCGContext *s, uint16_t v)
     s->code_ptr = p + 2;
 }
 
+static inline void tcg_patch16(uint8_t *p, uint16_t v)
+{
+    memcpy(p, &v, sizeof(v));
+}
+
 static inline void tcg_out32(TCGContext *s, uint32_t v)
 {
     uint8_t *p = s->code_ptr;
@@ -136,6 +146,11 @@ static inline void tcg_out32(TCGContext *s, uint32_t v)
     s->code_ptr = p + 4;
 }
 
+static inline void tcg_patch32(uint8_t *p, uint32_t v)
+{
+    memcpy(p, &v, sizeof(v));
+}
+
 static inline void tcg_out64(TCGContext *s, uint64_t v)
 {
     uint8_t *p = s->code_ptr;
@@ -143,6 +158,11 @@ static inline void tcg_out64(TCGContext *s, uint64_t v)
     s->code_ptr = p + 8;
 }
 
+static inline void tcg_patch64(uint8_t *p, uint64_t v)
+{
+    memcpy(p, &v, sizeof(v));
+}
+
 /* label relocation processing */
 
 static void tcg_out_reloc(TCGContext *s, uint8_t *code_ptr, int type,