Fix ems allocator unaligned memory access on riscv64 (#2140)

Make `hmu_tree_node` struct packed and add 4 padding bytes before `kfc_tree_root_buf`
field in `gc_heap_struct` struct to ensure the `left/right/parent` fields in `hmu_tree_node`
are 8-byte aligned on the 64-bit target which doesn't support unaligned memory access.

Fix the issue reported in #2136.

core/shared/mem-alloc/ems/ems_alloc.c

@@ -25,7 +25,7 @@ static bool
 remove_tree_node(gc_heap_t *heap, hmu_tree_node_t *p)
 {
     hmu_tree_node_t *q = NULL, **slot = NULL, *parent;
-    hmu_tree_node_t *root = &heap->kfc_tree_root;
+    hmu_tree_node_t *root = heap->kfc_tree_root;
     gc_uint8 *base_addr = heap->base_addr;
     gc_uint8 *end_addr = base_addr + heap->current_size;
 
@@ -38,13 +38,17 @@ remove_tree_node(gc_heap_t *heap, hmu_tree_node_t *p)
         goto fail;
     }
 
-    /* get the slot which holds pointer to node p*/
+    /* get the slot which holds pointer to node p */
     if (p == p->parent->right) {
-        slot = &p->parent->right;
+        /* Don't use `slot = &p->parent->right` to avoid compiler warning */
+        slot = (hmu_tree_node_t **)((uint8 *)p->parent
+                                    + offsetof(hmu_tree_node_t, right));
     }
     else if (p == p->parent->left) {
-        /* p should be a child of its parent*/
-        slot = &p->parent->left;
+        /* p should be a child of its parent */
+        /* Don't use `slot = &p->parent->left` to avoid compiler warning */
+        slot = (hmu_tree_node_t **)((uint8 *)p->parent
+                                    + offsetof(hmu_tree_node_t, left));
     }
     else {
         goto fail;
@@ -241,7 +245,7 @@ gci_add_fc(gc_heap_t *heap, hmu_t *hmu, gc_size_t size)
     node->left = node->right = node->parent = NULL;
 
     /* find proper node to link this new node to */
-    root = &heap->kfc_tree_root;
+    root = heap->kfc_tree_root;
     tp = root;
     bh_assert(tp->size < size);
     while (1) {
@@ -289,6 +293,7 @@ alloc_hmu(gc_heap_t *heap, gc_size_t size)
     uint32 node_idx = 0, init_node_idx = 0;
     hmu_tree_node_t *root = NULL, *tp = NULL, *last_tp = NULL;
     hmu_t *next, *rest;
+    uintptr_t tp_ret;
 
     bh_assert(gci_is_heap_valid(heap));
     bh_assert(size > 0 && !(size & 7));
@@ -354,7 +359,7 @@ alloc_hmu(gc_heap_t *heap, gc_size_t size)
     }
 
     /* need to find a node in tree*/
-    root = &heap->kfc_tree_root;
+    root = heap->kfc_tree_root;
 
     /* find the best node*/
     bh_assert(root);
@@ -402,7 +407,8 @@ alloc_hmu(gc_heap_t *heap, gc_size_t size)
             heap->highmark_size = heap->current_size - heap->total_free_size;
 
         hmu_set_size((hmu_t *)last_tp, size);
-        return (hmu_t *)last_tp;
+        tp_ret = (uintptr_t)last_tp;
+        return (hmu_t *)tp_ret;
     }
 
     return NULL;

core/shared/mem-alloc/ems/ems_gc_internal.h

@@ -204,13 +204,47 @@ set_hmu_normal_node_next(hmu_normal_node_t *node, hmu_normal_node_t *next)
     }
 }
 
+/**
+ * Define hmu_tree_node as a packed struct, since it is at the 4-byte
+ * aligned address and the size of hmu_head is 4, so in 64-bit target,
+ * the left/right/parent fields will be at 8-byte aligned address,
+ * we can access them directly.
+ */
+#if UINTPTR_MAX == UINT64_MAX
+#if defined(_MSC_VER)
+__pragma(pack(push, 1));
+#define __attr_packed
+#elif defined(__GNUC__) || defined(__clang__)
+#define __attr_packed __attribute__((packed))
+#else
+#error "packed attribute isn't used to define struct hmu_tree_node"
+#endif
+#else /* else of UINTPTR_MAX == UINT64_MAX */
+#define __attr_packed
+#endif
+
 typedef struct hmu_tree_node {
     hmu_t hmu_header;
-    gc_size_t size;
     struct hmu_tree_node *left;
     struct hmu_tree_node *right;
     struct hmu_tree_node *parent;
-} hmu_tree_node_t;
+    gc_size_t size;
+} __attr_packed hmu_tree_node_t;
+
+#if UINTPTR_MAX == UINT64_MAX
+#if defined(_MSC_VER)
+__pragma(pack(pop));
+#endif
+#endif
+
+bh_static_assert(sizeof(hmu_tree_node_t) == 8 + 3 * sizeof(void *));
+bh_static_assert(offsetof(hmu_tree_node_t, left) == 4);
+
+#define ASSERT_TREE_NODE_ALIGNED_ACCESS(tree_node)                          \
+    do {                                                                    \
+        bh_assert((((uintptr_t)&tree_node->left) & (sizeof(uintptr_t) - 1)) \
+                  == 0);                                                    \
+    } while (0)
 
 typedef struct gc_heap_struct {
     /* for double checking*/
@@ -223,8 +257,16 @@ typedef struct gc_heap_struct {
 
     hmu_normal_list_t kfc_normal_list[HMU_NORMAL_NODE_CNT];
 
-    /* order in kfc_tree is: size[left] <= size[cur] < size[right]*/
-    hmu_tree_node_t kfc_tree_root;
+#if UINTPTR_MAX == UINT64_MAX
+    /* make kfc_tree_root_buf 4-byte aligned and not 8-byte aligned,
+       so kfc_tree_root's left/right/parent fields are 8-byte aligned
+       and we can access them directly */
+    uint32 __padding;
+#endif
+    uint8 kfc_tree_root_buf[sizeof(hmu_tree_node_t)];
+    /* point to kfc_tree_root_buf, the order in kfc_tree is:
+         size[left] <= size[cur] < size[right] */
+    hmu_tree_node_t *kfc_tree_root;
 
     /* whether heap is corrupted, e.g. the hmu nodes are modified
        by user */

core/shared/mem-alloc/ems/ems_kfc.c

@@ -27,7 +27,7 @@ gc_init_internal(gc_heap_t *heap, char *base_addr, gc_size_t heap_max_size)
     heap->total_free_size = heap->current_size;
     heap->highmark_size = 0;
 
-    root = &heap->kfc_tree_root;
+    root = heap->kfc_tree_root = (hmu_tree_node_t *)heap->kfc_tree_root_buf;
     memset(root, 0, sizeof *root);
     root->size = sizeof *root;
     hmu_set_ut(&root->hmu_header, HMU_FC);
@@ -38,6 +38,9 @@ gc_init_internal(gc_heap_t *heap, char *base_addr, gc_size_t heap_max_size)
     hmu_set_ut(&q->hmu_header, HMU_FC);
     hmu_set_size(&q->hmu_header, heap->current_size);
 
+    ASSERT_TREE_NODE_ALIGNED_ACCESS(q);
+    ASSERT_TREE_NODE_ALIGNED_ACCESS(root);
+
     hmu_mark_pinuse(&q->hmu_header);
     root->right = q;
     q->parent = root;
@@ -165,6 +168,7 @@ gc_migrate(gc_handle_t handle, char *pool_buf_new, gc_size_t pool_buf_size)
     intptr_t offset = (uint8 *)base_addr_new - (uint8 *)heap->base_addr;
     hmu_t *cur = NULL, *end = NULL;
     hmu_tree_node_t *tree_node;
+    uint8 **p_left, **p_right, **p_parent;
     gc_size_t heap_max_size, size;
 
     if ((((uintptr_t)pool_buf_new) & 7) != 0) {
@@ -188,9 +192,18 @@ gc_migrate(gc_handle_t handle, char *pool_buf_new, gc_size_t pool_buf_size)
     }
 
     heap->base_addr = (uint8 *)base_addr_new;
-    adjust_ptr((uint8 **)&heap->kfc_tree_root.left, offset);
-    adjust_ptr((uint8 **)&heap->kfc_tree_root.right, offset);
-    adjust_ptr((uint8 **)&heap->kfc_tree_root.parent, offset);
+
+    ASSERT_TREE_NODE_ALIGNED_ACCESS(heap->kfc_tree_root);
+
+    p_left = (uint8 **)((uint8 *)heap->kfc_tree_root
+                        + offsetof(hmu_tree_node_t, left));
+    p_right = (uint8 **)((uint8 *)heap->kfc_tree_root
+                         + offsetof(hmu_tree_node_t, right));
+    p_parent = (uint8 **)((uint8 *)heap->kfc_tree_root
+                          + offsetof(hmu_tree_node_t, parent));
+    adjust_ptr(p_left, offset);
+    adjust_ptr(p_right, offset);
+    adjust_ptr(p_parent, offset);
 
     cur = (hmu_t *)heap->base_addr;
     end = (hmu_t *)((char *)heap->base_addr + heap->current_size);
@@ -206,12 +219,21 @@ gc_migrate(gc_handle_t handle, char *pool_buf_new, gc_size_t pool_buf_size)
 
         if (hmu_get_ut(cur) == HMU_FC && !HMU_IS_FC_NORMAL(size)) {
             tree_node = (hmu_tree_node_t *)cur;
-            adjust_ptr((uint8 **)&tree_node->left, offset);
-            adjust_ptr((uint8 **)&tree_node->right, offset);
-            if (tree_node->parent != &heap->kfc_tree_root)
+
+            ASSERT_TREE_NODE_ALIGNED_ACCESS(tree_node);
+
+            p_left = (uint8 **)((uint8 *)tree_node
+                                + offsetof(hmu_tree_node_t, left));
+            p_right = (uint8 **)((uint8 *)tree_node
+                                 + offsetof(hmu_tree_node_t, right));
+            p_parent = (uint8 **)((uint8 *)tree_node
+                                  + offsetof(hmu_tree_node_t, parent));
+            adjust_ptr(p_left, offset);
+            adjust_ptr(p_right, offset);
+            if (tree_node->parent != heap->kfc_tree_root)
                 /* The root node belongs to heap structure,
                    it is fixed part and isn't changed. */
-                adjust_ptr((uint8 **)&tree_node->parent, offset);
+                adjust_ptr(p_parent, offset);
         }
         cur = (hmu_t *)((char *)cur + size);
     }