bpf: create repeated fields for arrays.

The verifier uses field information for certain special types, such as
kptr, rbtree root, and list head. These types are treated
differently. However, we did not previously support these types in
arrays. This update examines arrays and duplicates field information the
same number of times as the length of the array if the element type is one
of the special types.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240523174202.461236-5-thinker.li@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -3494,6 +3494,41 @@ static int btf_get_field_type(const char *name, u32 field_mask, u32 *seen_mask,
 
 #undef field_mask_test_name
 
+/* Repeat a field for a specified number of times.
+ *
+ * Copy and repeat the first field for repeat_cnt
+ * times. The field is repeated by adding the offset of each field
+ * with
+ *   (i + 1) * elem_size
+ * where i is the repeat index and elem_size is the size of an element.
+ */
+static int btf_repeat_field(struct btf_field_info *info,
+			    u32 repeat_cnt, u32 elem_size)
+{
+	u32 i;
+	u32 cur;
+
+	/* Ensure not repeating fields that should not be repeated. */
+	switch (info[0].type) {
+	case BPF_KPTR_UNREF:
+	case BPF_KPTR_REF:
+	case BPF_KPTR_PERCPU:
+	case BPF_LIST_HEAD:
+	case BPF_RB_ROOT:
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	cur = 1;
+	for (i = 0; i < repeat_cnt; i++) {
+		memcpy(&info[cur], &info[0], sizeof(info[0]));
+		info[cur++].off += (i + 1) * elem_size;
+	}
+
+	return 0;
+}
+
 static int btf_find_field_one(const struct btf *btf,
 			      const struct btf_type *var,
 			      const struct btf_type *var_type,
@@ -3504,6 +3539,21 @@ static int btf_find_field_one(const struct btf *btf,
 {
 	int ret, align, sz, field_type;
 	struct btf_field_info tmp;
+	const struct btf_array *array;
+	u32 i, nelems = 1;
+
+	/* Walk into array types to find the element type and the number of
+	 * elements in the (flattened) array.
+	 */
+	for (i = 0; i < MAX_RESOLVE_DEPTH && btf_type_is_array(var_type); i++) {
+		array = btf_array(var_type);
+		nelems *= array->nelems;
+		var_type = btf_type_by_id(btf, array->type);
+	}
+	if (i == MAX_RESOLVE_DEPTH)
+		return -E2BIG;
+	if (nelems == 0)
+		return 0;
 
 	field_type = btf_get_field_type(__btf_name_by_offset(btf, var_type->name_off),
 					field_mask, seen_mask, &align, &sz);
@@ -3512,7 +3562,7 @@ static int btf_find_field_one(const struct btf *btf,
 	if (field_type < 0)
 		return field_type;
 
-	if (expected_size && expected_size != sz)
+	if (expected_size && expected_size != sz * nelems)
 		return 0;
 	if (off % align)
 		return 0;
@@ -3552,10 +3602,14 @@ static int btf_find_field_one(const struct btf *btf,
 
 	if (ret == BTF_FIELD_IGNORE)
 		return 0;
-	if (!info_cnt)
+	if (nelems > info_cnt)
 		return -E2BIG;
-
-	return 1;
+	if (nelems > 1) {
+		ret = btf_repeat_field(info, nelems - 1, sz);
+		if (ret < 0)
+			return ret;
+	}
+	return nelems;
 }
 
 static int btf_find_struct_field(const struct btf *btf,