libbpf: Convert st_ops->data to shadow type.

Convert st_ops->data to the shadow type of the struct_ops map. The shadow
type of a struct_ops type is a variant of the original struct type
providing a way to access/change the values in the maps of the struct_ops
type.

bpf_map__initial_value() will return st_ops->data for struct_ops types. The
skeleton is going to use it as the pointer to the shadow type of the
original struct type.

One of the main differences between the original struct type and the shadow
type is that all function pointers of the shadow type are converted to
pointers of struct bpf_program. Users can replace these bpf_program
pointers with other BPF programs. The st_ops->progs[] will be updated
before updating the value of a map to reflect the changes made by users.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-3-thinker.li@gmail.com

Author: ThinkerYzu1

tools/lib/bpf/libbpf.c

@@ -1014,6 +1014,19 @@ static bool bpf_map__is_struct_ops(const struct bpf_map *map)
 	return map->def.type == BPF_MAP_TYPE_STRUCT_OPS;
 }
 
+static bool is_valid_st_ops_program(struct bpf_object *obj,
+				    const struct bpf_program *prog)
+{
+	int i;
+
+	for (i = 0; i < obj->nr_programs; i++) {
+		if (&obj->programs[i] == prog)
+			return prog->type == BPF_PROG_TYPE_STRUCT_OPS;
+	}
+
+	return false;
+}
+
 /* Init the map's fields that depend on kern_btf */
 static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 {
@@ -1102,9 +1115,16 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 		if (btf_is_ptr(mtype)) {
 			struct bpf_program *prog;
 
-			prog = st_ops->progs[i];
+			/* Update the value from the shadow type */
+			prog = *(void **)mdata;
+			st_ops->progs[i] = prog;
 			if (!prog)
 				continue;
+			if (!is_valid_st_ops_program(obj, prog)) {
+				pr_warn("struct_ops init_kern %s: member %s is not a struct_ops program\n",
+					map->name, mname);
+				return -ENOTSUP;
+			}
 
 			kern_mtype = skip_mods_and_typedefs(kern_btf,
 							    kern_mtype->type,
@@ -9310,7 +9330,9 @@ static struct bpf_map *find_struct_ops_map_by_offset(struct bpf_object *obj,
 	return NULL;
 }
 
-/* Collect the reloc from ELF and populate the st_ops->progs[] */
+/* Collect the reloc from ELF, populate the st_ops->progs[], and update
+ * st_ops->data for shadow type.
+ */
 static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
 					    Elf64_Shdr *shdr, Elf_Data *data)
 {
@@ -9424,6 +9446,14 @@ static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
 		}
 
 		st_ops->progs[member_idx] = prog;
+
+		/* st_ops->data will be exposed to users, being returned by
+		 * bpf_map__initial_value() as a pointer to the shadow
+		 * type. All function pointers in the original struct type
+		 * should be converted to a pointer to struct bpf_program
+		 * in the shadow type.
+		 */
+		*((struct bpf_program **)(st_ops->data + moff)) = prog;
 	}
 
 	return 0;
@@ -9882,6 +9912,12 @@ int bpf_map__set_initial_value(struct bpf_map *map,
 
 void *bpf_map__initial_value(struct bpf_map *map, size_t *psize)
 {
+	if (bpf_map__is_struct_ops(map)) {
+		if (psize)
+			*psize = map->def.value_size;
+		return map->st_ops->data;
+	}
+
 	if (!map->mmaped)
 		return NULL;
 	*psize = map->def.value_size;