libbpf: Deprecate bpf_objects_list

Add a flag to `enum libbpf_strict_mode' to disable the global
`bpf_objects_list', preventing race conditions when concurrent threads
call bpf_object__open() or bpf_object__close().

bpf_object__next() will return NULL if this option is set.

Callers may achieve the same workflow by tracking bpf_objects in
application code.

  [0] Closes: https://github.com/libbpf/libbpf/issues/293

Signed-off-by: Joe Burton <jevburton@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211026223528.413950-1-jevburton.kernel@gmail.com

src/libbpf.c

@@ -1148,6 +1148,7 @@ static struct bpf_object *bpf_object__new(const char *path,
 					  size_t obj_buf_sz,
 					  const char *obj_name)
 {
+	bool strict = (libbpf_mode & LIBBPF_STRICT_NO_OBJECT_LIST);
 	struct bpf_object *obj;
 	char *end;
 
@@ -1188,7 +1189,8 @@ static struct bpf_object *bpf_object__new(const char *path,
 	obj->loaded = false;
 
 	INIT_LIST_HEAD(&obj->list);
-	list_add(&obj->list, &bpf_objects_list);
+	if (!strict)
+		list_add(&obj->list, &bpf_objects_list);
 	return obj;
 }
 
@@ -7936,6 +7938,10 @@ struct bpf_object *
 bpf_object__next(struct bpf_object *prev)
 {
 	struct bpf_object *next;
+	bool strict = (libbpf_mode & LIBBPF_STRICT_NO_OBJECT_LIST);
+
+	if (strict)
+		return NULL;
 
 	if (!prev)
 		next = list_first_entry(&bpf_objects_list,

src/libbpf.h

@@ -168,7 +168,8 @@ LIBBPF_API struct bpf_program *
 bpf_object__find_program_by_name(const struct bpf_object *obj,
 				 const char *name);
 
-LIBBPF_API struct bpf_object *bpf_object__next(struct bpf_object *prev);
+LIBBPF_API LIBBPF_DEPRECATED_SINCE(0, 7, "track bpf_objects in application code instead")
+struct bpf_object *bpf_object__next(struct bpf_object *prev);
 #define bpf_object__for_each_safe(pos, tmp)			\
 	for ((pos) = bpf_object__next(NULL),		\
 		(tmp) = bpf_object__next(pos);		\

src/libbpf_legacy.h

@@ -57,6 +57,12 @@ enum libbpf_strict_mode {
 	 * function name instead of section name.
 	 */
 	LIBBPF_STRICT_SEC_NAME = 0x04,
+	/*
+	 * Disable the global 'bpf_objects_list'. Maintaining this list adds
+	 * a race condition to bpf_object__open() and bpf_object__close().
+	 * Clients can maintain it on their own if it is valuable for them.
+	 */
+	LIBBPF_STRICT_NO_OBJECT_LIST = 0x08,
 
 	__LIBBPF_STRICT_LAST,
 };