bpf: Abstract away entire bpf_link clean up procedure

Instead of requiring users to do three steps for cleaning up bpf_link, its
anon_inode file, and unused fd, abstract that away into bpf_link_cleanup()
helper. bpf_link_defunct() is removed, as it shouldn't be needed as an
individual operation anymore.

v1->v2:
- keep bpf_link_cleanup() static for now (Daniel).

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200313002128.2028680-1-andriin@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -1075,7 +1075,6 @@ struct bpf_link_ops {
 
 void bpf_link_init(struct bpf_link *link, const struct bpf_link_ops *ops,
 		   struct bpf_prog *prog);
-void bpf_link_defunct(struct bpf_link *link);
 void bpf_link_inc(struct bpf_link *link);
 void bpf_link_put(struct bpf_link *link);
 int bpf_link_new_fd(struct bpf_link *link);

kernel/bpf/syscall.c

@@ -2188,9 +2188,17 @@ void bpf_link_init(struct bpf_link *link, const struct bpf_link_ops *ops,
 	link->prog = prog;
 }
 
-void bpf_link_defunct(struct bpf_link *link)
+/* Clean up bpf_link and corresponding anon_inode file and FD. After
+ * anon_inode is created, bpf_link can't be just kfree()'d due to deferred
+ * anon_inode's release() call. This helper manages marking bpf_link as
+ * defunct, releases anon_inode file and puts reserved FD.
+ */
+static void bpf_link_cleanup(struct bpf_link *link, struct file *link_file,
+			     int link_fd)
 {
 	link->prog = NULL;
+	fput(link_file);
+	put_unused_fd(link_fd);
 }
 
 void bpf_link_inc(struct bpf_link *link)
@@ -2383,9 +2391,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog)
 
 	err = bpf_trampoline_link_prog(prog);
 	if (err) {
-		bpf_link_defunct(&link->link);
-		fput(link_file);
-		put_unused_fd(link_fd);
+		bpf_link_cleanup(&link->link, link_file, link_fd);
 		goto out_put_prog;
 	}
 
@@ -2498,9 +2504,7 @@ static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
 
 	err = bpf_probe_register(link->btp, prog);
 	if (err) {
-		bpf_link_defunct(&link->link);
-		fput(link_file);
-		put_unused_fd(link_fd);
+		bpf_link_cleanup(&link->link, link_file, link_fd);
 		goto out_put_btp;
 	}