bpf: Factor out bpf_link_by_id() helper.

Refactor the code a bit to extract bpf_link_by_id() helper.
It's similar to existing bpf_prog_by_id().

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/bpf.h

···
 			 struct btf *btf, const struct btf_type *t);
 
 struct bpf_prog *bpf_prog_by_id(u32 id);
+struct bpf_link *bpf_link_by_id(u32 id);
 
 const struct bpf_func_proto *bpf_base_func_proto(enum bpf_func_id func_id);
 #else /* !CONFIG_BPF_SYSCALL */

kernel/bpf/syscall.c

···
 	return ret;
 }
 
-static int bpf_link_inc_not_zero(struct bpf_link *link)
+static struct bpf_link *bpf_link_inc_not_zero(struct bpf_link *link)
 {
-	return atomic64_fetch_add_unless(&link->refcnt, 1, 0) ? 0 : -ENOENT;
+	return atomic64_fetch_add_unless(&link->refcnt, 1, 0) ? link : ERR_PTR(-ENOENT);
+}
+
+struct bpf_link *bpf_link_by_id(u32 id)
+{
+	struct bpf_link *link;
+
+	if (!id)
+		return ERR_PTR(-ENOENT);
+
+	spin_lock_bh(&link_idr_lock);
+	/* before link is "settled", ID is 0, pretend it doesn't exist yet */
+	link = idr_find(&link_idr, id);
+	if (link) {
+		if (link->id)
+			link = bpf_link_inc_not_zero(link);
+		else
+			link = ERR_PTR(-EAGAIN);
+	} else {
+		link = ERR_PTR(-ENOENT);
+	}
+	spin_unlock_bh(&link_idr_lock);
+	return link;
 }
 
 #define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
···
 {
 	struct bpf_link *link;
 	u32 id = attr->link_id;
-	int fd, err;
+	int fd;
 
 	if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID))
 		return -EINVAL;
···
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	spin_lock_bh(&link_idr_lock);
-	link = idr_find(&link_idr, id);
-	/* before link is "settled", ID is 0, pretend it doesn't exist yet */
-	if (link) {
-		if (link->id)
-			err = bpf_link_inc_not_zero(link);
-		else
-			err = -EAGAIN;
-	} else {
-		err = -ENOENT;
-	}
-	spin_unlock_bh(&link_idr_lock);
-
-	if (err)
-		return err;
+	link = bpf_link_by_id(id);
+	if (IS_ERR(link))
+		return PTR_ERR(link);
 
 	fd = bpf_link_new_fd(link);
 	if (fd < 0)