Re: [PATCH v4 bpf-next 1/4] bpf: Factor out bpf_link_get_by_id() helper.

---
 include/linux/bpf.h  |  1 +
 kernel/bpf/syscall.c | 46 +++++++++++++++++++++++++++-----------------
 2 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 8357be349133..d5c4e2cc24a0 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1355,6 +1355,7 @@ int btf_check_type_match(struct bpf_verifier_env *env, struct bpf_prog *prog,
                         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 */

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ee290b1f2d9e..42eb0d622980 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -3984,40 +3984,50 @@ static int link_update(union bpf_attr *attr)
        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);
 }

-#define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
-
-static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
+struct bpf_link *bpf_link_by_id(u32 id)
 {
        struct bpf_link *link;
-       u32 id = attr->link_id;
-       int fd, err;

-       if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID))
-               return -EINVAL;
-
-       if (!capable(CAP_SYS_ADMIN))
-               return -EPERM;
+       if (!id)
+               return ERR_PTR(-ENOENT);

        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 */
+       link = idr_find(&link_idr, id);
        if (link) {
                if (link->id)
-                       err = bpf_link_inc_not_zero(link);
+                       link = bpf_link_inc_not_zero(link);
                else
-                       err = -EAGAIN;
+                       link = ERR_PTR(-EAGAIN);
        } else {
-               err = -ENOENT;
+               link = ERR_PTR(-ENOENT);
        }
        spin_unlock_bh(&link_idr_lock);
+       return link;
+}

-       if (err)
-               return err;
+#define BPF_LINK_GET_FD_BY_ID_LAST_FIELD link_id
+
+static int bpf_link_get_fd_by_id(const union bpf_attr *attr)
+{
+       struct bpf_link *link;
+       u32 id = attr->link_id;
+       int fd;
+
+       if (CHECK_ATTR(BPF_LINK_GET_FD_BY_ID))
+               return -EINVAL;
+
+       if (!capable(CAP_SYS_ADMIN))
+               return -EPERM;
+
+       link = bpf_link_by_id(id);
+       if (IS_ERR(link))
+               return PTR_ERR(link);

        fd = bpf_link_new_fd(link);
        if (fd < 0)
--

2.23.0