Re: [PATCH bpf-next v5 4/8] libbpf: Create a bpf_link in bpf_map__attach_struct_ops().

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On 3/9/23 10:16, Kui-Feng Lee wrote:


On 3/9/23 09:09, Martin KaFai Lau wrote:
On 3/8/23 4:22 PM, Kui-Feng Lee wrote:


On 3/8/23 13:42, Martin KaFai Lau wrote:
On 3/7/23 4:50 PM, Kui-Feng Lee wrote:
@@ -11566,22 +11591,34 @@ struct bpf_link *bpf_program__attach(const struct bpf_program *prog)
      return link;
  }
+struct bpf_link_struct_ops {
+    struct bpf_link link;
+    int map_fd;
+};
+
  static int bpf_link__detach_struct_ops(struct bpf_link *link)
  {
+    struct bpf_link_struct_ops *st_link;
      __u32 zero = 0;
-    if (bpf_map_delete_elem(link->fd, &zero))
-        return -errno;
+    st_link = container_of(link, struct bpf_link_struct_ops, link);
-    return 0;
+    if (st_link->map_fd < 0) {

map_fd < 0 should always be true?

If the user pass a wrong link, it can fail.

I may have missed something. How can user directly pass a link to this static function?

Ouch! You are right. This check is not necessary. I mixed it with the old detach feature.

By the way, I will keep this test here since this function will handle
the case w/o a link as well.




I check it here explicitly even the kernel returns
an error for deleting an element of a struct_ops w/ link.
Yep, the kernel should have stopped the delete if the user somehow corrupted the map_fd to -1.



+        /* Fake bpf_link */
+        if (bpf_map_delete_elem(link->fd, &zero))
+            return -errno;
+        return 0;
+    }
+
+    /* Doesn't support detaching. */
+    return -EOPNOTSUPP;




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