bpf: Implement BPF XDP link-specific introspection APIs

Implement XDP link-specific show_fdinfo and link_info to emit ifindex.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722064603.3350758-7-andriin@fb.com

include/uapi/linux/bpf.h

@@ -4069,6 +4069,9 @@ struct bpf_link_info {
 			__u32 netns_ino;
 			__u32 attach_type;
 		} netns;
+		struct {
+			__u32 ifindex;
+		} xdp;
 	};
 } __attribute__((aligned(8)));
 

net/core/dev.c

@@ -8996,6 +8996,35 @@ static void bpf_xdp_link_dealloc(struct bpf_link *link)
 	kfree(xdp_link);
 }
 
+static void bpf_xdp_link_show_fdinfo(const struct bpf_link *link,
+				     struct seq_file *seq)
+{
+	struct bpf_xdp_link *xdp_link = container_of(link, struct bpf_xdp_link, link);
+	u32 ifindex = 0;
+
+	rtnl_lock();
+	if (xdp_link->dev)
+		ifindex = xdp_link->dev->ifindex;
+	rtnl_unlock();
+
+	seq_printf(seq, "ifindex:\t%u\n", ifindex);
+}
+
+static int bpf_xdp_link_fill_link_info(const struct bpf_link *link,
+				       struct bpf_link_info *info)
+{
+	struct bpf_xdp_link *xdp_link = container_of(link, struct bpf_xdp_link, link);
+	u32 ifindex = 0;
+
+	rtnl_lock();
+	if (xdp_link->dev)
+		ifindex = xdp_link->dev->ifindex;
+	rtnl_unlock();
+
+	info->xdp.ifindex = ifindex;
+	return 0;
+}
+
 static int bpf_xdp_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
 			       struct bpf_prog *old_prog)
 {
@@ -9041,6 +9070,8 @@ static int bpf_xdp_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
 static const struct bpf_link_ops bpf_xdp_link_lops = {
 	.release = bpf_xdp_link_release,
 	.dealloc = bpf_xdp_link_dealloc,
+	.show_fdinfo = bpf_xdp_link_show_fdinfo,
+	.fill_link_info = bpf_xdp_link_fill_link_info,
 	.update_prog = bpf_xdp_link_update,
 };