bpftool: show flow_dissector attachment status

Right now there is no way to query whether BPF flow_dissector program
is attached to a network namespace or not. In previous commit, I added
support for querying that info, show it when doing `bpftool net`:

$ bpftool prog loadall ./bpf_flow.o \
	/sys/fs/bpf/flow type flow_dissector \
	pinmaps /sys/fs/bpf/flow
$ bpftool prog
3: flow_dissector  name _dissect  tag 8c9e917b513dd5cc  gpl
        loaded_at 2019-04-23T16:14:48-0700  uid 0
        xlated 656B  jited 461B  memlock 4096B  map_ids 1,2
        btf_id 1
...

$ bpftool net -j
[{"xdp":[],"tc":[],"flow_dissector":[]}]

$ bpftool prog attach pinned \
	/sys/fs/bpf/flow/flow_dissector flow_dissector
$ bpftool net -j
[{"xdp":[],"tc":[],"flow_dissector":["id":3]}]

Doesn't show up in a different net namespace:
$ ip netns add test
$ ip netns exec test bpftool net -j
[{"xdp":[],"tc":[],"flow_dissector":[]}]

Non-json output:
$ bpftool net
xdp:

tc:

flow_dissector:
id 3

v2:
* initialization order (Jakub Kicinski)
* clear errno for batch mode (Quentin Monnet)
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/net.c

@@ -3,6 +3,7 @@
 
 #define _GNU_SOURCE
 #include <errno.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
@@ -12,6 +13,8 @@
 #include <linux/rtnetlink.h>
 #include <linux/tc_act/tc_bpf.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 
 #include <bpf.h>
 #include <nlattr.h>
@@ -48,6 +51,10 @@ struct bpf_filter_t {
 	int		ifindex;
 };
 
+struct bpf_attach_info {
+	__u32 flow_dissector_id;
+};
+
 static int dump_link_nlmsg(void *cookie, void *msg, struct nlattr **tb)
 {
 	struct bpf_netdev_t *netinfo = cookie;
@@ -180,8 +187,45 @@ static int show_dev_tc_bpf(int sock, unsigned int nl_pid,
 	return 0;
 }
 
+static int query_flow_dissector(struct bpf_attach_info *attach_info)
+{
+	__u32 attach_flags;
+	__u32 prog_ids[1];
+	__u32 prog_cnt;
+	int err;
+	int fd;
+
+	fd = open("/proc/self/ns/net", O_RDONLY);
+	if (fd < 0) {
+		p_err("can't open /proc/self/ns/net: %d",
+		      strerror(errno));
+		return -1;
+	}
+	prog_cnt = ARRAY_SIZE(prog_ids);
+	err = bpf_prog_query(fd, BPF_FLOW_DISSECTOR, 0,
+			     &attach_flags, prog_ids, &prog_cnt);
+	close(fd);
+	if (err) {
+		if (errno == EINVAL) {
+			/* Older kernel's don't support querying
+			 * flow dissector programs.
+			 */
+			errno = 0;
+			return 0;
+		}
+		p_err("can't query prog: %s", strerror(errno));
+		return -1;
+	}
+
+	if (prog_cnt == 1)
+		attach_info->flow_dissector_id = prog_ids[0];
+
+	return 0;
+}
+
 static int do_show(int argc, char **argv)
 {
+	struct bpf_attach_info attach_info = {};
 	int i, sock, ret, filter_idx = -1;
 	struct bpf_netdev_t dev_array;
 	unsigned int nl_pid;
@@ -199,6 +243,10 @@ static int do_show(int argc, char **argv)
 		usage();
 	}
 
+	ret = query_flow_dissector(&attach_info);
+	if (ret)
+		return -1;
+
 	sock = libbpf_netlink_open(&nl_pid);
 	if (sock < 0) {
 		fprintf(stderr, "failed to open netlink sock\n");
@@ -227,6 +275,12 @@ static int do_show(int argc, char **argv)
 		}
 		NET_END_ARRAY("\n");
 	}
+
+	NET_START_ARRAY("flow_dissector", "%s:\n");
+	if (attach_info.flow_dissector_id > 0)
+		NET_DUMP_UINT("id", "id %u", attach_info.flow_dissector_id);
+	NET_END_ARRAY("\n");
+
 	NET_END_OBJECT;
 	if (json_output)
 		jsonw_end_array(json_wtr);