samples/bpf: make xdp_fwd more practically usable via devmap lookup

This address the TODO in samples/bpf/xdp_fwd_kern.c, which points out
that the chosen egress index should be checked for existence in the
devmap. This can now be done via taking advantage of Toke's work in
commit 0cdbb4b09a06 ("devmap: Allow map lookups from eBPF").

This change makes xdp_fwd more practically usable, as this allows for
a mixed environment, where IP-forwarding fallback to network stack, if
the egress device isn't configured to use XDP.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 22 insertions, 11 deletions

diff --git a/samples/bpf/xdp_fwd_user.c b/samples/bpf/xdp_fwd_user.c
index ba012d9f93dd..97ff1dad7669 100644
--- a/samples/bpf/xdp_fwd_user.c
+++ b/samples/bpf/xdp_fwd_user.c
@@ -27,14 +27,20 @@
 #include "libbpf.h"
 #include <bpf/bpf.h>
 
-
-static int do_attach(int idx, int fd, const char *name)
+static int do_attach(int idx, int prog_fd, int map_fd, const char *name)
 {
 	int err;
 
-	err = bpf_set_link_xdp_fd(idx, fd, 0);
-	if (err < 0)
+	err = bpf_set_link_xdp_fd(idx, prog_fd, 0);
+	if (err < 0) {
 		printf("ERROR: failed to attach program to %s\n", name);
+		return err;
+	}
+
+	/* Adding ifindex as a possible egress TX port */
+	err = bpf_map_update_elem(map_fd, &idx, &idx, 0);
+	if (err)
+		printf("ERROR: failed using device %s as TX-port\n", name);
 
 	return err;
 }
@@ -47,6 +53,9 @@ static int do_detach(int idx, const char *name)
 	if (err < 0)
 		printf("ERROR: failed to detach program from %s\n", name);
 
+	/* TODO: Remember to cleanup map, when adding use of shared map
+	 *  bpf_map_delete_elem((map_fd, &idx);
+	 */
 	return err;
 }
 
@@ -67,10 +76,10 @@ int main(int argc, char **argv)
 	};
 	const char *prog_name = "xdp_fwd";
 	struct bpf_program *prog;
+	int prog_fd, map_fd = -1;
 	char filename[PATH_MAX];
 	struct bpf_object *obj;
 	int opt, i, idx, err;
-	int prog_fd, map_fd;
 	int attach = 1;
 	int ret = 0;
 
@@ -103,8 +112,14 @@ int main(int argc, char **argv)
 			return 1;
 		}
 
-		if (bpf_prog_load_xattr(&prog_load_attr, &obj, &prog_fd))
+		err = bpf_prog_load_xattr(&prog_load_attr, &obj, &prog_fd);
+		if (err) {
+			printf("Does kernel support devmap lookup?\n");
+			/* If not, the error message will be:
+			 *  "cannot pass map_type 14 into func bpf_map_lookup_elem#1"
+			 */
 			return 1;
+		}
 
 		prog = bpf_object__find_program_by_title(obj, prog_name);
 		prog_fd = bpf_program__fd(prog);
@@ -119,10 +134,6 @@ int main(int argc, char **argv)
 			return 1;
 		}
 	}
-	if (attach) {
-		for (i = 1; i < 64; ++i)
-			bpf_map_update_elem(map_fd, &i, &i, 0);
-	}
 
 	for (i = optind; i < argc; ++i) {
 		idx = if_nametoindex(argv[i]);
@@ -138,7 +149,7 @@ int main(int argc, char **argv)
 			if (err)
 				ret = err;
 		} else {
-			err = do_attach(idx, prog_fd, argv[i]);
+			err = do_attach(idx, prog_fd, map_fd, argv[i]);
 			if (err)
 				ret = err;
 		}