bpftool: Register struct_ops with a link.

You can include an optional path after specifying the object name for the
'struct_ops register' subcommand.

Since the commit 226bc6ae6405 ("Merge branch 'Transit between BPF TCP
congestion controls.'") has been accepted, it is now possible to create a
link for a struct_ops. This can be done by defining a struct_ops in
SEC(".struct_ops.link") to make libbpf returns a real link. If we don't pin
the links before leaving bpftool, they will disappear. To instruct bpftool
to pin the links in a directory with the names of the maps, we need to
provide the path of that directory.

Signed-off-by: Kui-Feng Lee <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/bpf/bpftool/common.c

···
 	default:	return libbpf_bpf_attach_type_str(t);
 	}
 }
+
+int pathname_concat(char *buf, int buf_sz, const char *path,
+		    const char *name)
+{
+	int len;
+
+	len = snprintf(buf, buf_sz, "%s/%s", path, name);
+	if (len < 0)
+		return -EINVAL;
+	if (len >= buf_sz)
+		return -ENAMETOOLONG;
+
+	return 0;
+}

tools/bpf/bpftool/main.h

···
 	return map ? hashmap__size(map) == 0 : true;
 }
 
+int pathname_concat(char *buf, int buf_sz, const char *path,
+		    const char *name);
+
 #endif

tools/bpf/bpftool/prog.c

···
 	return err;
 }
 
-static int pathname_concat(char *buf, size_t buf_sz, const char *path, const char *name)
-{
-	int len;
-
-	len = snprintf(buf, buf_sz, "%s/%s", path, name);
-	if (len < 0)
-		return -EINVAL;
-	if ((size_t)len >= buf_sz)
-		return -ENAMETOOLONG;
-
-	return 0;
-}
-
 static int
 auto_attach_programs(struct bpf_object *obj, const char *path)
 {

tools/bpf/bpftool/struct_ops.c

···
 	return cmd_retval(&res, true);
 }
 
+static int pin_link(struct bpf_link *link, const char *pindir,
+		    const char *name)
+{
+	char pinfile[PATH_MAX];
+	int err;
+
+	err = pathname_concat(pinfile, sizeof(pinfile), pindir, name);
+	if (err)
+		return -1;
+
+	return bpf_link__pin(link, pinfile);
+}
+
 static int do_register(int argc, char **argv)
 {
 	LIBBPF_OPTS(bpf_object_open_opts, open_opts);
+	__u32 link_info_len = sizeof(struct bpf_link_info);
+	struct bpf_link_info link_info = {};
 	struct bpf_map_info info = {};
 	__u32 info_len = sizeof(info);
 	int nr_errs = 0, nr_maps = 0;
+	const char *linkdir = NULL;
 	struct bpf_object *obj;
 	struct bpf_link *link;
 	struct bpf_map *map;
 	const char *file;
 
-	if (argc != 1)
+	if (argc != 1 && argc != 2)
 		usage();
 
 	file = GET_ARG();
+	if (argc == 1)
+		linkdir = GET_ARG();
+
+	if (linkdir && mount_bpffs_for_pin(linkdir)) {
+		p_err("can't mount bpffs for pinning");
+		return -1;
+	}
 
 	if (verifier_logs)
 		/* log_level1 + log_level2 + stats, but not stable UAPI */
···
 		}
 		nr_maps++;
 
-		bpf_link__disconnect(link);
-		bpf_link__destroy(link);
-
-		if (!bpf_map_get_info_by_fd(bpf_map__fd(map), &info,
-					    &info_len))
-			p_info("Registered %s %s id %u",
-			       get_kern_struct_ops_name(&info),
-			       bpf_map__name(map),
-			       info.id);
-		else
+		if (bpf_map_get_info_by_fd(bpf_map__fd(map), &info,
+					   &info_len)) {
 			/* Not p_err.  The struct_ops was attached
 			 * successfully.
 			 */
 			p_info("Registered %s but can't find id: %s",
 			       bpf_map__name(map), strerror(errno));
+			goto clean_link;
+		}
+		if (!(bpf_map__map_flags(map) & BPF_F_LINK)) {
+			p_info("Registered %s %s id %u",
+			       get_kern_struct_ops_name(&info),
+			       info.name,
+			       info.id);
+			goto clean_link;
+		}
+		if (bpf_link_get_info_by_fd(bpf_link__fd(link),
+					    &link_info,
+					    &link_info_len)) {
+			p_err("Registered %s but can't find link id: %s",
+			      bpf_map__name(map), strerror(errno));
+			nr_errs++;
+			goto clean_link;
+		}
+		if (linkdir && pin_link(link, linkdir, info.name)) {
+			p_err("can't pin link %u for %s: %s",
+			      link_info.id, info.name,
+			      strerror(errno));
+			nr_errs++;
+			goto clean_link;
+		}
+		p_info("Registered %s %s map id %u link id %u",
+		       get_kern_struct_ops_name(&info),
+		       info.name, info.id, link_info.id);
+
+clean_link:
+		bpf_link__disconnect(link);
+		bpf_link__destroy(link);
 	}
 
 	bpf_object__close(obj);
···
 	fprintf(stderr,
 		"Usage: %1$s %2$s { show | list } [STRUCT_OPS_MAP]\n"
 		"       %1$s %2$s dump [STRUCT_OPS_MAP]\n"
-		"       %1$s %2$s register OBJ\n"
+		"       %1$s %2$s register OBJ [LINK_DIR]\n"
 		"       %1$s %2$s unregister STRUCT_OPS_MAP\n"
 		"       %1$s %2$s help\n"
 		"\n"