tools/bpftool: Add bpftool support for bpf map element iterator

The optional parameter "map MAP" can be added to "bpftool iter"
command to create a bpf iterator for map elements. For example,
bpftool iter pin ./prog.o /sys/fs/bpf/p1 map id 333

For map element bpf iterator "map MAP" parameter is required.
Otherwise, bpf link creation will return an error.

Quentin Monnet kindly provided bash-completion implementation
for new "map MAP" option.

Signed-off-by: Yonghong Song <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpftool/Documentation/bpftool-iter.rst

···
 ITER COMMANDS
 ===================
 
-|	**bpftool** **iter pin** *OBJ* *PATH*
+|	**bpftool** **iter pin** *OBJ* *PATH* [**map** *MAP*]
 |	**bpftool** **iter help**
 |
 |	*OBJ* := /a/file/of/bpf_iter_target.o
+|	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
 
 DESCRIPTION
 ===========
-	**bpftool iter pin** *OBJ* *PATH*
+	**bpftool iter pin** *OBJ* *PATH* [**map** *MAP*]
 		  A bpf iterator combines a kernel iterating of
 		  particular kernel data (e.g., tasks, bpf_maps, etc.)
 		  and a bpf program called for each kernel data object
···
 		  in *bpffs* mount. It must not contain a dot
 		  character ('.'), which is reserved for future extensions
 		  of *bpffs*.
+
+		  Map element bpf iterator requires an additional parameter
+		  *MAP* so bpf program can iterate over map elements for
+		  that map. User can have a bpf program in kernel to run
+		  with each map element, do checking, filtering, aggregation,
+		  etc. without copying data to user space.
 
 		  User can then *cat PATH* to see the bpf iterator output.
 
···
 
    Create a file-based bpf iterator from bpf_iter_netlink.o and pin it
    to /sys/fs/bpf/my_netlink
+
+**# bpftool iter pin bpf_iter_hashmap.o /sys/fs/bpf/my_hashmap map id 20**
+
+::
+
+   Create a file-based bpf iterator from bpf_iter_hashmap.o and map with
+   id 20, and pin it to /sys/fs/bpf/my_hashmap
 
 SEE ALSO
 ========

tools/bpf/bpftool/bash-completion/bpftool

···
         iter)
             case $command in
                 pin)
-                    _filedir
+                    case $prev in
+                        $command)
+                            _filedir
+                            ;;
+                        id)
+                            _bpftool_get_map_ids
+                            ;;
+                        name)
+                            _bpftool_get_map_names
+                            ;;
+                        pinned)
+                            _filedir
+                            ;;
+                        *)
+                            _bpftool_one_of_list $MAP_TYPE
+                            ;;
+                    esac
                     return 0
                     ;;
                 *)

tools/bpf/bpftool/iter.c

···
 // Copyright (C) 2020 Facebook
 
 #define _GNU_SOURCE
+#include <unistd.h>
 #include <linux/err.h>
 #include <bpf/libbpf.h>
 
···
 
 static int do_pin(int argc, char **argv)
 {
+	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, iter_opts);
 	const char *objfile, *path;
 	struct bpf_program *prog;
 	struct bpf_object *obj;
 	struct bpf_link *link;
-	int err;
+	int err = -1, map_fd = -1;
 
 	if (!REQ_ARGS(2))
 		usage();
···
 	objfile = GET_ARG();
 	path = GET_ARG();
 
+	/* optional arguments */
+	if (argc) {
+		if (is_prefix(*argv, "map")) {
+			NEXT_ARG();
+
+			if (!REQ_ARGS(2)) {
+				p_err("incorrect map spec");
+				return -1;
+			}
+
+			map_fd = map_parse_fd(&argc, &argv);
+			if (map_fd < 0)
+				return -1;
+		}
+	}
+
 	obj = bpf_object__open(objfile);
 	if (IS_ERR(obj)) {
 		p_err("can't open objfile %s", objfile);
-		return -1;
+		goto close_map_fd;
 	}
 
 	err = bpf_object__load(obj);
···
 		goto close_obj;
 	}
 
-	link = bpf_program__attach_iter(prog, NULL);
+	if (map_fd >= 0)
+		iter_opts.map_fd = map_fd;
+
+	link = bpf_program__attach_iter(prog, &iter_opts);
 	if (IS_ERR(link)) {
 		err = PTR_ERR(link);
 		p_err("attach_iter failed for program %s",
···
 	bpf_link__destroy(link);
 close_obj:
 	bpf_object__close(obj);
+close_map_fd:
+	if (map_fd >= 0)
+		close(map_fd);
 	return err;
 }
 
 static int do_help(int argc, char **argv)
 {
 	fprintf(stderr,
-		"Usage: %1$s %2$s pin OBJ PATH\n"
+		"Usage: %1$s %2$s pin OBJ PATH [map MAP]\n"
 		"       %1$s %2$s help\n"
+		"       " HELP_SPEC_MAP "\n"
 		"",
 		bin_name, "iter");