bpftool: add peek command

This is intended to be used with queues and stacks and be more
user-friendly than 'lookup' without key/value.

Example:
bpftool map create /sys/fs/bpf/q type queue value 4 entries 10 name q
bpftool map update pinned /sys/fs/bpf/q value 0 1 2 3
bpftool map peek pinned /sys/fs/bpf/q
value: 00 01 02 03
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

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

@@ -31,6 +31,7 @@ MAP COMMANDS
 |	**bpftool** **map delete**     *MAP*  **key** *DATA*
 |	**bpftool** **map pin**        *MAP*  *FILE*
 |	**bpftool** **map event_pipe** *MAP* [**cpu** *N* **index** *M*]
+|	**bpftool** **map peek**       *MAP*
 |	**bpftool** **map help**
 |
 |	*MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
@@ -107,6 +108,9 @@ DESCRIPTION
 		  replace any existing ring.  Any other application will stop
 		  receiving events if it installed its rings earlier.
 
+	**bpftool map peek**  *MAP*
+		  Peek next **value** in the queue or stack.
+
 	**bpftool map help**
 		  Print short help message.
 

tools/bpf/bpftool/map.c

@@ -1168,6 +1168,7 @@ static int do_help(int argc, char **argv)
 		"       %s %s delete     MAP  key DATA\n"
 		"       %s %s pin        MAP  FILE\n"
 		"       %s %s event_pipe MAP [cpu N index M]\n"
+		"       %s %s peek       MAP\n"
 		"       %s %s help\n"
 		"\n"
 		"       " HELP_SPEC_MAP "\n"
@@ -1185,7 +1186,7 @@ static int do_help(int argc, char **argv)
 		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2],
 		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2],
 		bin_name, argv[-2], bin_name, argv[-2], bin_name, argv[-2],
-		bin_name, argv[-2]);
+		bin_name, argv[-2], bin_name, argv[-2]);
 
 	return 0;
 }
@@ -1202,6 +1203,7 @@ static const struct cmd cmds[] = {
 	{ "pin",	do_pin },
 	{ "event_pipe",	do_event_pipe },
 	{ "create",	do_create },
+	{ "peek",	do_lookup },
 	{ 0 }
 };