tools: bpftool: warn about risky prog array updates

When prog array is updated with bpftool users often refer
to the map via the ID.  Unfortunately, that's likely
to lead to confusion because prog arrays get flushed when
the last user reference is gone.  If there is no other
reference bpftool will create one, update successfully
just to close the map again and have it flushed.

Warn about this case in non-JSON mode.

If the problem continues causing confusion we can remove
the support for referring to a map by ID for prog array
update completely.  For now it seems like the potential
inconvenience to users who know what they're doing outweighs
the benefit.
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/map.c

@@ -426,6 +426,9 @@ static int parse_elem(char **argv, struct bpf_map_info *info,
 				p_err("not enough value arguments for map of progs");
 				return -1;
 			}
+			if (is_prefix(*argv, "id"))
+				p_info("Warning: updating program array via MAP_ID, make sure this map is kept open\n"
+				       "         by some process or pinned otherwise update will be lost");
 
 			fd = prog_parse_fd(&argc, &argv);
 			if (fd < 0)