tools: bpftool: protect against races with disappearing objects

On program/map show we may get an ID of an object from GETNEXT,
but the object may disappear before we call GET_FD_BY_ID.  If
that happens, ignore the object and continue.

Fixes: 71bb428f ("tools: bpf: add bpftool")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/map.c

@@ -528,6 +528,8 @@ static int do_show(int argc, char **argv)
 
 		fd = bpf_map_get_fd_by_id(id);
 		if (fd < 0) {
+			if (errno == ENOENT)
+				continue;
 			p_err("can't get map by id (%u): %s",
 			      id, strerror(errno));
 			break;

tools/bpf/bpftool/prog.c

@@ -382,6 +382,8 @@ static int do_show(int argc, char **argv)
 
 		fd = bpf_prog_get_fd_by_id(id);
 		if (fd < 0) {
+			if (errno == ENOENT)
+				continue;
 			p_err("can't get prog by id (%u): %s",
 			      id, strerror(errno));
 			err = -1;