"bpftool map dump" walks the map with bpf_map_get_next_key() and treats
ENOENT as the end of the map. Map types that can't be iterated fail that
call on the very first key: local storage and ringbuf with ENOTSUPP,
bloom filter and arena with EOPNOTSUPP, queue and stack with EINVAL.
bpftool then ends the walk without a word and prints what it prints for
an empty map, "[]" with BTF or "Found 0 elements" without, and only the
exit status tells that the map was not read. On a socket storage map
that holds storage for a socket:

  # bpftool map dump pinned /sys/fs/bpf/sk_storage
  []

Ask for the first key before printing anything, and when that fails with
anything but ENOENT, fail with the map type and the error instead of a
dump:

  # bpftool map dump pinned /sys/fs/bpf/sk_storage
  Error: can't dump sk_storage map: Unknown error 524
  # bpftool -j map dump pinned /sys/fs/bpf/sk_storage
  {"error":"can't dump sk_storage map: Unknown error 524"}

An error later in the walk is now reported too, as "bpftool map getnext"
reports it, and the element count is printed only after a complete walk.

Assisted-by: LLM
Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
---
 tools/bpf/bpftool/map.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/map.c b/tools/bpf/bpftool/map.c
index 20d59eab09a1..b7f62fcbb6fc 100644
--- a/tools/bpf/bpftool/map.c
+++ b/tools/bpf/bpftool/map.c
@@ -856,6 +856,22 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
 		}
 	}
 
+	/* Fail early to avoid an empty dump if map type cannot be iterated */
+	if (bpf_map_get_next_key(fd, NULL, key) && errno != ENOENT) {
+		const char *map_type_str;
+		int saved_errno = errno;
+
+		map_type_str = libbpf_bpf_map_type_str(info->type);
+		if (map_type_str)
+			p_err("can't dump %s map: %s", map_type_str,
+			      strerror(saved_errno));
+		else
+			p_err("can't dump map of type %u: %s", info->type,
+			      strerror(saved_errno));
+		err = -1;
+		goto exit_free;
+	}
+
 	if (wtr) {
 		err = get_map_kv_btf(info, &btf);
 		if (err) {
@@ -883,8 +899,11 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
 	while (true) {
 		err = bpf_map_get_next_key(fd, prev_key, key);
 		if (err) {
-			if (errno == ENOENT)
+			if (errno == ENOENT) {
 				err = 0;
+				break;
+			}
+			p_err("can't get next key: %s", strerror(errno));
 			break;
 		}
 		if (!dump_map_elem(fd, key, value, info, btf, wtr,
@@ -897,7 +916,7 @@ map_dump(int fd, struct bpf_map_info *info, json_writer_t *wtr,
 		jsonw_end_array(wtr);	/* elements */
 		if (show_header)
 			jsonw_end_object(wtr);	/* map object */
-	} else {
+	} else if (!err) {
 		printf("Found %u element%s\n", num_elems,
 		       num_elems != 1 ? "s" : "");
 	}
-- 
2.47.3