bpftool: Print ENOTSUPP as "Operation not supported"

Some BPF commands return ENOTSUPP (524) to user space: iterating local
storage maps, element operations on ringbuf maps, creating an LRU hash
map without preallocation, freezing a struct_ops map, or running a
program type that has no test_run. ENOTSUPP is kernel internal, so
strerror() only gives "Unknown error 524":

  # bpftool map getnext pinned /sys/fs/bpf/sk_storage
  Error: can't get next key: Unknown error 524

Add a small helper that knows ENOTSUPP and use it where bpftool prints
the error of map create, map element operations, map freeze and prog
run.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://patch.msgid.link/20261016194825.230433-2-skokovmaksimevg@gmail.com

tools/bpf/bpftool/common.c

@@ -72,6 +72,17 @@ void p_info(const char *fmt, ...)
 	va_end(ap);
 }
 
+/*
+ * Like strerror(), but also knows ENOTSUPP (524), which the kernel returns
+ * to user space from some BPF commands although it is not a userspace errno.
+ */
+const char *bpf_strerror(int err)
+{
+	if (err == 524 /* ENOTSUPP */)
+		return "Operation not supported";
+	return strerror(err);
+}
+
 static bool is_bpffs(const char *path)
 {
 	struct statfs st_fs;

tools/bpf/bpftool/main.h

@@ -98,6 +98,7 @@ extern const char *cert_path;
 
 void __printf(1, 2) p_err(const char *fmt, ...);
 void __printf(1, 2) p_info(const char *fmt, ...);
+const char *bpf_strerror(int err);
 
 bool is_prefix(const char *pfx, const char *str);
 int detect_common_prefix(const char *arg, ...);

tools/bpf/bpftool/map.c

@@ -228,7 +228,7 @@ print_entry_error(struct bpf_map_info *map_info, void *key, int lookup_errno)
 		print_hex_data_json(key, map_info->key_size);
 		jsonw_name(json_wtr, "value");
 		jsonw_start_object(json_wtr);	/* error */
-		jsonw_string_field(json_wtr, "error", strerror(lookup_errno));
+		jsonw_string_field(json_wtr, "error", bpf_strerror(lookup_errno));
 		jsonw_end_object(json_wtr);	/* error */
 		jsonw_end_object(json_wtr);	/* entry */
 	} else {
@@ -241,7 +241,7 @@ print_entry_error(struct bpf_map_info *map_info, void *key, int lookup_errno)
 			msg = "<cannot read>";
 
 		print_entry_error_msg(map_info, key,
-				      msg ? : strerror(lookup_errno));
+				      msg ? : bpf_strerror(lookup_errno));
 	}
 }
 
@@ -1032,7 +1032,7 @@ static int do_update(int argc, char **argv)
 
 	err = bpf_map_update_elem(fd, key, value, flags);
 	if (err) {
-		p_err("update failed: %s", strerror(errno));
+		p_err("update failed: %s", bpf_strerror(errno));
 		goto exit_free;
 	}
 
@@ -1136,7 +1136,7 @@ static int do_lookup(int argc, char **argv)
 				printf("\n\nNot found\n");
 			}
 		} else {
-			p_err("lookup failed: %s", strerror(errno));
+			p_err("lookup failed: %s", bpf_strerror(errno));
 		}
 
 		goto exit_free;
@@ -1188,7 +1188,7 @@ static int do_getnext(int argc, char **argv)
 
 	err = bpf_map_get_next_key(fd, key, nextkey);
 	if (err) {
-		p_err("can't get next key: %s", strerror(errno));
+		p_err("can't get next key: %s", bpf_strerror(errno));
 		goto exit_free;
 	}
 
@@ -1253,7 +1253,7 @@ static int do_delete(int argc, char **argv)
 
 	err = bpf_map_delete_elem(fd, key);
 	if (err)
-		p_err("delete failed: %s", strerror(errno));
+		p_err("delete failed: %s", bpf_strerror(errno));
 
 exit_free:
 	free(key);
@@ -1381,7 +1381,7 @@ static int do_create(int argc, char **argv)
 
 	fd = bpf_map_create(map_type, map_name, key_size, value_size, max_entries, &attr);
 	if (fd < 0) {
-		p_err("map create failed: %s", strerror(errno));
+		p_err("map create failed: %s", bpf_strerror(errno));
 		goto exit;
 	}
 
@@ -1427,7 +1427,7 @@ static int do_pop_dequeue(int argc, char **argv)
 			else
 				printf("Error: empty map\n");
 		} else {
-			p_err("pop failed: %s", strerror(errno));
+			p_err("pop failed: %s", bpf_strerror(errno));
 		}
 
 		goto exit_free;
@@ -1462,7 +1462,7 @@ static int do_freeze(int argc, char **argv)
 	err = bpf_map_freeze(fd);
 	close(fd);
 	if (err) {
-		p_err("failed to freeze map: %s", strerror(errno));
+		p_err("failed to freeze map: %s", bpf_strerror(errno));
 		return err;
 	}
 

tools/bpf/bpftool/prog.c

@@ -1499,7 +1499,7 @@ static int do_run(int argc, char **argv)
 
 	err = bpf_prog_test_run_opts(fd, &test_attr);
 	if (err) {
-		p_err("failed to run program: %s", strerror(errno));
+		p_err("failed to run program: %s", bpf_strerror(errno));
 		goto free_ctx_out;
 	}