Socket storage can't be iterated from user space. Create a
BPF_MAP_TYPE_SK_STORAGE map, attach storage to a socket, pin it, and
check that "bpftool map dump" fails with nothing but an error: a JSON
error object with -j, no elements and no count on stdout in plain mode,
and the reason on stderr.

Assisted-by: LLM
Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
---
 .../bpf/prog_tests/bpftool_map_dump.c         | 88 +++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/bpftool_map_dump.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpftool_map_dump.c b/tools/testing/selftests/bpf/prog_tests/bpftool_map_dump.c
new file mode 100644
index 0000000000..0a0fa8926b
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/bpftool_map_dump.c
@@ -0,0 +1,88 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <sys/socket.h>
+#include <unistd.h>
+#include <bpf/btf.h>
+#include <test_progs.h>
+#include "bpftool_helpers.h"
+
+#define PIN_PATH	"/sys/fs/bpf/test_bpftool_map_dump_sk_storage"
+#define JSON_ERROR	"{\"error\":\"can't dump sk_storage map"
+
+/* A socket storage map with a single element, keyed on a socket we own. */
+static int create_sk_storage_map(void)
+{
+	LIBBPF_OPTS(bpf_map_create_opts, opts,
+		    .map_flags = BPF_F_NO_PREALLOC);
+	struct btf *btf;
+	int int_id, fd = -1;
+
+	btf = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf, "btf__new_empty"))
+		return -1;
+
+	int_id = btf__add_int(btf, "int", 4, BTF_INT_SIGNED);
+	if (!ASSERT_GT(int_id, 0, "btf__add_int"))
+		goto out;
+	if (!ASSERT_OK(btf__load_into_kernel(btf), "btf__load_into_kernel"))
+		goto out;
+
+	opts.btf_fd = btf__fd(btf);
+	opts.btf_key_type_id = int_id;
+	opts.btf_value_type_id = int_id;
+	fd = bpf_map_create(BPF_MAP_TYPE_SK_STORAGE, "sk_storage", sizeof(int),
+			    sizeof(int), 0, &opts);
+	ASSERT_OK_FD(fd, "bpf_map_create");
+out:
+	btf__free(btf);
+	return fd;
+}
+
+void test_bpftool_map_dump(void)
+{
+	int map_fd, sk_fd = -1, value = 42, ret;
+	char output[1024] = {};
+
+	map_fd = create_sk_storage_map();
+	if (map_fd < 0)
+		return;
+
+	sk_fd = socket(AF_INET, SOCK_STREAM, 0);
+	if (!ASSERT_OK_FD(sk_fd, "socket"))
+		goto out;
+	if (!ASSERT_OK(bpf_map_update_elem(map_fd, &sk_fd, &value, BPF_NOEXIST),
+		       "add socket storage"))
+		goto out;
+	if (!ASSERT_OK(bpf_obj_pin(map_fd, PIN_PATH), "pin map"))
+		goto out;
+
+	/*
+	 * Socket storage can't be iterated, so the dump must fail with an
+	 * error and nothing else, rather than print an empty map that holds
+	 * an element. The buffer is cleared before each run so that a run
+	 * that fails to start can't match stale or uninitialised output.
+	 */
+	ret = get_bpftool_command_output("-j map dump pinned " PIN_PATH,
+					 output, sizeof(output));
+	ASSERT_NEQ(ret, 0, "json dump fails");
+	ASSERT_STRNEQ(output, JSON_ERROR, strlen(JSON_ERROR),
+		      "json dump is only the error");
+
+	memset(output, 0, sizeof(output));
+	ret = get_bpftool_command_output("map dump pinned " PIN_PATH,
+					 output, sizeof(output));
+	ASSERT_NEQ(ret, 0, "plain dump fails");
+	ASSERT_NULL(strchr(output, '['), "plain dump prints no elements");
+	ASSERT_NULL(strstr(output, "Found"), "plain dump prints no count");
+
+	memset(output, 0, sizeof(output));
+	get_bpftool_command_output("map dump pinned " PIN_PATH " 2>&1 >/dev/null",
+				   output, sizeof(output));
+	ASSERT_HAS_SUBSTR(output, "can't dump sk_storage map",
+			  "plain dump explains why on stderr");
+
+	unlink(PIN_PATH);
+out:
+	if (sk_fd >= 0)
+		close(sk_fd);
+	close(map_fd);
+}
-- 
2.47.3