[PATCH bpf 2/2] selftests/bpf: Add test for key-less BTF hash map

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Create a hash and an rhash map with btf_key_type_id == 0 and expect
bpf_map_create() to fail with -EINVAL. On an unfixed kernel the map is
created; the test then pins and reads it back to walk the bpffs dump path,
which reproduces the btf_type_show() NULL-deref - so running this test on
an unfixed kernel panics it.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../bpf/prog_tests/btf_map_keyless.c          | 69 +++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_map_keyless.c

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_map_keyless.c b/tools/testing/selftests/bpf/prog_tests/btf_map_keyless.c
new file mode 100644
index 000000000000..6e1496cbd8bf
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/btf_map_keyless.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+/*
+ * A key-less BTF (btf_key_type_id == 0) is only meant for array maps. A hash
+ * map with one used to NULL-deref in btf_type_show() when dumped via bpffs.
+ * A fixed kernel rejects such a map at creation; on an unfixed kernel the
+ * pin-and-read below deliberately walks that bpffs dump path, so running this
+ * test on an unfixed kernel panics it.
+ */
+static void check_keyless(int map_type, __u32 map_flags, int btf_fd, int val_id)
+{
+	const char *path = "/sys/fs/bpf/keyless_map";
+	LIBBPF_OPTS(bpf_map_create_opts, opts);
+	__u32 key = 1, val = 0x41424344;
+	char buf[256];
+	int map_fd;
+	FILE *f;
+
+	opts.map_flags = map_flags;
+	opts.btf_fd = btf_fd;
+	opts.btf_key_type_id = 0;
+	opts.btf_value_type_id = val_id;
+	map_fd = bpf_map_create(map_type, "keyless_map", 4, 4, 8, &opts);
+
+	if (map_fd >= 0) {
+		/* Unfixed kernel: reproduce the oops via the bpffs dump path. */
+		(void)bpf_map_update_elem(map_fd, &key, &val, 0);
+		if (bpf_obj_pin(map_fd, path) == 0) {
+			f = fopen(path, "r");
+			if (f) {
+				while (fgets(buf, sizeof(buf), f))
+					;
+				fclose(f);
+			}
+			unlink(path);
+		}
+		close(map_fd);
+	}
+
+	ASSERT_EQ(map_fd, -EINVAL, "key-less create is rejected");
+}
+
+void test_btf_map_keyless(void)
+{
+	int btf_fd, val_id;
+	struct btf *btf;
+
+	btf = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf, "btf__new_empty"))
+		return;
+
+	val_id = btf__add_int(btf, "int", 4, BTF_INT_SIGNED);
+	if (!ASSERT_GT(val_id, 0, "btf__add_int"))
+		goto out;
+
+	if (!ASSERT_OK(btf__load_into_kernel(btf), "btf__load_into_kernel"))
+		goto out;
+	btf_fd = btf__fd(btf);
+
+	if (test__start_subtest("hash"))
+		check_keyless(BPF_MAP_TYPE_HASH, 0, btf_fd, val_id);
+	if (test__start_subtest("rhash"))
+		check_keyless(BPF_MAP_TYPE_RHASH, BPF_F_NO_PREALLOC, btf_fd, val_id);
+out:
+	btf__free(btf);
+}
-- 
2.43.0