selftests/landlock: Add test for invalid ruleset file descriptor

To align with fs_test's layout1.inval and layout0.proc_nsfs which test
EBADFD for landlock_add_rule(2), create a new base_test's
restrict_self_fd which test EBADFD for landlock_restrict_self(2).

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20250320190717.2287696-21-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

tools/testing/selftests/landlock/base_test.c

@@ -277,6 +277,17 @@ TEST(restrict_self_checks_ordering)
 	ASSERT_EQ(0, close(ruleset_fd));
 }
 
+TEST(restrict_self_fd)
+{
+	int fd;
+
+	fd = open("/dev/null", O_RDONLY | O_CLOEXEC);
+	ASSERT_LE(0, fd);
+
+	EXPECT_EQ(-1, landlock_restrict_self(fd, 0));
+	EXPECT_EQ(EBADFD, errno);
+}
+
 TEST(ruleset_fd_io)
 {
 	struct landlock_ruleset_attr ruleset_attr = {