selftests/landlock: Add layout1.refer_mount_root

Add tests to check error codes when linking or renaming a mount root
directory.  This previously triggered a kernel warning, but it is fixed
with the previous commit.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20240516181935.1645983-3-mic@digikod.netSigned-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/fs_test.c

@@ -35,6 +35,7 @@
  * See https://sourceware.org/glibc/wiki/Synchronizing_Headers.
  */
 #include <linux/fs.h>
+#include <linux/mount.h>
 
 #include "common.h"
 
@@ -47,6 +48,13 @@ int renameat2(int olddirfd, const char *oldpath, int newdirfd,
 }
 #endif
 
+#ifndef open_tree
+int open_tree(int dfd, const char *filename, unsigned int flags)
+{
+	return syscall(__NR_open_tree, dfd, filename, flags);
+}
+#endif
+
 #ifndef RENAME_EXCHANGE
 #define RENAME_EXCHANGE (1 << 1)
 #endif
@@ -2400,6 +2408,43 @@ TEST_F_FORK(layout1, refer_denied_by_default4)
 				layer_dir_s1d1_refer);
 }
 
+/*
+ * Tests walking through a denied root mount.
+ */
+TEST_F_FORK(layout1, refer_mount_root_deny)
+{
+	const struct landlock_ruleset_attr ruleset_attr = {
+		.handled_access_fs = LANDLOCK_ACCESS_FS_MAKE_DIR,
+	};
+	int root_fd, ruleset_fd;
+
+	/* Creates a mount object from a non-mount point. */
+	set_cap(_metadata, CAP_SYS_ADMIN);
+	root_fd =
+		open_tree(AT_FDCWD, dir_s1d1,
+			  AT_EMPTY_PATH | OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC);
+	clear_cap(_metadata, CAP_SYS_ADMIN);
+	ASSERT_LE(0, root_fd);
+
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	ASSERT_LE(0, ruleset_fd);
+
+	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
+	ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+	EXPECT_EQ(0, close(ruleset_fd));
+
+	/* Link denied by Landlock: EACCES. */
+	EXPECT_EQ(-1, linkat(root_fd, ".", root_fd, "does_not_exist", 0));
+	EXPECT_EQ(EACCES, errno);
+
+	/* renameat2() always returns EBUSY. */
+	EXPECT_EQ(-1, renameat2(root_fd, ".", root_fd, "does_not_exist", 0));
+	EXPECT_EQ(EBUSY, errno);
+
+	EXPECT_EQ(0, close(root_fd));
+}
+
 TEST_F_FORK(layout1, reparent_link)
 {
 	const struct rule layer1[] = {