[LTP] [PATCH STAGING 12/16] fchroot09: test setns escape from failfs root

[Andrea Cervesato]

From: Andrea Cervesato <andrea.cervesato@suse.com>

Verify that entering failfs with fchroot() is hard to undo: a process
inside counts as chrooted, so the remaining way out is a pre-opened
mount namespace fd. setns() into it resets both the root and the
working directory.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot09.c | 70 +++++++++++++++++++++++++++
 3 files changed, 72 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 8dfe95c4a..23f6c6a20 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -8,3 +8,4 @@ fchroot05 fchroot05
 fchroot06 fchroot06
 fchroot07 fchroot07
 fchroot08 fchroot08
+fchroot09 fchroot09
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index c099b4b3a..e803fa2b7 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -6,3 +6,4 @@ fchroot05
 fchroot06
 fchroot07
 fchroot08
+fchroot09
diff --git a/testcases/kernel/syscalls/fchroot/fchroot09.c b/testcases/kernel/syscalls/fchroot/fchroot09.c
new file mode 100644
index 000000000..196a6844d
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot09.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Test that :manpage:`setns(2)` escapes the failfs root.
+ *
+ * Entering failfs with :manpage:`fchroot(2)` is hard to undo: a process
+ * inside counts as chrooted, so :manpage:`chroot(2)` and fchroot() back
+ * out require CAP_SYS_CHROOT. The remaining way out is a pre-opened mount
+ * namespace file descriptor: setns() into it resets both the root and the
+ * working directory.
+ *
+ * Root is required because entering failfs requires CAP_SYS_CHROOT and
+ * :manpage:`setns(2)` into the mount namespace requires CAP_SYS_ADMIN.
+ *
+ * The test runs in a forked child so the root of the parent process is
+ * left untouched.
+ */
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sys/stat.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/sched.h"
+#include "lapi/setns.h"
+#include "lapi/syscalls.h"
+
+static void run(void)
+{
+	if (!SAFE_FORK()) {
+		struct stat realroot, realcwd, st;
+		int nsfd;
+
+		SAFE_STAT("/", &realroot);
+		SAFE_STAT(".", &realcwd);
+		nsfd = SAFE_OPEN("/proc/self/ns/mnt", O_RDONLY);
+
+		TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+			"fchroot() with the FD_FAILFS_ROOT sentinel");
+
+		TST_EXP_FAIL2(open("/etc", O_PATH), EOPNOTSUPP,
+			"absolute lookup after entering failfs");
+
+		TST_EXP_PASS(setns(nsfd, CLONE_NEWNS),
+			"setns() back into the mount namespace");
+
+		SAFE_CLOSE(nsfd);
+
+		SAFE_STAT("/", &st);
+		TST_EXP_EXPR(st.st_dev == realroot.st_dev &&
+			st.st_ino == realroot.st_ino,
+			"root restored after setns()");
+
+		SAFE_STAT(".", &st);
+		TST_EXP_EXPR(st.st_dev == realcwd.st_dev &&
+			st.st_ino == realcwd.st_ino,
+			"working directory restored after setns()");
+
+		exit(0);
+	}
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.needs_root = 1,
+	.forks_child = 1,
+};

-- 
2.51.0