[PATCH STAGING v2 12/16] fchroot09: test setns escape from failfs root

From: Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>

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-IBi9RG/[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot09.c | 87 +++++++++++++++++++++++++++
 3 files changed, 89 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..adcc47352
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot09.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
+ */
+
+/*\
+ * 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"
+
+#define ABSDIR "abs"
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	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(ABSDIR, 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(tst_tmpdir_path(), &st);
+	TST_EXP_EXPR(st.st_dev == realcwd.st_dev &&
+		st.st_ino == realcwd.st_ino,
+		"working directory restored after setns()");
+
+	exit(0);
+}
+
+static void setup(void)
+{
+	char *tmpdir;
+	char abs_path[PATH_MAX];
+
+	tmpdir = tst_tmpdir_path();
+	snprintf(abs_path, sizeof(abs_path), "%s/%s", tmpdir, "target");
+
+	SAFE_TOUCH("target", 0644, NULL);
+	SAFE_SYMLINK(abs_path, ABSDIR);
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.setup = setup,
+	.needs_root = 1,
+	.forks_child = 1,
+	.needs_tmpdir = 1,
+};

-- 
2.51.0


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