[PATCH STAGING v2 07/16] fchroot04: test fchroot() into failfs as root

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

Verify that entering failfs with the FD_FAILFS_ROOT sentinel succeeds
with CAP_SYS_CHROOT and that every absolute path lookup then fails
with EOPNOTSUPP, while the working directory is reported as
unreachable by getcwd().

Signed-off-by: Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot04.c | 70 +++++++++++++++++++++++++++
 3 files changed, 72 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 13635037b..91dfdba08 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -3,3 +3,4 @@
 fchroot01 fchroot01
 fchroot02 fchroot02
 fchroot03 fchroot03
+fchroot04 fchroot04
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index 235befd99..9270c1408 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -1,3 +1,4 @@
 fchroot01
 fchroot02
 fchroot03
+fchroot04
diff --git a/testcases/kernel/syscalls/fchroot/fchroot04.c b/testcases/kernel/syscalls/fchroot/fchroot04.c
new file mode 100644
index 000000000..33097d8fd
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot04.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
+ */
+
+/*\
+ * Test :manpage:`fchroot(2)` with the ``FD_FAILFS_ROOT`` sentinel as a
+ * privileged process.
+ *
+ * :manpage:`fchroot(2)` was introduced in Linux v7.3 together with failfs,
+ * a kernel internal filesystem where every operation fails with
+ * ``EOPNOTSUPP``. The ``FD_FAILFS_ROOT`` sentinel moves the process root
+ * there without needing a file descriptor: it is the ``fs_struct`` equivalent
+ * of ``RESOLVE_BENEATH``.
+ *
+ * The test verifies that entering failfs succeeds with ``CAP_SYS_CHROOT`` and
+ * that every absolute path lookup then fails with ``EOPNOTSUPP``. The working
+ * directory, left behind in the real filesystem, is consequently reported
+ * as unreachable by :manpage:`getcwd(2)`.
+ *
+ * The syscall runs in a forked child because leaving failfs requires a
+ * mount namespace file descriptor and the parent needs its root for the
+ * test framework cleanup.
+ */
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <limits.h>
+#include <sys/stat.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+#define UNR_TAG "(unreachable)/"
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	char buf[PATH_MAX];
+
+	TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+		"fchroot() with the FD_FAILFS_ROOT sentinel");
+
+	TST_EXP_FAIL2(open("/etc/passwd", O_RDONLY), EOPNOTSUPP,
+		"absolute file open");
+
+	TST_EXP_FAIL(mkdir("/foo", 0700), EOPNOTSUPP,
+		"absolute directory creation");
+
+	/*
+	 * The libc getcwd() wrapper rejects the "(unreachable)"
+	 * prefix produced by the kernel, so call the raw syscall.
+	 */
+	TEST(tst_syscall(__NR_getcwd, buf, sizeof(buf)));
+	if (TST_RET > 0)
+		TST_EXP_EQ_STRN(buf, UNR_TAG, sizeof(UNR_TAG) - 1);
+	else
+		tst_res(TFAIL | TTERRNO, "getcwd() failed");
+
+	exit(0);
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.needs_root = 1,
+	.needs_tmpdir = 1,
+	.forks_child = 1,
+};

-- 
2.51.0


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