[LTP] [PATCH v5 09/14] fchroot08: test failfs root fork inheritance

[Andrea Cervesato]

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

Verify that the root moved into failfs by fchroot() lives in the
fs_struct which is duplicated on fork, so a child of a process with
the failfs root also fails every absolute path lookup with
EOPNOTSUPP.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
 runtest/syscalls                              |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot08.c | 52 +++++++++++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/runtest/syscalls b/runtest/syscalls
index bc2dffdca..24d9b1b4a 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -323,6 +323,7 @@ fchroot04 fchroot04
 fchroot05 fchroot05
 fchroot06 fchroot06
 fchroot07 fchroot07
+fchroot08 fchroot08
 
 fcntl01 fcntl01
 fcntl01_64 fcntl01_64
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index bfa2a8ce1..0a2510d8a 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -6,3 +6,4 @@ fchroot05
 fchroot06
 fchroot07
 fchroot07_child
+fchroot08
diff --git a/testcases/kernel/syscalls/fchroot/fchroot08.c b/testcases/kernel/syscalls/fchroot/fchroot08.c
new file mode 100644
index 000000000..1b20e3162
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot08.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Test that the failfs root is inherited across :manpage:`fork(2)`.
+ *
+ * The root moved into failfs by :manpage:`fchroot(2)` lives in the
+ * fs_struct which is duplicated on :manpage:`fork(2)`, so a child of a
+ * process with the failfs root also fails every absolute path lookup with
+ * ``EOPNOTSUPP``.
+ *
+ * Root is required because entering failfs with the ``FD_FAILFS_ROOT``
+ * sentinel requires ``CAP_SYS_CHROOT``.
+ *
+ * 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/wait.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	pid_t pid;
+
+	TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+		"fchroot() with the FD_FAILFS_ROOT sentinel");
+
+	pid = SAFE_FORK();
+	if (!pid) {
+		TST_EXP_FAIL2(open("/", O_PATH), EOPNOTSUPP,
+			"absolute lookup in a forked child");
+		return;
+	}
+
+	SAFE_WAITPID(pid, NULL, 0);
+}
+
+static struct tst_test test = {
+	.test_all = run,
+	.needs_root = 1,
+	.forks_child = 1,
+};

-- 
2.51.0