[LTP] [PATCH STAGING v2 11/16] fchroot08: test failfs root fork inheritance

From: Andrea Cervesato <[email protected]>

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 <[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot08.c | 69 +++++++++++++++++++++++++++
 3 files changed, 71 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index b8954bb5a..8dfe95c4a 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -7,3 +7,4 @@ fchroot04 fchroot04
 fchroot05 fchroot05
 fchroot06 fchroot06
 fchroot07 fchroot07
+fchroot08 fchroot08
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index b68069d9a..c099b4b3a 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -5,3 +5,4 @@ fchroot04
 fchroot05
 fchroot06
 fchroot07
+fchroot08
diff --git a/testcases/kernel/syscalls/fchroot/fchroot08.c b/testcases/kernel/syscalls/fchroot/fchroot08.c
new file mode 100644
index 000000000..3efe8d85f
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot08.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <[email protected]>
+ */
+
+/*\
+ * 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 fork, 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"
+
+#define ABSDIR "abs"
+
+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(ABSDIR, O_PATH), EOPNOTSUPP,
+			"absolute lookup in a forked child");
+		exit(0);
+	}
+
+	SAFE_WAITPID(pid, NULL, 0);
+
+	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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