[LTP] [PATCH STAGING 09/16] fchroot06: test path walks under failfs root

[Andrea Cervesato]

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

Verify that once the root is in failfs only lookups anchored at a
file descriptor keep working: relative lookups from the working
directory and from a pre-opened directory fd succeed, absolute
symlinks fail with EOPNOTSUPP and ".." walks clamp at the top of the
mount tree, landing on the real root.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot06.c | 90 +++++++++++++++++++++++++++
 3 files changed, 92 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 49f51d637..8b6b1ecd9 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -5,3 +5,4 @@ fchroot02 fchroot02
 fchroot03 fchroot03
 fchroot04 fchroot04
 fchroot05 fchroot05
+fchroot06 fchroot06
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index 0697f10eb..12151270a 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -3,3 +3,4 @@ fchroot02
 fchroot03
 fchroot04
 fchroot05
+fchroot06
diff --git a/testcases/kernel/syscalls/fchroot/fchroot06.c b/testcases/kernel/syscalls/fchroot/fchroot06.c
new file mode 100644
index 000000000..a1293b932
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot06.c
@@ -0,0 +1,90 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Test path walks under the failfs root.
+ *
+ * Once :manpage:`fchroot(2)` moved the process root into failfs, only
+ * lookups anchored at a file descriptor keep working:
+ *
+ * - lookups relative to the working directory, which stays in the real
+ *   filesystem, keep working
+ * - lookups anchored at a pre-opened directory fd keep working, including
+ *   resolution of relative symlinks
+ * - absolute symlinks restart the walk at the failfs root and fail with
+ *   EOPNOTSUPP
+ * - ".." walks clamp at the top of the mount tree, not at the failfs
+ *   root, so walking up from the working directory lands on the real
+ *   root
+ *
+ * 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/stat.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+#include "tst_safe_file_at.h"
+
+#define UPWARDS "../../../../../../../../../.."
+
+static void run(void)
+{
+	if (!SAFE_FORK()) {
+		struct stat realroot, st;
+		int dfd, fd;
+
+		SAFE_STAT("/", &realroot);
+		dfd = SAFE_OPEN(".", O_RDONLY | O_DIRECTORY);
+
+		TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+			"fchroot() with the FD_FAILFS_ROOT sentinel");
+
+		fd = SAFE_OPENAT(AT_FDCWD, ".", O_RDONLY | O_DIRECTORY);
+		SAFE_CLOSE(fd);
+
+		fd = SAFE_OPENAT(dfd, "canary", O_WRONLY | O_CREAT, 0600);
+		SAFE_WRITE(SAFE_WRITE_ALL, fd, "x", 1);
+		SAFE_CLOSE(fd);
+
+		fd = SAFE_OPENAT(dfd, "rel", O_RDONLY);
+		SAFE_CLOSE(fd);
+
+		TST_EXP_FAIL2(openat(dfd, "abs", O_RDONLY), EOPNOTSUPP,
+			"resolution of an absolute symlink");
+
+		fd = SAFE_OPENAT(AT_FDCWD, UPWARDS, O_PATH);
+		SAFE_FSTAT(fd, &st);
+		TST_EXP_EXPR(st.st_dev == realroot.st_dev &&
+			st.st_ino == realroot.st_ino,
+			"'..' walk clamps at the top of the mount tree");
+		SAFE_CLOSE(fd);
+
+		SAFE_CLOSE(dfd);
+
+		exit(0);
+	}
+}
+
+static void setup(void)
+{
+	SAFE_TOUCH("target", 0644, NULL);
+	SAFE_SYMLINK("target", "rel");
+	SAFE_SYMLINK("/etc", "abs");
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.test_all = run,
+	.needs_root = 1,
+	.needs_tmpdir = 1,
+	.forks_child = 1,
+};

-- 
2.51.0