[LTP] [PATCH v5 07/14] fchroot06: test path walks under failfs root

[Andrea Cervesato <andrea.cervesato@suse.de>]

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/syscalls                              |   1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |   1 +
 testcases/kernel/syscalls/fchroot/fchroot06.c | 124 ++++++++++++++++++++++++++
 3 files changed, 126 insertions(+)

diff --git a/runtest/syscalls b/runtest/syscalls
index a5ed6792e..8b688cbe2 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -321,6 +321,7 @@ fchroot02 fchroot02
 fchroot03 fchroot03
 fchroot04 fchroot04
 fchroot05 fchroot05
+fchroot06 fchroot06
 
 fcntl01 fcntl01
 fcntl01_64 fcntl01_64
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..d7d7dfc50
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot06.c
@@ -0,0 +1,124 @@
+// 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 <string.h>
+#include <sys/stat.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+#define RELDIR "rel"
+#define ABSDIR "abs"
+
+static char upwards[PATH_MAX];
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	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 = TST_EXP_FD(openat(AT_FDCWD, ".", O_RDONLY | O_DIRECTORY));
+	if (fd != -1)
+		SAFE_CLOSE(fd);
+
+	fd = TST_EXP_FD(openat(dfd, "canary", O_WRONLY | O_CREAT, 0600));
+	if (fd != -1) {
+		SAFE_WRITE(SAFE_WRITE_ALL, fd, "x", 1);
+		SAFE_CLOSE(fd);
+	}
+
+	fd = TST_EXP_FD(openat(dfd, RELDIR, O_RDONLY));
+	if (fd != -1)
+		SAFE_CLOSE(fd);
+
+	TST_EXP_FAIL2(openat(dfd, ABSDIR, O_RDONLY), EOPNOTSUPP,
+		"resolution of an absolute symlink");
+
+	fd = TST_EXP_FD(openat(AT_FDCWD, upwards, O_PATH));
+	if (fd != -1) {
+		SAFE_FSTAT(fd, &st);
+		SAFE_CLOSE(fd);
+
+		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(dfd);
+}
+
+static void setup(void)
+{
+	char *tmpdir;
+	char abs_path[PATH_MAX];
+	struct stat root_st, st;
+	int off;
+
+	tmpdir = tst_tmpdir_path();
+	snprintf(abs_path, sizeof(abs_path), "%s/%s", tmpdir, "target");
+
+	SAFE_TOUCH("target", 0644, NULL);
+	SAFE_SYMLINK("target", RELDIR);
+	SAFE_SYMLINK(abs_path, ABSDIR);
+
+	SAFE_STAT("/", &root_st);
+
+	strcpy(upwards, "..");
+	off = 2;
+	while (1) {
+		SAFE_STAT(upwards, &st);
+
+		if (st.st_dev == root_st.st_dev && st.st_ino == root_st.st_ino)
+			break;
+
+		if ((size_t)off + sizeof("/..") > sizeof(upwards))
+			tst_brk(TBROK, "Path to root too long");
+
+		strcpy(upwards + off, "/..");
+		off += 3;
+	}
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.test_all = run,
+	.needs_root = 1,
+	.needs_tmpdir = 1,
+	.forks_child = 1,
+};

-- 
2.51.0


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