[PATCH STAGING v2 05/16] fchroot02: test fchroot() invalid arguments

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

Verify the fchroot() error paths: a non-zero flags argument fails with
EINVAL before anything else, an invalid fd fails with EBADF, including
the FD_PIDFS_ROOT and FD_NSFS_ROOT sentinels, and a fd referring to a
regular file fails with ENOTDIR. All these checks happen before the
CAP_SYS_CHROOT check, so no privileges are needed.

Signed-off-by: Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot02.c | 80 +++++++++++++++++++++++++++
 3 files changed, 82 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 733d5609a..e12230218 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -1,3 +1,4 @@
 # Tests for features that are not yet in the stable kernel ABI
 
 fchroot01 fchroot01
+fchroot02 fchroot02
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index 03ebdbe7a..fb3287612 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -1 +1,2 @@
 fchroot01
+fchroot02
diff --git a/testcases/kernel/syscalls/fchroot/fchroot02.c b/testcases/kernel/syscalls/fchroot/fchroot02.c
new file mode 100644
index 000000000..96be00d70
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot02.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
+ */
+
+/*\
+ * Test the :manpage:`fchroot(2)` error paths for invalid arguments.
+ *
+ * :manpage:`fchroot(2)` was introduced in Linux v7.3. The syscall checks its
+ * arguments in this order:
+ *
+ * - a non-zero flags argument fails with ``EINVAL`` before anything else,
+ *   including with the ``FD_FAILFS_ROOT`` sentinel and with an invalid fd
+ * - an invalid fd fails with ``EBADF``, including the ``FD_PIDFS_ROOT`` and
+ *   ``FD_NSFS_ROOT`` sentinels which :manpage:`fchroot(2)` does not accept
+ * - a fd referring to a regular file fails with ``ENOTDIR``
+ *
+ * All these checks happen before the ``CAP_SYS_CHROOT`` check, so the test
+ * needs no privileges.
+ */
+
+#include <fcntl.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+#define FILENAME "file.txt"
+
+static int dir_fd = -1;
+static int file_fd = -1;
+static int bad_fd = -1;
+static int failfs_root = FD_FAILFS_ROOT;
+static int pidfs_root = FD_PIDFS_ROOT;
+static int nsfs_root = FD_NSFS_ROOT;
+
+static struct tcase {
+	int *fd;
+	unsigned int flags;
+	int exp_errno;
+	const char *desc;
+} tcases[] = {
+	{&dir_fd, 1, EINVAL, "non-zero flags with a directory fd"},
+	{&failfs_root, 1, EINVAL, "non-zero flags with FD_FAILFS_ROOT"},
+	{&bad_fd, 1, EINVAL, "non-zero flags with an invalid fd"},
+	{&bad_fd, 0, EBADF, "invalid fd"},
+	{&pidfs_root, 0, EBADF, "FD_PIDFS_ROOT sentinel"},
+	{&nsfs_root, 0, EBADF, "FD_NSFS_ROOT sentinel"},
+	{&file_fd, 0, ENOTDIR, "fd referring to a regular file"},
+};
+
+static void run(unsigned int i)
+{
+	struct tcase *tc = &tcases[i];
+
+	TST_EXP_FAIL(tst_syscall(__NR_fchroot, *tc->fd, tc->flags),
+		tc->exp_errno, "fchroot() with %s", tc->desc);
+}
+
+static void setup(void)
+{
+	dir_fd = SAFE_OPEN(".", O_PATH | O_DIRECTORY);
+	file_fd = SAFE_OPEN(FILENAME, O_CREAT | O_EXCL | O_WRONLY, 0644);
+}
+
+static void cleanup(void)
+{
+	if (dir_fd != -1)
+		SAFE_CLOSE(dir_fd);
+
+	if (file_fd != -1)
+		SAFE_CLOSE(file_fd);
+}
+
+static struct tst_test test = {
+	.test = run,
+	.tcnt = ARRAY_SIZE(tcases),
+	.setup = setup,
+	.cleanup = cleanup,
+	.needs_tmpdir = 1,
+};

-- 
2.51.0


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