[LTP] [PATCH STAGING 06/16] fchroot03: test fchroot() permission checks

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

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

Verify that with a regular directory fd the kernel first checks the
execute permission on the directory and then CAP_SYS_CHROOT: an
unprivileged process with an accessible directory fails with EPERM and
a process without execute permission fails with EACCES.

Signed-off-by: Andrea Cervesato <andrea.cervesato@suse.com>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot03.c | 77 +++++++++++++++++++++++++++
 3 files changed, 79 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index e12230218..13635037b 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -2,3 +2,4 @@
 
 fchroot01 fchroot01
 fchroot02 fchroot02
+fchroot03 fchroot03
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index fb3287612..235befd99 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -1,2 +1,3 @@
 fchroot01
 fchroot02
+fchroot03
diff --git a/testcases/kernel/syscalls/fchroot/fchroot03.c b/testcases/kernel/syscalls/fchroot/fchroot03.c
new file mode 100644
index 000000000..4c1350e21
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot03.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato@suse.com>
+ */
+
+/*\
+ * Test the :manpage:`fchroot(2)` permission checks with a regular directory
+ * fd.
+ *
+ * fchroot() was introduced in Linux v7.3. With a regular directory fd the
+ * kernel first checks that the caller has execute permission on the
+ * directory, then that it holds CAP_SYS_CHROOT:
+ *
+ * - an unprivileged process with an accessible directory fails with EPERM
+ * - a process without execute permission on the directory fails with
+ *   EACCES, proving the permission check comes before the capability
+ *   check
+ *
+ * Root is required to open the directory file descriptors before dropping
+ * to an unprivileged user in forked children.
+ */
+
+#include <fcntl.h>
+#include <pwd.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+static struct tcase {
+	const char *dir;
+	mode_t mode;
+	int exp_errno;
+	const char *desc;
+} tcases[] = {
+	{"pubdir", 0755, EPERM, "no CAP_SYS_CHROOT"},
+	{"privdir", 0600, EACCES, "no execute permission"},
+};
+
+static struct passwd *ltpuser;
+
+static void run(unsigned int i)
+{
+	struct tcase *tc = &tcases[i];
+
+	if (!SAFE_FORK()) {
+		int dfd = SAFE_OPEN(tc->dir, O_PATH | O_DIRECTORY);
+
+		SAFE_SETRESUID(ltpuser->pw_uid, ltpuser->pw_uid,
+			ltpuser->pw_uid);
+
+		TST_EXP_FAIL(tst_syscall(__NR_fchroot, dfd, 0),
+			tc->exp_errno, "fchroot() with %s", tc->desc);
+
+		exit(0);
+	}
+}
+
+static void setup(void)
+{
+	unsigned int i;
+
+	ltpuser = SAFE_GETPWNAM("nobody");
+
+	for (i = 0; i < ARRAY_SIZE(tcases); i++) {
+		SAFE_MKDIR(tcases[i].dir, tcases[i].mode);
+		SAFE_CHMOD(tcases[i].dir, tcases[i].mode);
+	}
+}
+
+static struct tst_test test = {
+	.test = run,
+	.tcnt = ARRAY_SIZE(tcases),
+	.setup = setup,
+	.needs_root = 1,
+	.needs_tmpdir = 1,
+	.forks_child = 1,
+};

-- 
2.51.0


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