[PATCH STAGING v2 10/16] fchroot07: test execve blocked by failfs root

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

Verify that after fchroot() moved the root into failfs, loading a
binary by absolute path fails with EOPNOTSUPP. The exec runs in a
grandchild so a wrongly successful exec is still detected through
the exit code.

Signed-off-by: Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot07.c | 72 +++++++++++++++++++++++++++
 3 files changed, 74 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 8b6b1ecd9..b8954bb5a 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -6,3 +6,4 @@ fchroot03 fchroot03
 fchroot04 fchroot04
 fchroot05 fchroot05
 fchroot06 fchroot06
+fchroot07 fchroot07
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index 12151270a..b68069d9a 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -4,3 +4,4 @@ fchroot03
 fchroot04
 fchroot05
 fchroot06
+fchroot07
diff --git a/testcases/kernel/syscalls/fchroot/fchroot07.c b/testcases/kernel/syscalls/fchroot/fchroot07.c
new file mode 100644
index 000000000..d5ebfc576
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot07.c
@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <andrea.cervesato-IBi9RG/[email protected]>
+ */
+
+/*\
+ * Test that :manpage:`execve(2)` is blocked under the failfs root.
+ *
+ * After :manpage:`fchroot(2)` moved the process root into failfs, loading
+ * a binary by absolute path fails with ``EOPNOTSUPP``.
+ *
+ * Root is required because entering failfs with the ``FD_FAILFS_ROOT``
+ * sentinel requires ``CAP_SYS_CHROOT``.
+ *
+ * The exec runs in a grandchild: a wrongly successful exec would replace
+ * the test image, so the outcome can only be reported when the exec call
+ * returns, and the grandchild exit code tells the parent whether the
+ * image was replaced.
+ */
+
+#define _GNU_SOURCE
+#include <sys/wait.h>
+#include <unistd.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/syscalls.h"
+
+/* Marker exit code proving the grandchild image was not replaced. */
+#define EXEC_NOT_REPLACED 42
+
+static void check_exec_blocked(void)
+{
+	pid_t pid = SAFE_FORK();
+	int status;
+
+	if (!pid) {
+		TST_EXP_FAIL(execl("/bin/true", "true", NULL), EOPNOTSUPP,
+			"absolute exec blocked by the failfs root");
+
+		exit(EXEC_NOT_REPLACED);
+	}
+
+	SAFE_WAITPID(pid, &status, 0);
+	if (!WIFEXITED(status) || WEXITSTATUS(status) != EXEC_NOT_REPLACED)
+		tst_res(TFAIL, "exec replaced the test image");
+}
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+		"fchroot() with the FD_FAILFS_ROOT sentinel");
+
+	check_exec_blocked();
+
+	exit(0);
+}
+
+static void setup(void)
+{
+	if (access("/bin/true", X_OK))
+		tst_brk(TCONF | TERRNO, "/bin/true is not available");
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.test_all = run,
+	.needs_root = 1,
+	.forks_child = 1,
+};

-- 
2.51.0


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