[PATCH v4 15/21] mm/ksw: add test module

[Jinchao Wang <wangjinchao600@gmail.com>]

Introduce a separate test module to validate functionality in controlled
scenarios, such as stack canary writes and simulated corruption.

The module provides a proc interface (/proc/kstackwatch_test) that allows
triggering specific test cases via simple commands:

 - test0: directly corrupt the canary to verify watch/fire behavior

Test module is built with optimizations disabled to ensure predictable
behavior.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/Kconfig.debug        |  10 ++++
 mm/kstackwatch/Makefile |   6 +++
 mm/kstackwatch/test.c   | 115 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 131 insertions(+)
 create mode 100644 mm/kstackwatch/test.c

diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
index fdfc6e6d0dec..46c280280980 100644
--- a/mm/Kconfig.debug
+++ b/mm/Kconfig.debug
@@ -320,3 +320,13 @@ config KSTACK_WATCH
 	  the recursive depth of the monitored function.
 
 	  If unsure, say N.
+
+config KSTACK_WATCH_TEST
+	tristate "KStackWatch Test Module"
+	depends on KSTACK_WATCH
+	help
+	  This module provides controlled stack exhaustion and overflow scenarios
+	  to verify the functionality of KStackWatch. It is particularly useful
+	  for development and validation of the KStachWatch mechanism.
+
+	  If unsure, say N.
diff --git a/mm/kstackwatch/Makefile b/mm/kstackwatch/Makefile
index 84a46cb9a766..d007b8dcd1c6 100644
--- a/mm/kstackwatch/Makefile
+++ b/mm/kstackwatch/Makefile
@@ -1,2 +1,8 @@
 obj-$(CONFIG_KSTACK_WATCH)	+= kstackwatch.o
 kstackwatch-y := kernel.o stack.o watch.o
+
+obj-$(CONFIG_KSTACK_WATCH_TEST)	+= kstackwatch_test.o
+kstackwatch_test-y := test.o
+CFLAGS_test.o := -fno-inline \
+		-fno-optimize-sibling-calls \
+		-fno-pic -fno-pie -O0 -Og
diff --git a/mm/kstackwatch/test.c b/mm/kstackwatch/test.c
new file mode 100644
index 000000000000..76dbfb042067
--- /dev/null
+++ b/mm/kstackwatch/test.c
@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/delay.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+#include <linux/prandom.h>
+#include <linux/printk.h>
+#include <linux/proc_fs.h>
+#include <linux/string.h>
+#include <linux/uaccess.h>
+
+#include "kstackwatch.h"
+
+MODULE_AUTHOR("Jinchao Wang");
+MODULE_DESCRIPTION("Simple KStackWatch Test Module");
+MODULE_LICENSE("GPL");
+
+static struct proc_dir_entry *test_proc;
+#define BUFFER_SIZE 4
+#define MAX_DEPTH 6
+
+/*
+ * Test Case 0: Write to the canary position directly (Canary Test)
+ * use a u64 buffer array to ensure the canary will be placed
+ * corrupt the stack canary using the debug function
+ */
+static void canary_test_write(void)
+{
+	u64 buffer[BUFFER_SIZE];
+
+	pr_info("starting %s\n", __func__);
+	ksw_watch_show();
+	ksw_watch_fire();
+
+	buffer[0] = 0;
+
+	/* make sure the compiler do not drop assign action */
+	barrier_data(buffer);
+	pr_info("canary write test completed\n");
+}
+
+static ssize_t test_proc_write(struct file *file, const char __user *buffer,
+			       size_t count, loff_t *pos)
+{
+	char cmd[256];
+	int test_num;
+
+	if (count >= sizeof(cmd))
+		return -EINVAL;
+
+	if (copy_from_user(cmd, buffer, count))
+		return -EFAULT;
+
+	cmd[count] = '\0';
+	strim(cmd);
+
+	pr_info("received command: %s\n", cmd);
+
+	if (sscanf(cmd, "test%d", &test_num) == 1) {
+		switch (test_num) {
+		case 0:
+			pr_info("triggering canary write test\n");
+			canary_test_write();
+			break;
+		default:
+			pr_err("Unknown test number %d\n", test_num);
+			return -EINVAL;
+		}
+	} else {
+		pr_err("invalid command format. Use 'test1', 'test2', or 'test3'.\n");
+		return -EINVAL;
+	}
+
+	return count;
+}
+
+static ssize_t test_proc_read(struct file *file, char __user *buffer,
+			      size_t count, loff_t *pos)
+{
+	static const char usage[] =
+		"KStackWatch Simplified Test Module\n"
+		"==================================\n"
+		"Usage:\n"
+		"  echo 'test0' > /proc/kstackwatch_test  - Canary write test\n";
+
+	return simple_read_from_buffer(buffer, count, pos, usage,
+				       strlen(usage));
+}
+
+static const struct proc_ops test_proc_ops = {
+	.proc_read = test_proc_read,
+	.proc_write = test_proc_write,
+};
+
+static int __init kstackwatch_test_init(void)
+{
+	test_proc = proc_create("kstackwatch_test", 0600, NULL, &test_proc_ops);
+	if (!test_proc) {
+		pr_err("Failed to create proc entry\n");
+		return -ENOMEM;
+	}
+	pr_info("module loaded\n");
+	return 0;
+}
+
+static void __exit kstackwatch_test_exit(void)
+{
+	if (test_proc)
+		remove_proc_entry("kstackwatch_test", NULL);
+	pr_info("module unloaded\n");
+}
+
+module_init(kstackwatch_test_init);
+module_exit(kstackwatch_test_exit);
-- 
2.43.0