[PATCH v4 16/21] mm/ksw: add stack overflow test

[Jinchao Wang <wangjinchao600@gmail.com>]

Extend the test module with a new test case (test1) that intentionally
overflows a local u64 buffer to corrupt the stack canary. This helps
validate detection of stack corruption under overflow conditions.

The proc interface is updated to document the new test:

 - test1: stack canary overflow test

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/test.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/mm/kstackwatch/test.c b/mm/kstackwatch/test.c
index 76dbfb042067..ab1a3f92b5e8 100644
--- a/mm/kstackwatch/test.c
+++ b/mm/kstackwatch/test.c
@@ -40,6 +40,27 @@ static void canary_test_write(void)
 	pr_info("canary write test completed\n");
 }
 
+/*
+ * Test Case 1: Stack Overflow (Canary Test)
+ * This function uses a u64 buffer 64-bit write
+ * to corrupt the stack canary with a single operation
+ */
+static void canary_test_overflow(void)
+{
+	u64 buffer[BUFFER_SIZE];
+
+	pr_info("starting %s\n", __func__);
+	pr_info("buffer 0x%lx\n", (unsigned long)buffer);
+
+	/* intentionally overflow the u64 buffer. */
+	((u64 *)buffer + BUFFER_SIZE)[0] = 0xdeadbeefdeadbeef;
+
+	/* make sure the compiler do not drop assign action */
+	barrier_data(buffer);
+
+	pr_info("canary overflow test completed\n");
+}
+
 static ssize_t test_proc_write(struct file *file, const char __user *buffer,
 			       size_t count, loff_t *pos)
 {
@@ -63,6 +84,10 @@ static ssize_t test_proc_write(struct file *file, const char __user *buffer,
 			pr_info("triggering canary write test\n");
 			canary_test_write();
 			break;
+		case 1:
+			pr_info("triggering canary overflow test\n");
+			canary_test_overflow();
+			break;
 		default:
 			pr_err("Unknown test number %d\n", test_num);
 			return -EINVAL;
@@ -82,7 +107,8 @@ static ssize_t test_proc_read(struct file *file, char __user *buffer,
 		"KStackWatch Simplified Test Module\n"
 		"==================================\n"
 		"Usage:\n"
-		"  echo 'test0' > /proc/kstackwatch_test  - Canary write test\n";
+		"  echo 'test0' > /proc/kstackwatch_test  - Canary write test\n"
+		"  echo 'test1' > /proc/kstackwatch_test  - Canary overflow test\n";
 
 	return simple_read_from_buffer(buffer, count, pos, usage,
 				       strlen(usage));
-- 
2.43.0