Add helpers to find the stack canary or a local variable addr and len
for the probed function based on ksw_get_config(). For canary search,
limits search to a fixed number of steps to avoid scanning the entire
stack. Validates that the computed address and length are within the
kernel stack.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/stack.c | 77 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 74 insertions(+), 3 deletions(-)

diff --git a/mm/kstackwatch/stack.c b/mm/kstackwatch/stack.c
index e596ef97222d..3c4cb6d5b58a 100644
--- a/mm/kstackwatch/stack.c
+++ b/mm/kstackwatch/stack.c
@@ -9,6 +9,7 @@
 
 #include "kstackwatch.h"
 
+#define MAX_CANARY_SEARCH_STEPS 128
 static struct kprobe entry_probe;
 static struct fprobe exit_probe;
 
@@ -59,13 +60,83 @@ static bool ksw_stack_check_ctx(bool entry)
 		return false;
 }
 
+static unsigned long ksw_find_stack_canary_addr(struct pt_regs *regs)
+{
+	unsigned long *stack_ptr, *stack_end, *stack_base;
+	unsigned long expected_canary;
+	unsigned int i;
+
+	stack_ptr = (unsigned long *)kernel_stack_pointer(regs);
+
+	stack_base = (unsigned long *)(current->stack);
+
+	// TODO: limit it to the current frame
+	stack_end = (unsigned long *)((char *)current->stack + THREAD_SIZE);
+
+	expected_canary = current->stack_canary;
+
+	if (stack_ptr < stack_base || stack_ptr >= stack_end) {
+		pr_err("Stack pointer 0x%lx out of bounds [0x%lx, 0x%lx)\n",
+		       (unsigned long)stack_ptr, (unsigned long)stack_base,
+		       (unsigned long)stack_end);
+		return 0;
+	}
+
+	for (i = 0; i < MAX_CANARY_SEARCH_STEPS; i++) {
+		if (&stack_ptr[i] >= stack_end)
+			break;
+
+		if (stack_ptr[i] == expected_canary) {
+			pr_debug("canary found i:%d 0x%lx\n", i,
+				 (unsigned long)&stack_ptr[i]);
+			return (unsigned long)&stack_ptr[i];
+		}
+	}
+
+	pr_debug("canary not found in first %d steps\n",
+		 MAX_CANARY_SEARCH_STEPS);
+	return 0;
+}
+
+static int ksw_stack_validate_addr(unsigned long addr, size_t size)
+{
+	unsigned long stack_start, stack_end;
+
+	if (!addr || !size)
+		return -EINVAL;
+
+	stack_start = (unsigned long)current->stack;
+	stack_end = stack_start + THREAD_SIZE;
+
+	if (addr < stack_start || (addr + size) > stack_end)
+		return -ERANGE;
+
+	return 0;
+}
+
 static int ksw_stack_prepare_watch(struct pt_regs *regs,
 				   const struct ksw_config *config,
 				   ulong *watch_addr, u16 *watch_len)
 {
-	/* implement logic will be added in following patches */
-	*watch_addr = 0;
-	*watch_len = 0;
+	ulong addr;
+	u16 len;
+
+	// default is to watch the canary
+	if (!ksw_get_config()->watch_len) {
+		addr = ksw_find_stack_canary_addr(regs);
+		len = sizeof(ulong);
+	} else {
+		addr = kernel_stack_pointer(regs) + ksw_get_config()->sp_offset;
+		len = ksw_get_config()->watch_len;
+	}
+
+	if (ksw_stack_validate_addr(addr, len)) {
+		pr_err("invalid stack addr:0x%lx len :%u\n", addr, len);
+		return -EINVAL;
+	}
+
+	*watch_addr = addr;
+	*watch_len = len;
 	return 0;
 }
 
-- 
2.43.0