[PATCH v3 10/19] mm/ksw: resolve stack watch addr and len

Jinchao Wang
2025-09-10T05:32:24Z

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 | 86 ++++++++++++++++++++++++++++++++++++++++--
1 file changed, 83 insertions(+), 3 deletions(-)

diff --git a/mm/kstackwatch/stack.c b/mm/kstackwatch/stack.c
index 72409156458f..3ea0f9de698e 100644
--- a/mm/kstackwatch/stack.c
+++ b/mm/kstackwatch/stack.c
@@ -13,14 +13,94 @@ static struct kprobe entry_probe;
static struct fprobe exit_probe;
#define INVALID_PID -1
static atomic_t ksw_stack_pid = ATOMIC_INIT(INVALID_PID);
+#define MAX_CANARY_SEARCH_STEPS 128
+
+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,
u64 *watch_addr, u64 *watch_len)
{
- /* implement logic will be added in following patches */
- *watch_addr = 0;
- *watch_len = 0;
+ u64 addr;
+ u64 len;
+
+ /* Resolve addresses for all active watches */
+ switch (ksw_get_config()->type) {
+ case WATCH_CANARY:
+ addr = ksw_find_stack_canary_addr(regs);
+ len = sizeof(unsigned long);
+ break;
+
+ case WATCH_LOCAL_VAR:
+ addr = kernel_stack_pointer(regs) +
+ ksw_get_config()->local_var_offset;
+ len = ksw_get_config()->local_var_len;
+ break;
+
+ default:
+ pr_err("Unknown watch type %d\n", ksw_get_config()->type);
+ return -EINVAL;
+ }
+
+ if (ksw_stack_validate_addr(addr, len)) {
+ pr_err("invalid stack addr:0x%llx len :%llu\n", addr, len);
+ return -EINVAL;
+ }
+
+ *watch_addr = addr;
+ *watch_len = len;
return 0;
}

--
2.43.0