[PATCH v2 07/18] mm/ksw: add atomic watch on/off operations

Jinchao Wang
2025-09-04T00:22:55Z

Add support to atomically turn the hardware watch on and off without
allocation overhead.

The watch is pre-allocated and later retargeted using hw_breakpoint_modify_local().
The current CPU is updated directly, while other CPUs are updated
asynchronously via smp_call_function_single_async().

This allows KStackWatch to switch the watch in kprobe/fprobe handlers.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
mm/kstackwatch/kstackwatch.h | 2 +
mm/kstackwatch/watch.c | 97 ++++++++++++++++++++++++++++++++++++
2 files changed, 99 insertions(+)

diff --git a/mm/kstackwatch/kstackwatch.h b/mm/kstackwatch/kstackwatch.h
index 2318779bde70..13ef8c79f855 100644
--- a/mm/kstackwatch/kstackwatch.h
+++ b/mm/kstackwatch/kstackwatch.h
@@ -41,5 +41,7 @@ extern bool panic_on_catch;
/* watch management */
int ksw_watch_init(struct ksw_config *config);
void ksw_watch_exit(void);
+int ksw_watch_on(u64 watch_addr, u64 watch_len);
+void ksw_watch_off(void);

#endif /* _KSTACKWATCH_H */
diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index e7ed88700b49..c563f8d17829 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -16,9 +16,21 @@

struct perf_event *__percpu *watch_events;
struct ksw_config *watch_config;
+static DEFINE_SPINLOCK(watch_lock);

static unsigned long long watch_holder;

+static struct watch_info {
+ u64 addr;
+ u64 len;
+} watch_info;
+
+static void ksw_watch_on_local_cpu(void *info);
+
+static DEFINE_PER_CPU(call_single_data_t,
+ watch_csd) = CSD_INIT(ksw_watch_on_local_cpu,
+ &watch_info);
+
static void ksw_watch_handler(struct perf_event *bp,
struct perf_sample_data *data,
struct pt_regs *regs)
@@ -32,6 +44,91 @@ static void ksw_watch_handler(struct perf_event *bp,
panic("KSW: Stack corruption detected");
}

+/*
+ * set up watchon current CPU
+ * addr and len updated by ksw_watch_on() already
+ */
+static void ksw_watch_on_local_cpu(void *data)
+{
+ struct perf_event *bp;
+ struct perf_event_attr attr;
+ struct watch_info *watch_info = data;
+
+ int cpu = smp_processor_id();
+ int ret;
+
+ bp = *per_cpu_ptr(watch_events, cpu);
+ if (!bp)
+ return;
+ attr.bp_addr = watch_info->addr;
+ attr.bp_len = watch_info->len;
+ attr.bp_type = bp->attr.bp_type;
+
+ ret = hw_breakpoint_modify_local(bp, &attr);
+ if (ret) {
+ pr_err("KSW: failed to reinstall HWBP on CPU %d ret %d\n", cpu,
+ ret);
+ return;
+ }
+
+ if (bp->attr.bp_addr == (unsigned long)&watch_holder) {
+ pr_debug("KSW: watch off CPU %d\n", cpu);
+ } else {
+ pr_debug("KSW: watch on CPU %d at 0x%px (len %llu)\n", cpu,
+ (void *)bp->attr.bp_addr, bp->attr.bp_len);
+ }
+}
+
+int ksw_watch_on(u64 watch_addr, u64 watch_len)
+{
+ unsigned long flags;
+ int cpu;
+
+ if (!watch_addr) {
+ pr_err("KSW: watch with invalid address\n");
+ return -EINVAL;
+ }
+
+ spin_lock_irqsave(&watch_lock, flags);
+
+ /*
+ * check if already watched
+ */
+ if (watch_info.addr != 0 && // not uninit
+ watch_info.addr != (unsigned long)&watch_holder && // installed
+ watch_addr != (unsigned long)&watch_holder) { //not restore
+ spin_unlock_irqrestore(&watch_lock, flags);
+ return -EBUSY;
+ }
+
+ watch_info.addr = watch_addr;
+ watch_info.len = watch_len;
+
+ spin_unlock_irqrestore(&watch_lock, flags);
+
+ if (watch_addr == (unsigned long)&watch_holder)
+ pr_debug("KSW: watch off starting\n");
+ else
+ pr_debug("KSW: watch on starting\n");
+
+ for_each_online_cpu(cpu) {
+ if (cpu == raw_smp_processor_id()) {
+ ksw_watch_on_local_cpu(&watch_info);
+ } else {
+ call_single_data_t *csd = &per_cpu(watch_csd, cpu);
+
+ smp_call_function_single_async(cpu, csd);
+ }
+ }
+
+ return 0;
+}
+
+void ksw_watch_off(void)
+{
+ ksw_watch_on((unsigned long)&watch_holder, sizeof(watch_holder));
+}
+
int ksw_watch_init(struct ksw_config *config)
{
struct perf_event_attr attr;
--
2.43.0