[PATCH v4 08/21] mm/ksw: Add atomic ksw_watch_on() and ksw_watch_off()

Jinchao Wang
2025-09-12T10:12:37Z

The atomic_long_cmpxchg() ensures at most one active watchpoint exists at
any time, with ksw_watch_on() succeeding only when no watch is active
(current address is placeholder) and ksw_watch_off() succeeding only when
the caller knows the active watch address.

For cross-CPU synchronization, updates are propagated using direct
modification on the local CPU and asynchronous IPIs for remote CPUs.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
mm/kstackwatch/kstackwatch.h | 2 +
mm/kstackwatch/watch.c | 73 +++++++++++++++++++++++++++++++++++-
2 files changed, 74 insertions(+), 1 deletion(-)

diff --git a/mm/kstackwatch/kstackwatch.h b/mm/kstackwatch/kstackwatch.h
index 3ea191370970..0786fa961011 100644
--- a/mm/kstackwatch/kstackwatch.h
+++ b/mm/kstackwatch/kstackwatch.h
@@ -41,5 +41,7 @@ const struct ksw_config *ksw_get_config(void);
/* watch management */
int ksw_watch_init(void);
void ksw_watch_exit(void);
+int ksw_watch_on(ulong watch_addr, u16 watch_len);
+int ksw_watch_off(ulong watch_addr, u16 watch_len);

#endif /* _KSTACKWATCH_H */
diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index d3399ac840b2..14549e02faf1 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -2,6 +2,7 @@
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/hw_breakpoint.h>
+#include <linux/irqflags.h>
#include <linux/perf_event.h>
#include <linux/printk.h>

@@ -9,10 +10,16 @@

static struct perf_event *__percpu *watch_events;

-static unsigned long watch_holder;
+static ulong watch_holder;
+static atomic_long_t watched_addr = ATOMIC_LONG_INIT((ulong)&watch_holder);

static struct perf_event_attr watch_attr;

+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, NULL);
+
bool panic_on_catch;
module_param(panic_on_catch, bool, 0644);
MODULE_PARM_DESC(panic_on_catch, "panic immediately on corruption catch");
@@ -29,6 +36,70 @@ static void ksw_watch_handler(struct perf_event *bp,
panic("Stack corruption detected");
}

+static void ksw_watch_on_local_cpu(void *data)
+{
+ struct perf_event *bp;
+ ulong flags;
+ int cpu;
+ int ret;
+
+ local_irq_save(flags);
+ cpu = raw_smp_processor_id();
+ bp = *per_cpu_ptr(watch_events, cpu);
+ if (!bp) {
+ local_irq_restore(flags);
+ return;
+ }
+
+ ret = modify_wide_hw_breakpoint_local(bp, &watch_attr);
+ local_irq_restore(flags);
+
+ if (ret) {
+ pr_err("failed to reinstall HWBP on CPU %d ret %d\n", cpu,
+ ret);
+ return;
+ }
+}
+
+static void __ksw_watch_target(ulong addr, u16 len)
+{
+ int cpu;
+ call_single_data_t *csd;
+
+ watch_attr.bp_addr = addr;
+ watch_attr.bp_len = len;
+
+ /* ensure watchpoint update is visible to other CPUs before IPI */
+ smp_wmb();
+
+ for_each_online_cpu(cpu) {
+ if (cpu == raw_smp_processor_id()) {
+ ksw_watch_on_local_cpu(NULL);
+ } else {
+ csd = &per_cpu(watch_csd, cpu);
+ smp_call_function_single_async(cpu, csd);
+ }
+ }
+}
+
+static int ksw_watch_target(ulong old_addr, ulong new_addr, u16 watch_len)
+{
+ if (atomic_long_cmpxchg(&watched_addr, old_addr, new_addr) != old_addr)
+ return -EINVAL;
+ __ksw_watch_target(new_addr, watch_len);
+ return 0;
+}
+
+int ksw_watch_on(ulong watch_addr, u16 watch_len)
+{
+ return ksw_watch_target((ulong)&watch_holder, watch_addr, watch_len);
+}
+
+int ksw_watch_off(ulong watch_addr, u16 watch_len)
+{
+ return ksw_watch_target(watch_addr, (ulong)&watch_holder, watch_len);
+}
+
int ksw_watch_init(void)
{
int ret;
--
2.43.0