[RFC PATCH 04/13] mm/kstackwatch: Add HWBP pre-allocation infrastructure

[Jinchao Wang <wangjinchao600@gmail.com>]

Implement the core HWBP management for kstackwatch with
pre-allocation strategy to enable atomic context operation.

This patch introduces the fundamental breakthrough that allows kstackwatch
to work in atomic contexts: pre-allocating hardware breakpoints across
all CPUs during initialization, then using arch_reinstall_hw_breakpoint()
to atomically update breakpoint targets without allocation overhead.

Key features:
- Pre-allocate per-CPU hardware breakpoints using register_wide_hw_breakpoint()
- Initialize with a dummy marker address that will be dynamically updated
- Comprehensive corruption detection handler with register dumps and
  optional panic trigger
- Clean resource management on module exit

The pre-allocation approach is critical because:
1. Hardware breakpoint allocation can fail or sleep in atomic contexts
2. kprobes run in atomic context where allocation is not permitted
3. Pre-allocated breakpoints can be instantly retargeted using
   arch_reinstall_hw_breakpoint() without any blocking operations

This foundation enables the subsequent kprobe integration to atomically
arm/disarm breakpoints on function entry/exit, providing real-time
stack corruption detection without the limitations of traditional
allocation-based approaches.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/kstackwatch.h |  6 ++++
 mm/kstackwatch/watch.c       | 65 ++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/mm/kstackwatch/kstackwatch.h b/mm/kstackwatch/kstackwatch.h
index f58af36e64a7..256574cd9cb2 100644
--- a/mm/kstackwatch/kstackwatch.h
+++ b/mm/kstackwatch/kstackwatch.h
@@ -36,4 +36,10 @@ struct ksw_config {
 	char config_str[MAX_CONFIG_STR_LEN];
 };
 
+extern bool panic_on_catch;
+
+/* watch management */
+int ksw_watch_init(struct ksw_config *config);
+void ksw_watch_exit(void);
+
 #endif /* _KSTACKWATCH_H */
diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index e69de29bb2d1..5cc2dfef140b 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kprobes.h>
+#include <linux/hw_breakpoint.h>
+#include <linux/perf_event.h>
+#include <linux/sched/debug.h>
+#include <linux/smp.h>
+#include <linux/slab.h>
+#include <asm/hw_breakpoint.h>
+#include <linux/stacktrace.h>
+#include <linux/delay.h>
+
+#include "kstackwatch.h"
+
+struct perf_event *__percpu *watch_events;
+struct ksw_config *watch_config;
+
+static unsigned long long marker;
+
+/* Enhanced breakpoint handler with watch identification */
+static void ksw_watch_handler(struct perf_event *bp,
+			      struct perf_sample_data *data,
+			      struct pt_regs *regs)
+{
+	pr_emerg("========== KStackWatch: Caught stack corruption =======\n");
+	pr_emerg("KSW: config %s\n", watch_config->config_str);
+	show_regs(regs);
+	pr_emerg("========== KStackWatch End ==========\n");
+	mdelay(100);
+
+	if (panic_on_catch)
+		panic("KSW: Stack corruption detected");
+}
+
+/* Initialize hardware breakpoint  */
+int ksw_watch_init(struct ksw_config *config)
+{
+	struct perf_event_attr attr;
+
+	/* Initialize default breakpoint attributes */
+	hw_breakpoint_init(&attr);
+	attr.bp_addr = (unsigned long)&marker;
+	attr.bp_len = HW_BREAKPOINT_LEN_8;
+	attr.bp_type = HW_BREAKPOINT_W;
+	watch_events =
+		register_wide_hw_breakpoint(&attr, ksw_watch_handler, NULL);
+	if (IS_ERR((void *)watch_events)) {
+		int ret = PTR_ERR((void *)watch_events);
+
+		pr_err("KSW: Failed to register wide hw breakpoint: %d\n", ret);
+		return ret;
+	}
+
+	watch_config = config;
+	pr_info("KSW: HWBP  initialized\n");
+	return 0;
+}
+
+/* Cleanup hardware breakpoint  */
+void ksw_watch_exit(void)
+{
+	unregister_wide_hw_breakpoint(watch_events);
+	watch_events = NULL;
+
+	pr_info("KSW: HWBP  cleaned up\n");
+}
-- 
2.43.0