[RFC PATCH 09/13] mm/kwatch: add probe lifecycle runtime

[Jinchao Wang <wangjinchao600@gmail.com>]

Open and close the watch window with a kretprobe on the target
function: the entry handler tracks per-task nesting depth and, when
the configured depth is reached, resolves the watch expression and
arms a watchpoint; the exit handler disarms it. An optional kprobe
at func_offset arms mid-function instead of at entry.

Functions running in a real NMI(-like) context are rejected once, at
function entry, by comparing the NMI nesting count against the one
NMI-like layer that int3-based kprobe delivery itself adds; a
companion kprobe with a post_handler pins the probe point so jump
optimization cannot change the delivery mechanism after it is
sampled. Rejections are counted and exposed to the control plane.

A global epoch versioning scheme invalidates stale per-task state
across reconfigurations, and a per-CPU mute flag keeps window
management quiet while a CPU rewrites its own debug registers.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kwatch/Makefile |   2 +-
 mm/kwatch/probe.c  | 263 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 264 insertions(+), 1 deletion(-)
 create mode 100644 mm/kwatch/probe.c

diff --git a/mm/kwatch/Makefile b/mm/kwatch/Makefile
index b2bc3003c89b..f04673cc5b1c 100644
--- a/mm/kwatch/Makefile
+++ b/mm/kwatch/Makefile
@@ -1,3 +1,3 @@
 obj-$(CONFIG_KWATCH) += kwatch.o
 
-kwatch-y := deref.o task_ctx.o hwbp.o
+kwatch-y := deref.o task_ctx.o hwbp.o probe.o
diff --git a/mm/kwatch/probe.c b/mm/kwatch/probe.c
new file mode 100644
index 000000000000..af6e0af45c10
--- /dev/null
+++ b/mm/kwatch/probe.c
@@ -0,0 +1,263 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/atomic.h>
+#include <linux/kprobes.h>
+#include <linux/kallsyms.h>
+#include <linux/percpu.h>
+#include <linux/preempt.h>
+#include <linux/sched.h>
+
+#include "kwatch.h"
+#define TRAMPOLINE_CHECK_DEPTH 16
+static DEFINE_PER_CPU(bool, kwatch_probe_cpu_muted);
+
+struct kwatch_probe_ctx {
+	struct kprobe kp;
+	struct kretprobe rp;
+	struct kprobe pin_kp;
+	const struct kwatch_config *cfg;
+	bool rp_via_int3;
+
+	u32 epoch;
+};
+
+static struct kwatch_probe_ctx kwatch_probe_ctx;
+static atomic_long_t kwatch_nmi_rejected;
+
+unsigned long kwatch_probe_nmi_rejected(void)
+{
+	return atomic_long_read(&kwatch_nmi_rejected);
+}
+
+/*
+ * True if the probed function itself runs in an NMI-like context.
+ * int3-based kprobe delivery adds one NMI-like layer of its own;
+ * delivery is pinned at registration so the subtraction stays exact.
+ */
+static bool kwatch_probed_ctx_in_nmi(bool via_int3)
+{
+	return (preempt_count() & NMI_MASK) > (via_int3 ? NMI_OFFSET : 0);
+}
+
+static void kwatch_pin_post_handler(struct kprobe *p, struct pt_regs *regs,
+				    unsigned long flags)
+{
+	/* a post_handler pins the probepoint: no jump optimization */
+}
+
+bool kwatch_probe_validate_hit(struct pt_regs *regs,
+			       struct task_struct *arm_tsk)
+{
+	struct kwatch_tsk_ctx *ctx = kwatch_tsk_ctx_get(false);
+
+	if (unlikely(!ctx))
+		return true;
+
+	if (arm_tsk != current ||
+	    ctx->depth != kwatch_probe_ctx.cfg->depth + 1)
+		return true;
+
+	return false;
+}
+
+void kwatch_probe_mute(bool mute)
+{
+	__this_cpu_write(kwatch_probe_cpu_muted, mute);
+}
+
+static inline bool kwatch_probe_is_muted(void)
+{
+	return __this_cpu_read(kwatch_probe_cpu_muted);
+}
+
+enum kwatch_probe_position {
+	KWATCH_PROBE_POSITION_ENTRY,
+	KWATCH_PROBE_POSITION_ACTIVE,
+	KWATCH_PROBE_POSITION_EXIT
+};
+
+static bool kwatch_tsk_ctx_check(enum kwatch_probe_position pos)
+{
+	struct kwatch_tsk_ctx *ctx = kwatch_tsk_ctx_get(true);
+	u32 epoch;
+
+	if (unlikely(!ctx))
+		return false;
+
+	/* Pairs with smp_store_release() in kwatch_probe_start/stop() */
+	epoch = smp_load_acquire(&kwatch_probe_ctx.epoch);
+
+	if (unlikely(ctx->epoch != epoch))
+		kwatch_tsk_ctx_reset(ctx, epoch);
+
+	if (unlikely(!epoch))
+		return false;
+
+	switch (pos) {
+	case KWATCH_PROBE_POSITION_ENTRY:
+		ctx->depth++;
+		return true;
+	case KWATCH_PROBE_POSITION_ACTIVE:
+		return true;
+	case KWATCH_PROBE_POSITION_EXIT:
+		if (unlikely(ctx->depth == 0)) {
+			kwatch_tsk_ctx_put();
+			return false;
+		}
+
+		ctx->depth--;
+		if (ctx->depth == 0) {
+			kwatch_tsk_ctx_put();
+			return false;
+		}
+		return true;
+	}
+	return false;
+}
+
+static int kwatch_activate_handler(struct kprobe *p, struct pt_regs *regs)
+{
+	struct kwatch_tsk_ctx *ctx = kwatch_tsk_ctx_get(false);
+	unsigned long watch_addr;
+	u16 watch_len;
+
+	if (unlikely(!ctx))
+		return 0;
+
+	if (unlikely(kwatch_probe_is_muted()))
+		return 0;
+
+	if (unlikely(!kwatch_tsk_ctx_check(KWATCH_PROBE_POSITION_ACTIVE)))
+		return 0;
+
+	if (ctx->depth != kwatch_probe_ctx.cfg->depth + 1 || ctx->wp)
+		return 0;
+
+	if (kwatch_deref_resolve(kwatch_probe_ctx.cfg, regs, &watch_addr,
+				 &watch_len))
+		return 0;
+
+	if (kwatch_hwbp_get(&ctx->wp))
+		return 0;
+
+	kwatch_hwbp_arm(ctx->wp, watch_addr, watch_len);
+	return 0;
+}
+
+static int kwatch_lifecycle_entry(struct kretprobe_instance *ri,
+				  struct pt_regs *regs)
+{
+	/*
+	 * Single policy point: the target function's context is judged once
+	 * here. A rejected invocation never increments depth, so the offset
+	 * kprobe path inherits the verdict through the depth check.
+	 */
+	if (unlikely(kwatch_probed_ctx_in_nmi(kwatch_probe_ctx.rp_via_int3))) {
+		atomic_long_inc(&kwatch_nmi_rejected);
+		return 1; /* NMI context is unsupported: no window, no return hook */
+	}
+
+	if (!kwatch_tsk_ctx_check(KWATCH_PROBE_POSITION_ENTRY))
+		return 0;
+
+	if (kwatch_probe_ctx.cfg->func_offset == 0)
+		kwatch_activate_handler(NULL, regs);
+
+	return 0;
+}
+
+static int kwatch_lifecycle_exit(struct kretprobe_instance *ri,
+				 struct pt_regs *regs)
+{
+	struct kwatch_tsk_ctx *ctx = kwatch_tsk_ctx_get(false);
+
+	if (unlikely(!ctx))
+		return 0;
+
+	if (!kwatch_tsk_ctx_check(KWATCH_PROBE_POSITION_EXIT))
+		return 0;
+
+	if (ctx->depth == kwatch_probe_ctx.cfg->depth) {
+		struct kwatch_watchpoint *wp = xchg(&ctx->wp, NULL);
+
+		if (wp)
+			kwatch_hwbp_put(wp);
+	}
+
+	return 0;
+}
+
+int kwatch_probe_start(struct kwatch_config *cfg)
+{
+	static u32 next_epoch;
+	u32 current_epoch;
+	int ret;
+
+	/*
+	 * Lockless check to prevent concurrent starts. Strictly serialized
+	 * by the control plane mutex, but serves as a sanity check.
+	 */
+	if (smp_load_acquire(&kwatch_probe_ctx.epoch) != 0)
+		return -EBUSY;
+
+	memset(&kwatch_probe_ctx, 0, sizeof(kwatch_probe_ctx));
+	kwatch_probe_ctx.cfg = cfg;
+
+	/*
+	 * Pin the entry probepoint before the kretprobe registers, so its
+	 * delivery (int3 vs ftrace) can never change under jump optimization.
+	 * register_kretprobe() clears kp.post_handler, hence the companion.
+	 */
+	kwatch_probe_ctx.pin_kp.symbol_name = cfg->func_name;
+	kwatch_probe_ctx.pin_kp.post_handler = kwatch_pin_post_handler;
+	ret = register_kprobe(&kwatch_probe_ctx.pin_kp);
+	if (ret < 0)
+		return ret;
+
+	kwatch_probe_ctx.rp.entry_handler = kwatch_lifecycle_entry;
+	kwatch_probe_ctx.rp.handler = kwatch_lifecycle_exit;
+	kwatch_probe_ctx.rp.kp.symbol_name = cfg->func_name;
+
+	ret = register_kretprobe(&kwatch_probe_ctx.rp);
+	if (ret < 0) {
+		unregister_kprobe(&kwatch_probe_ctx.pin_kp);
+		return ret;
+	}
+	kwatch_probe_ctx.rp_via_int3 = !kprobe_ftrace(&kwatch_probe_ctx.rp.kp);
+
+	if (cfg->func_offset) {
+		kwatch_probe_ctx.kp.symbol_name = cfg->func_name;
+		kwatch_probe_ctx.kp.offset = cfg->func_offset;
+		kwatch_probe_ctx.kp.pre_handler = kwatch_activate_handler;
+
+		ret = register_kprobe(&kwatch_probe_ctx.kp);
+		if (ret) {
+			unregister_kretprobe(&kwatch_probe_ctx.rp);
+			unregister_kprobe(&kwatch_probe_ctx.pin_kp);
+			return ret;
+		}
+	}
+
+	current_epoch = ++next_epoch;
+	if (unlikely(!current_epoch))
+		current_epoch = ++next_epoch;
+
+	/* Pairs with smp_load_acquire() in kwatch_tsk_ctx_check() */
+	smp_store_release(&kwatch_probe_ctx.epoch, current_epoch);
+
+	return 0;
+}
+
+void kwatch_probe_stop(void)
+{
+	if (!kwatch_probe_ctx.epoch)
+		return;
+
+	/* Pairs with smp_load_acquire() in kwatch_tsk_ctx_check() */
+	smp_store_release(&kwatch_probe_ctx.epoch, 0);
+
+	if (kwatch_probe_ctx.cfg->func_offset > 0)
+		unregister_kprobe(&kwatch_probe_ctx.kp);
+
+	unregister_kretprobe(&kwatch_probe_ctx.rp);
+	unregister_kprobe(&kwatch_probe_ctx.pin_kp);
+}
-- 
2.53.0