[RFC PATCH 3/5] tracing/kprobe: Separate inc recursion count out of perf_trace_buf_prepare

[He Kuang <hekuang@huawei.com>]

Inside perf_trace_buf_prepare(), a recursion count is increased, the
count was first introduced by commit 444a2a3bcd6d ("tracing,
perf_events: Protect the buffer from recursion in perf") to protect
the percpu data buffer from being overwritten.

For future patch to enable eBPF saving data into perf extra trace
buffer and prevent data buffer being filled recursively, the recursion
count is increased outside before entering trace_call_bpf() and
decreased in case of error. In this condition, we use the new function
perf_trace_buf_prepare_rctx() for not increasing the recursion count a
second time.

Signed-off-by: He Kuang <hekuang@huawei.com>
---
 include/linux/ftrace_event.h    |  2 ++
 kernel/trace/trace_event_perf.c | 27 ++++++++++++++++++++++-----
 kernel/trace/trace_kprobe.c     | 28 ++++++++++++++++++++++------
 3 files changed, 46 insertions(+), 11 deletions(-)

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index b6cf5a3..085d236 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -615,6 +615,8 @@ extern int  ftrace_profile_set_filter(struct perf_event *event, int event_id,
 extern void ftrace_profile_free_filter(struct perf_event *event);
 extern void *perf_trace_buf_prepare(int size, unsigned short type,
 				    struct pt_regs **regs, int *rctxp);
+extern void *perf_trace_buf_prepare_rctx(int size, unsigned short type,
+					 struct pt_regs **regs, int rctx);
 extern void *perf_extra_trace_buf_prepare(int size, int rctx);
 extern void *get_perf_extra_trace_buf(int rctx);
 
diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 603b3da..e683cb6 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -306,8 +306,9 @@ void *get_perf_extra_trace_buf(int rctx)
 EXPORT_SYMBOL_GPL(get_perf_extra_trace_buf);
 NOKPROBE_SYMBOL(get_perf_extra_trace_buf);
 
-void *perf_trace_buf_prepare(int size, unsigned short type,
-			     struct pt_regs **regs, int *rctxp)
+static void *__perf_trace_buf_prepare(int size, unsigned short type,
+			       struct pt_regs **regs, int *rctxp,
+			       bool update_rctx)
 {
 	struct trace_entry *entry;
 	unsigned long flags;
@@ -322,9 +323,11 @@ void *perf_trace_buf_prepare(int size, unsigned short type,
 
 	pc = preempt_count();
 
-	*rctxp = perf_swevent_get_recursion_context();
-	if (*rctxp < 0)
-		return NULL;
+	if (update_rctx) {
+		*rctxp = perf_swevent_get_recursion_context();
+		if (*rctxp < 0)
+			return NULL;
+	}
 
 	if (regs)
 		*regs = this_cpu_ptr(&__perf_regs[*rctxp]);
@@ -340,9 +343,23 @@ void *perf_trace_buf_prepare(int size, unsigned short type,
 
 	return raw_data;
 }
+
+void *perf_trace_buf_prepare(int size, unsigned short type,
+			     struct pt_regs **regs, int *rctxp)
+{
+	return __perf_trace_buf_prepare(size, type, regs, rctxp, true);
+}
 EXPORT_SYMBOL_GPL(perf_trace_buf_prepare);
 NOKPROBE_SYMBOL(perf_trace_buf_prepare);
 
+void *perf_trace_buf_prepare_rctx(int size, unsigned short type,
+			     struct pt_regs **regs, int rctx)
+{
+	return __perf_trace_buf_prepare(size, type, regs, &rctx, false);
+}
+EXPORT_SYMBOL_GPL(perf_trace_buf_prepare_rctx);
+NOKPROBE_SYMBOL(perf_trace_buf_prepare_rctx);
+
 #ifdef CONFIG_FUNCTION_TRACER
 static void
 perf_ftrace_function_call(unsigned long ip, unsigned long parent_ip,
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 5600df8..16ad88e 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -1145,22 +1145,30 @@ kprobe_perf_func(struct trace_kprobe *tk, struct pt_regs *regs)
 	if (hlist_empty(head))
 		return;
 
-	if (prog && !trace_call_bpf(prog, regs))
+	rctx = perf_swevent_get_recursion_context();
+	if (rctx < 0)
 		return;
 
+	if (prog && !trace_call_bpf(prog, regs))
+		goto out;
+
 	dsize = __get_data_size(&tk->tp, regs);
 	__size = sizeof(*entry) + tk->tp.size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
-	entry = perf_trace_buf_prepare(size, call->event.type, NULL, &rctx);
+	entry = perf_trace_buf_prepare_rctx(size, call->event.type, NULL, rctx);
 	if (!entry)
-		return;
+		goto out;
 
 	entry->ip = (unsigned long)tk->rp.kp.addr;
 	memset(&entry[1], 0, dsize);
 	store_trace_args(sizeof(*entry), &tk->tp, regs, (u8 *)&entry[1], dsize);
 	perf_trace_buf_submit(entry, size, rctx, 0, 1, regs, head, NULL);
+
+	return;
+out:
+	perf_swevent_put_recursion_context(rctx);
 }
 NOKPROBE_SYMBOL(kprobe_perf_func);
 
@@ -1180,22 +1188,30 @@ kretprobe_perf_func(struct trace_kprobe *tk, struct kretprobe_instance *ri,
 	if (hlist_empty(head))
 		return;
 
-	if (prog && !trace_call_bpf(prog, regs))
+	rctx = perf_swevent_get_recursion_context();
+	if (rctx < 0)
 		return;
 
+	if (prog && !trace_call_bpf(prog, regs))
+		goto out;
+
 	dsize = __get_data_size(&tk->tp, regs);
 	__size = sizeof(*entry) + tk->tp.size + dsize;
 	size = ALIGN(__size + sizeof(u32), sizeof(u64));
 	size -= sizeof(u32);
 
-	entry = perf_trace_buf_prepare(size, call->event.type, NULL, &rctx);
+	entry = perf_trace_buf_prepare_rctx(size, call->event.type, NULL, rctx);
 	if (!entry)
-		return;
+		goto out;
 
 	entry->func = (unsigned long)tk->rp.kp.addr;
 	entry->ret_ip = (unsigned long)ri->ret_addr;
 	store_trace_args(sizeof(*entry), &tk->tp, regs, (u8 *)&entry[1], dsize);
 	perf_trace_buf_submit(entry, size, rctx, 0, 1, regs, head, NULL);
+
+	return;
+out:
+	perf_swevent_put_recursion_context(rctx);
 }
 NOKPROBE_SYMBOL(kretprobe_perf_func);
 #endif	/* CONFIG_PERF_EVENTS */
-- 
1.8.5.2