linux-kernel - [PATCH] tracing: Do not take trace_event_sem in print_event

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Message-ID: <20250501224128.0b1f0571@batman.local.home>
Date: Thu, 1 May 2025 22:41:28 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux trace kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>,
 syzbot+441582c1592938fccf09@...kaller.appspotmail.com
Subject: [PATCH] tracing: Do not take trace_event_sem in
 print_event_fields()

From: Steven Rostedt <rostedt@...dmis.org>

On some paths in print_event_fields() it takes the trace_event_sem for
read, even though it should always be held when the function is called.

Remove the taking of that mutex and add a lockdep_assert_held_read() to
make sure the trace_event_sem is held when print_event_fields() is called.

Cc: stable@...r.kernel.org
Fixes: 80a76994b2d88 ("tracing: Add "fields" option to show raw trace event fields")
Reported-by: syzbot+441582c1592938fccf09@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/6813ff5e.050a0220.14dd7d.001b.GAE@google.com/
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/trace_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index c5e8299d1842..aa0d3663f754 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1056,11 +1056,12 @@ enum print_line_t print_event_fields(struct trace_iterator *iter,
 	struct trace_event_call *call;
 	struct list_head *head;
 
+	lockdep_assert_held_read(&trace_event_sem);
+
 	/* ftrace defined events have separate call structures */
 	if (event->type <= __TRACE_LAST_TYPE) {
 		bool found = false;
 
-		down_read(&trace_event_sem);
 		list_for_each_entry(call, &ftrace_events, list) {
 			if (call->event.type == event->type) {
 				found = true;
@@ -1070,7 +1071,6 @@ enum print_line_t print_event_fields(struct trace_iterator *iter,
 			if (call->event.type > __TRACE_LAST_TYPE)
 				break;
 		}
-		up_read(&trace_event_sem);
 		if (!found) {
 			trace_seq_printf(&iter->seq, "UNKNOWN TYPE %d\n", event->type);
 			goto out;
-- 
2.47.2