linux-kernel - [PATCH 1/5] tracing: Fix checking of freed trace_event

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Message-ID: <20260210113427.1068932-2-petr.pavlu@suse.com>
Date: Tue, 10 Feb 2026 12:28:16 +0100
From: Petr Pavlu <petr.pavlu@...e.com>
To: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tom Zanussi <zanussi@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Petr Pavlu <petr.pavlu@...e.com>
Subject: [PATCH 1/5] tracing: Fix checking of freed trace_event_file for hist files

The event_hist_open() and event_hist_poll() functions currently retrieve
a trace_event_file pointer from a file struct by invoking
event_file_data(), which simply returns file->f_inode->i_private. The
functions then check if the pointer is NULL to determine whether the event
is still valid. This approach is flawed because i_private is assigned when
an eventfs inode is allocated and remains set throughout its lifetime.
Instead, the code should call event_file_file(), which checks for
EVENT_FILE_FL_FREED. Using the incorrect access function may result in the
code potentially opening a hist file for an event that is being removed or
becoming stuck while polling on this file.

A related issue is that although event_hist_poll() attempts to verify
whether an event file is being removed, this check may not occur or could
be unnecessarily delayed. This happens because hist_poll_wakeup() is
currently invoked only from event_hist_trigger() when a hist command is
triggered. If the event file is being removed, no associated hist command
will be triggered and a waiter will be woken up only after an unrelated
hist command is triggered.

Fix these issues by changing the access method to event_file_file() and
adding a call to hist_poll_wakeup() in remove_event_file_dir() after
setting the EVENT_FILE_FL_FREED flag. This ensures that a task polling on
a hist file is woken up and receives EPOLLERR.

Fixes: 1bd13edbbed6 ("tracing/hist: Add poll(POLLIN) support on hist file")
Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
---
 kernel/trace/trace_events.c      | 3 +++
 kernel/trace/trace_events_hist.c | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 137b4d9bb116..e8ed6ba155cf 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1295,6 +1295,9 @@ static void remove_event_file_dir(struct trace_event_file *file)
 	free_event_filter(file->filter);
 	file->flags |= EVENT_FILE_FL_FREED;
 	event_file_put(file);
+
+	/* Wake up hist poll waiters to notice the EVENT_FILE_FL_FREED flag. */
+	hist_poll_wakeup();
 }

 /*
diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index c97bb2fda5c0..744c2aa3d668 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -5778,7 +5778,7 @@ static __poll_t event_hist_poll(struct file *file, struct poll_table_struct *wai

 	guard(mutex)(&event_mutex);

-	event_file = event_file_data(file);
+	event_file = event_file_file(file);
 	if (!event_file)
 		return EPOLLERR;

@@ -5816,7 +5816,7 @@ static int event_hist_open(struct inode *inode, struct file *file)

 	guard(mutex)(&event_mutex);

-	event_file = event_file_data(file);
+	event_file = event_file_file(file);
 	if (!event_file) {
 		ret = -ENODEV;
 		goto err;
-- 
2.52.0