On Thu, 23 Jul 2026 18:02:12 +0100
David Carlier <[email protected]> wrote:
> Commit 61d445af0a7c ("tracing: Add bulk garbage collection of freeing
> event_trigger_data") moved the kfree() of event_trigger_data to a kthread
> that runs tracepoint_synchronize_unregister() before freeing. That removed
> the synchronize_rcu() the trigger .free callbacks used to get implicitly and
> inline from trigger_data_free().
>
> event_hist_trigger_free(), event_hist_trigger_named_free() and
> event_enable_trigger_free() free their satellite data (hist_data, cmd_ops,
> enable_data) right after trigger_data_free() returns. With the synchronize
> now deferred to the kthread, a concurrent tracepoint handler can still reach
> that data through the list_del_rcu()'d trigger, causing a use-after-free.
>
> The teardown must also stay synchronous for correctness: the histogram
> cleanup (remove_hist_vars(), unregister_field_var_hists()) has to detach a
> synthetic event from the histogram before the trigger-removal write returns.
> Deferring it lets a following command race in and fail the synthetic-event
> removal with -EBUSY, as caught by
> tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-eprobe.tc.
>
> Call synchronize_rcu() directly before freeing the satellite data instead.
>
> Suggested-by: Steven Rostedt <[email protected]>
> Fixes: 61d445af0a7c ("tracing: Add bulk garbage collection of freeing
> event_trigger_data")
> Signed-off-by: David Carlier <[email protected]>
Hi David,
Instead of adding just a synchronize_rcu(), can you reconsider to introduce
.private_data_free() to event_trigger_data? I hit the same issue on wprobe
and fixed it with ".private_free()" (I think private_data_free() is better
naming)
https://patchwork.kernel.org/project/linux-trace-kernel/patch/178476144141.26117.8004746214529353062.stgit@devnote2/
Thank you,
> ---
> kernel/trace/trace_events_hist.c | 3 +++
> kernel/trace/trace_events_trigger.c | 1 +
> 2 files changed, 4 insertions(+)
>
> diff --git a/kernel/trace/trace_events_hist.c
> b/kernel/trace/trace_events_hist.c
> index 82ce492ab268..ddd2f70dac4f 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -6349,6 +6349,8 @@ static void event_hist_trigger_free(struct
> event_trigger_data *data)
>
> trigger_data_free(data);
>
> + synchronize_rcu();
> +
> remove_hist_vars(hist_data);
>
> unregister_field_var_hists(hist_data);
> @@ -6388,6 +6390,7 @@ static void event_hist_trigger_named_free(struct
> event_trigger_data *data)
>
> del_named_trigger(data);
> trigger_data_free(data);
> + synchronize_rcu();
> kfree(cmd_ops);
> }
> }
> diff --git a/kernel/trace/trace_events_trigger.c
> b/kernel/trace/trace_events_trigger.c
> index 655db2e82513..c3f54f2540b6 100644
> --- a/kernel/trace/trace_events_trigger.c
> +++ b/kernel/trace/trace_events_trigger.c
> @@ -1730,6 +1730,7 @@ void event_enable_trigger_free(struct
> event_trigger_data *data)
> trace_event_enable_disable(enable_data->file, 0, 1);
> trace_event_put_ref(enable_data->file->event_call);
> trigger_data_free(data);
> + synchronize_rcu();
> kfree(enable_data);
> }
> }
> --
> 2.53.0
>
--
Masami Hiramatsu (Google) <[email protected]>
