2020-02-05 14:44:36

by Steven Rostedt

Subject: [PATCH] ftrace: Protect ftrace_graph_hash with ftrace_sync

From: "Steven Rostedt (VMware)" <[email protected]>

As function_graph tracer can run when RCU is not "watching", it can not be
protected by synchronize_rcu() it requires running a task on each CPU before
it can be freed. Calling schedule_on_each_cpu(ftrace_sync) needs to be used.

Link: https://lore.kernel.org/r/20200205131110.GT2935@paulmck-ThinkPad-P72

Cc: [email protected]
Fixes: b9b0c831bed26 ("ftrace: Convert graph filter to use hash tables")
Reported-by: "Paul E. McKenney" <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
---
kernel/trace/ftrace.c | 11 +++++++++--
kernel/trace/trace.h | 2 ++
2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 481ede3eac13..3f7ee102868a 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5867,8 +5867,15 @@ ftrace_graph_release(struct inode *inode, struct file *file)

mutex_unlock(&graph_lock);

- /* Wait till all users are no longer using the old hash */
- synchronize_rcu();
+ /*
+ * We need to do a hard force of sched synchronization.
+ * This is because we use preempt_disable() to do RCU, but
+ * the function tracers can be called where RCU is not watching
+ * (like before user_exit()). We can not rely on the RCU
+ * infrastructure to do the synchronization, thus we must do it
+ * ourselves.
+ */
+ schedule_on_each_cpu(ftrace_sync);

free_ftrace_hash(old_hash);
}
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 8c52f5de9384..3c75d29bd861 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -979,6 +979,7 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
* Have to open code "rcu_dereference_sched()" because the
* function graph tracer can be called when RCU is not
* "watching".
+ * Protected with schedule_on_each_cpu(ftrace_sync)
*/
hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());

@@ -1031,6 +1032,7 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
* Have to open code "rcu_dereference_sched()" because the
* function graph tracer can be called when RCU is not
* "watching".
+ * Protected with schedule_on_each_cpu(ftrace_sync)
*/
notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
!preemptible());
--
2.20.1

2020-02-05 21:59:09

by Joel Fernandes

Subject: Re: [PATCH] ftrace: Protect ftrace_graph_hash with ftrace_sync

On Wed, Feb 05, 2020 at 09:41:56AM -0500, Steven Rostedt wrote:
>
> From: "Steven Rostedt (VMware)" <[email protected]>
>
> As function_graph tracer can run when RCU is not "watching", it can not be
> protected by synchronize_rcu() it requires running a task on each CPU before
> it can be freed. Calling schedule_on_each_cpu(ftrace_sync) needs to be used.
>
> Link: https://lore.kernel.org/r/20200205131110.GT2935@paulmck-ThinkPad-P72
>
> Cc: [email protected]
> Fixes: b9b0c831bed26 ("ftrace: Convert graph filter to use hash tables")
> Reported-by: "Paul E. McKenney" <[email protected]>
> Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Reviewed-by: Joel Fernandes (Google) <[email protected]>

thanks,

- Joel

> ---
> kernel/trace/ftrace.c | 11 +++++++++--
> kernel/trace/trace.h | 2 ++
> 2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 481ede3eac13..3f7ee102868a 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5867,8 +5867,15 @@ ftrace_graph_release(struct inode *inode, struct file *file)
>
> mutex_unlock(&graph_lock);
>
> - /* Wait till all users are no longer using the old hash */
> - synchronize_rcu();
> + /*
> + * We need to do a hard force of sched synchronization.
> + * This is because we use preempt_disable() to do RCU, but
> + * the function tracers can be called where RCU is not watching
> + * (like before user_exit()). We can not rely on the RCU
> + * infrastructure to do the synchronization, thus we must do it
> + * ourselves.
> + */
> + schedule_on_each_cpu(ftrace_sync);
>
> free_ftrace_hash(old_hash);
> }
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 8c52f5de9384..3c75d29bd861 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -979,6 +979,7 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
> * Have to open code "rcu_dereference_sched()" because the
> * function graph tracer can be called when RCU is not
> * "watching".
> + * Protected with schedule_on_each_cpu(ftrace_sync)
> */
> hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());
>
> @@ -1031,6 +1032,7 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
> * Have to open code "rcu_dereference_sched()" because the
> * function graph tracer can be called when RCU is not
> * "watching".
> + * Protected with schedule_on_each_cpu(ftrace_sync)
> */
> notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
> !preemptible());
> --
> 2.20.1
>