Re: [for-next][PATCH 1/5] ftrace: Protect ftrace_graph_hash with ftrace_sync

On Wed, Feb 05, 2020 at 05:21:11PM -0500, Steven Rostedt wrote:

From: "Steven Rostedt (VMware)" rostedt@goodmis.org

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: stable@vger.kernel.org Fixes: b9b0c831bed26 ("ftrace: Convert graph filter to use hash tables") Reported-by: "Paul E. McKenney" paulmck@kernel.org Reviewed-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org

Nice! If there is much more call for this, perhaps I should take a hint from the ftrace_sync() comment and add synchronize_rcu_rude(). ;-)

Reviewed-by: "Paul E. McKenney" paulmck@kernel.org

---

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());

-- 2.24.1