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Subject[PATCH 4.14 061/173] ftrace: Protect ftrace_graph_hash with ftrace_sync
Date
From: Steven Rostedt (VMware) <rostedt@goodmis.org>

[ Upstream commit 54a16ff6f2e50775145b210bcd94d62c3c2af117 ]

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>
Signed-off-by: Sasha Levin <sashal@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 dd9fdb52e24ad..8974ecbcca3cb 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5419,8 +5419,15 @@ 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_sched();
+ /*
+ * 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 99af95e294d8d..c4c61ebb8d054 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -883,6 +883,7 @@ 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());

@@ -935,6 +936,7 @@ 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


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