[PATCH 3.16.y-ckt 119/165] ftrace: Fix en(dis)able graph caller when en(dis)abling record via sysctl

Luis Henriques luis.henriques at canonical.com
Wed Mar 25 14:01:32 UTC 2015

3.16.7-ckt9 -stable review patch.  If anyone has any objections, please let me know.


From: Pratyush Anand <panand at redhat.com>

commit 1619dc3f8f555ee1cdd3c75db3885d5715442b12 upstream.

When ftrace is enabled globally through the proc interface, we must check if
ftrace_graph_active is set. If it is set, then we should also pass the
FTRACE_START_FUNC_RET command to ftrace_run_update_code(). Similarly, when
ftrace is disabled globally through the proc interface, we must check if
ftrace_graph_active is set. If it is set, then we should also pass the
FTRACE_STOP_FUNC_RET command to ftrace_run_update_code().

Consider the following situation.

 # echo 0 > /proc/sys/kernel/ftrace_enabled

After this ftrace_enabled = 0.

 # echo function_graph > /sys/kernel/debug/tracing/current_tracer

Since ftrace_enabled = 0, ftrace_enable_ftrace_graph_caller() is never

 # echo 1 > /proc/sys/kernel/ftrace_enabled

Now ftrace_enabled will be set to true, but still
ftrace_enable_ftrace_graph_caller() will not be called, which is not

Further if we execute the following after this:
  # echo nop > /sys/kernel/debug/tracing/current_tracer

Now since ftrace_enabled is set it will call
ftrace_disable_ftrace_graph_caller(), which causes a kernel warning on
the ARM platform.

On the ARM platform, when ftrace_enable_ftrace_graph_caller() is called,
it checks whether the old instruction is a nop or not. If it's not a nop,
then it returns an error. If it is a nop then it replaces instruction at
that address with a branch to ftrace_graph_caller.
ftrace_disable_ftrace_graph_caller() behaves just the opposite. Therefore,
if generic ftrace code ever calls either ftrace_enable_ftrace_graph_caller()
or ftrace_disable_ftrace_graph_caller() consecutively two times in a row,
then it will return an error, which will cause the generic ftrace code to
raise a warning.

Note, x86 does not have an issue with this because the architecture
specific code for ftrace_enable_ftrace_graph_caller() and
ftrace_disable_ftrace_graph_caller() does not check the previous state,
and calling either of these functions twice in a row has no ill effect.

Link: http://lkml.kernel.org/r/e4fbe64cdac0dd0e86a3bf914b0f83c0b419f146.1425666454.git.panand@redhat.com

Signed-off-by: Pratyush Anand <panand at redhat.com>
  removed extra if (ftrace_start_up) and defined ftrace_graph_active as 0
Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 kernel/trace/ftrace.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index ca167e660e10..054f9bbe5f12 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1048,6 +1048,12 @@ static __init void ftrace_profile_debugfs(struct dentry *d_tracer)
 static struct pid * const ftrace_swapper_pid = &init_struct_pid;
+static int ftrace_graph_active;
+# define ftrace_graph_active 0
@@ -2211,24 +2217,36 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
 static void ftrace_startup_sysctl(void)
+	int command;
 	if (unlikely(ftrace_disabled))
 	/* Force update next time */
 	saved_ftrace_func = NULL;
 	/* ftrace_start_up is true if we want ftrace running */
-	if (ftrace_start_up)
-		ftrace_run_update_code(FTRACE_UPDATE_CALLS);
+	if (ftrace_start_up) {
+		if (ftrace_graph_active)
+			command |= FTRACE_START_FUNC_RET;
+		ftrace_run_update_code(command);
+	}
 static void ftrace_shutdown_sysctl(void)
+	int command;
 	if (unlikely(ftrace_disabled))
 	/* ftrace_start_up is true if ftrace is running */
-	if (ftrace_start_up)
-		ftrace_run_update_code(FTRACE_DISABLE_CALLS);
+	if (ftrace_start_up) {
+		if (ftrace_graph_active)
+			command |= FTRACE_STOP_FUNC_RET;
+		ftrace_run_update_code(command);
+	}
 static cycle_t		ftrace_update_time;
@@ -4924,8 +4942,6 @@ static struct ftrace_ops graph_ops = {
 	ASSIGN_OPS_HASH(graph_ops, &global_ops.local_hash)
-static int ftrace_graph_active;
 int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace)
 	return 0;

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