ftrace: Fix en(dis)able graph caller when en(dis)abling record via sysctl

commit 1619dc3f 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
called.

 # 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
desired.

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.comSigned-off-by: Pratyush Anand <panand@redhat.com>
[
  removed extra if (ftrace_start_up) and defined ftrace_graph_active as 0
  if CONFIG_FUNCTION_GRAPH_TRACER is not set.
]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/trace/ftrace.c

@@ -935,6 +935,12 @@ static __init void ftrace_profile_debugfs(struct dentry *d_tracer)
 
 static struct pid * const ftrace_swapper_pid = &init_struct_pid;
 
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+static int ftrace_graph_active;
+#else
+# define ftrace_graph_active 0
+#endif
+
 static loff_t
 ftrace_filter_lseek(struct file *file, loff_t offset, int whence)
 {
@@ -1810,24 +1816,36 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
 
 static void ftrace_startup_sysctl(void)
 {
+	int command;
+
 	if (unlikely(ftrace_disabled))
 		return;
 
 	/* 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) {
+		command = FTRACE_UPDATE_CALLS;
+		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))
 		return;
 
 	/* ftrace_start_up is true if ftrace is running */
-	if (ftrace_start_up)
-		ftrace_run_update_code(FTRACE_DISABLE_CALLS);
+	if (ftrace_start_up) {
+		command = FTRACE_DISABLE_CALLS;
+		if (ftrace_graph_active)
+			command |= FTRACE_STOP_FUNC_RET;
+		ftrace_run_update_code(command);
+	}
 }
 
 static cycle_t		ftrace_update_time;
@@ -4044,7 +4062,6 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 
-static int ftrace_graph_active;
 static struct notifier_block ftrace_suspend_notifier;
 
 int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace)