ftrace: Fix breakage of set_ftrace_pid

commit e3eea140 upstream.

Commit 4104d326 ("ftrace: Remove global function list and call function
directly") simplified the ftrace code by removing the global_ops list with a
new design. But this cleanup also broke the filtering of PIDs that are added
to the set_ftrace_pid file.

Add back the proper hooks to have pid filtering working once again.
Reported-by: Matt Fleming <matt@console-pimps.org>
Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
Tested-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

include/linux/ftrace.h

@@ -116,6 +116,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
  *            SAVE_REGS. If another ops with this flag set is already registered
  *            for any of the functions that this ops will be registered for, then
  *            this ops will fail to register or set_filter_ip.
+ * PID     - Is affected by set_ftrace_pid (allows filtering on those pids)
  */
 enum {
 	FTRACE_OPS_FL_ENABLED			= 1 << 0,
@@ -132,6 +133,7 @@ enum {
 	FTRACE_OPS_FL_MODIFYING			= 1 << 11,
 	FTRACE_OPS_FL_ALLOC_TRAMP		= 1 << 12,
 	FTRACE_OPS_FL_IPMODIFY			= 1 << 13,
+	FTRACE_OPS_FL_PID			= 1 << 14,
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE
@@ -159,6 +161,7 @@ struct ftrace_ops {
 	struct ftrace_ops		*next;
 	unsigned long			flags;
 	void				*private;
+	ftrace_func_t			saved_func;
 	int __percpu			*disabled;
 #ifdef CONFIG_DYNAMIC_FTRACE
 	int				nr_trampolines;

kernel/trace/ftrace.c

@@ -98,6 +98,13 @@ struct ftrace_pid {
 	struct pid *pid;
 };
 
+static bool ftrace_pids_enabled(void)
+{
+	return !list_empty(&ftrace_pids);
+}
+
+static void ftrace_update_trampoline(struct ftrace_ops *ops);
+
 /*
  * ftrace_disabled is set when an anomaly is discovered.
  * ftrace_disabled is much stronger than ftrace_enabled.
@@ -109,7 +116,6 @@ static DEFINE_MUTEX(ftrace_lock);
 static struct ftrace_ops *ftrace_control_list __read_mostly = &ftrace_list_end;
 static struct ftrace_ops *ftrace_ops_list __read_mostly = &ftrace_list_end;
 ftrace_func_t ftrace_trace_function __read_mostly = ftrace_stub;
-ftrace_func_t ftrace_pid_function __read_mostly = ftrace_stub;
 static struct ftrace_ops global_ops;
 static struct ftrace_ops control_ops;
 
@@ -183,14 +189,7 @@ static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip,
 	if (!test_tsk_trace_trace(current))
 		return;
 
-	ftrace_pid_function(ip, parent_ip, op, regs);
-}
-
-static void set_ftrace_pid_function(ftrace_func_t func)
-{
-	/* do not set ftrace_pid_function to itself! */
-	if (func != ftrace_pid_func)
-		ftrace_pid_function = func;
+	op->saved_func(ip, parent_ip, op, regs);
 }
 
 /**
@@ -202,7 +201,6 @@ static void set_ftrace_pid_function(ftrace_func_t func)
 void clear_ftrace_function(void)
 {
 	ftrace_trace_function = ftrace_stub;
-	ftrace_pid_function = ftrace_stub;
 }
 
 static void control_ops_disable_all(struct ftrace_ops *ops)
@@ -423,6 +421,12 @@ static int __register_ftrace_function(struct ftrace_ops *ops)
 	} else
 		add_ftrace_ops(&ftrace_ops_list, ops);
 
+	/* Always save the function, and reset at unregistering */
+	ops->saved_func = ops->func;
+
+	if (ops->flags & FTRACE_OPS_FL_PID && ftrace_pids_enabled())
+		ops->func = ftrace_pid_func;
+
 	ftrace_update_trampoline(ops);
 
 	if (ftrace_enabled)
@@ -450,15 +454,28 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops)
 	if (ftrace_enabled)
 		update_ftrace_function();
 
+	ops->func = ops->saved_func;
+
 	return 0;
 }
 
 static void ftrace_update_pid_func(void)
 {
+	bool enabled = ftrace_pids_enabled();
+	struct ftrace_ops *op;
+
 	/* Only do something if we are tracing something */
 	if (ftrace_trace_function == ftrace_stub)
 		return;
 
+	do_for_each_ftrace_op(op, ftrace_ops_list) {
+		if (op->flags & FTRACE_OPS_FL_PID) {
+			op->func = enabled ? ftrace_pid_func :
+				op->saved_func;
+			ftrace_update_trampoline(op);
+		}
+	} while_for_each_ftrace_op(op);
+
 	update_ftrace_function();
 }
 
@@ -1120,7 +1137,8 @@ static struct ftrace_ops global_ops = {
 	.local_hash.filter_hash		= EMPTY_HASH,
 	INIT_OPS_HASH(global_ops)
 	.flags				= FTRACE_OPS_FL_RECURSION_SAFE |
-					  FTRACE_OPS_FL_INITIALIZED,
+					  FTRACE_OPS_FL_INITIALIZED |
+					  FTRACE_OPS_FL_PID,
 };
 
 /*
@@ -5010,7 +5028,9 @@ static void ftrace_update_trampoline(struct ftrace_ops *ops)
 
 static struct ftrace_ops global_ops = {
 	.func			= ftrace_stub,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_INITIALIZED,
+	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
+				  FTRACE_OPS_FL_INITIALIZED |
+				  FTRACE_OPS_FL_PID,
 };
 
 static int __init ftrace_nodyn_init(void)
@@ -5067,11 +5087,6 @@ void ftrace_init_array_ops(struct trace_array *tr, ftrace_func_t func)
 		if (WARN_ON(tr->ops->func != ftrace_stub))
 			printk("ftrace ops had %pS for function\n",
 			       tr->ops->func);
-		/* Only the top level instance does pid tracing */
-		if (!list_empty(&ftrace_pids)) {
-			set_ftrace_pid_function(func);
-			func = ftrace_pid_func;
-		}
 	}
 	tr->ops->func = func;
 	tr->ops->private = tr;
@@ -5365,7 +5380,7 @@ static void *fpid_start(struct seq_file *m, loff_t *pos)
 {
 	mutex_lock(&ftrace_lock);
 
-	if (list_empty(&ftrace_pids) && (!*pos))
+	if (!ftrace_pids_enabled() && (!*pos))
 		return (void *) 1;
 
 	return seq_list_start(&ftrace_pids, *pos);
@@ -5604,6 +5619,7 @@ static struct ftrace_ops graph_ops = {
 	.func			= ftrace_stub,
 	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
 				   FTRACE_OPS_FL_INITIALIZED |
+				   FTRACE_OPS_FL_PID |
 				   FTRACE_OPS_FL_STUB,
 #ifdef FTRACE_GRAPH_TRAMP_ADDR
 	.trampoline		= FTRACE_GRAPH_TRAMP_ADDR,