ftrace: Add ftrace_find_direct_func()

As function_graph tracer modifies the return address to insert a trampoline
to trace the return of a function, it must be aware of a direct caller, as
when it gets called, the function's return address may not be at on the
stack where it expects. It may have to see if that return address points to
the a direct caller and adjust if it is.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/ftrace.h

@@ -51,6 +51,7 @@ static inline void early_trace_init(void) { }
 
 struct module;
 struct ftrace_hash;
+struct ftrace_direct_func;
 
 #if defined(CONFIG_FUNCTION_TRACER) && defined(CONFIG_MODULES) && \
 	defined(CONFIG_DYNAMIC_FTRACE)
@@ -248,6 +249,7 @@ static inline void ftrace_free_mem(struct module *mod, void *start, void *end) {
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 int register_ftrace_direct(unsigned long ip, unsigned long addr);
 int unregister_ftrace_direct(unsigned long ip, unsigned long addr);
+struct ftrace_direct_func *ftrace_find_direct_func(unsigned long addr);
 #else
 static inline int register_ftrace_direct(unsigned long ip, unsigned long addr)
 {
@@ -257,6 +259,10 @@ static inline int unregister_ftrace_direct(unsigned long ip, unsigned long addr)
 {
 	return -ENODEV;
 }
+static inline struct ftrace_direct_func *ftrace_find_direct_func(unsigned long addr)
+{
+	return NULL;
+}
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
 
 #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS

kernel/trace/ftrace.c

@@ -4938,6 +4938,46 @@ ftrace_set_addr(struct ftrace_ops *ops, unsigned long ip, int remove,
 }
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+
+struct ftrace_direct_func {
+	struct list_head	next;
+	unsigned long		addr;
+	int			count;
+};
+
+static LIST_HEAD(ftrace_direct_funcs);
+
+/**
+ * ftrace_find_direct_func - test an address if it is a registered direct caller
+ * @addr: The address of a registered direct caller
+ *
+ * This searches to see if a ftrace direct caller has been registered
+ * at a specific address, and if so, it returns a descriptor for it.
+ *
+ * This can be used by architecture code to see if an address is
+ * a direct caller (trampoline) attached to a fentry/mcount location.
+ * This is useful for the function_graph tracer, as it may need to
+ * do adjustments if it traced a location that also has a direct
+ * trampoline attached to it.
+ */
+struct ftrace_direct_func *ftrace_find_direct_func(unsigned long addr)
+{
+	struct ftrace_direct_func *entry;
+	bool found = false;
+
+	/* May be called by fgraph trampoline (protected by rcu tasks) */
+	list_for_each_entry_rcu(entry, &ftrace_direct_funcs, next) {
+		if (entry->addr == addr) {
+			found = true;
+			break;
+		}
+	}
+	if (found)
+		return entry;
+
+	return NULL;
+}
+
 /**
  * register_ftrace_direct - Call a custom trampoline directly
  * @ip: The address of the nop at the beginning of a function
@@ -4957,6 +4997,7 @@ ftrace_set_addr(struct ftrace_ops *ops, unsigned long ip, int remove,
  */
 int register_ftrace_direct(unsigned long ip, unsigned long addr)
 {
+	struct ftrace_direct_func *direct;
 	struct ftrace_func_entry *entry;
 	struct ftrace_hash *free_hash = NULL;
 	struct dyn_ftrace *rec;
@@ -5005,6 +5046,18 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 	if (!entry)
 		goto out_unlock;
 
+	direct = ftrace_find_direct_func(addr);
+	if (!direct) {
+		direct = kmalloc(sizeof(*direct), GFP_KERNEL);
+		if (!direct) {
+			kfree(entry);
+			goto out_unlock;
+		}
+		direct->addr = addr;
+		direct->count = 0;
+		list_add_rcu(&direct->next, &ftrace_direct_funcs);
+	}
+
 	entry->ip = ip;
 	entry->direct = addr;
 	__add_hash_entry(direct_functions, entry);
@@ -5019,8 +5072,20 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 			ftrace_set_filter_ip(&direct_ops, ip, 1, 0);
 	}
 
-	if (ret)
+	if (ret) {
 		kfree(entry);
+		if (!direct->count) {
+			list_del_rcu(&direct->next);
+			synchronize_rcu_tasks();
+			kfree(direct);
+			if (free_hash)
+				free_ftrace_hash(free_hash);
+			free_hash = NULL;
+		}
+	} else {
+		if (!direct->count)
+			direct->count++;
+	}
  out_unlock:
 	mutex_unlock(&direct_mutex);
 
@@ -5036,6 +5101,7 @@ EXPORT_SYMBOL_GPL(register_ftrace_direct);
 int unregister_ftrace_direct(unsigned long ip, unsigned long addr)
 {
 	struct ftrace_func_entry *entry;
+	struct ftrace_direct_func *direct;
 	struct dyn_ftrace *rec;
 	int ret = -ENODEV;
 
@@ -5066,6 +5132,17 @@ int unregister_ftrace_direct(unsigned long ip, unsigned long addr)
 
 	remove_hash_entry(direct_functions, entry);
 
+	direct = ftrace_find_direct_func(addr);
+	if (!WARN_ON(!direct)) {
+		/* This is the good path (see the ! before WARN) */
+		direct->count--;
+		WARN_ON(direct->count < 0);
+		if (!direct->count) {
+			list_del_rcu(&direct->next);
+			synchronize_rcu_tasks();
+			kfree(direct);
+		}
+	}
  out_unlock:
 	mutex_unlock(&direct_mutex);