Re: [PATCH v3 1/3] ftrace: Introduce PERMANENT ftrace_ops flag

[Steven Rostedt]

On Wed, 16 Oct 2019 13:33:13 +0200
Miroslav Benes <mbenes@xxxxxxx> wrote:

> Livepatch uses ftrace for redirection to new patched functions. It means
> that if ftrace is disabled, all live patched functions are disabled as
> well. Toggling global 'ftrace_enabled' sysctl thus affect it directly.
> It is not a problem per se, because only administrator can set sysctl
> values, but it still may be surprising.
> 
> Introduce PERMANENT ftrace_ops flag to amend this. If the
> FTRACE_OPS_FL_PERMANENT is set on any ftrace ops, the tracing cannot be
> disabled by disabling ftrace_enabled. Equally, a callback with the flag
> set cannot be registered if ftrace_enabled is disabled.
> 
> Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> Reviewed-by: Kamalesh Babulal <kamalesh@xxxxxxxxxxxxxxxxxx>
> ---
>

I pulled in this patch as is, but then I realized we have a race. This
race has been there before this patch series, and actually, been there
since the dawn of ftrace.

I realized that the user can modify ftrace_enabled out of lock context.
That is, the ftrace_enabled is modified directly from the sysctl code,
without taking the ftrace_lock mutex. Which means if the user was
starting and stopping function tracing while playing with the
ftrace_enabled switch, it could potentially cause an accounting failure.

I'm going to apply this patch on top of yours. It reverses the role of
how ftrace_enabled is set in the sysctl handler. Instead of having it
directly modify ftrace_enabled, I have it modify a new variable called
sysctl_ftrace_enabled. I no longer need the last_ftrace_enabled. This
way we only need to set or disable ftrace_enabled on a change and if
all conditions are met.

Thoughts?

-- Steve

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 8385cafe4f9f..aa2e2c7cef9e 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -79,6 +79,7 @@ static inline int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *val
 #ifdef CONFIG_FUNCTION_TRACER
 
 extern int ftrace_enabled;
+extern int sysctl_ftrace_enabled;
 extern int
 ftrace_enable_sysctl(struct ctl_table *table, int write,
 		     void __user *buffer, size_t *lenp,
@@ -638,6 +639,7 @@ static inline void tracer_disable(void)
 {
 #ifdef CONFIG_FUNCTION_TRACER
 	ftrace_enabled = 0;
+	sysctl_ftrace_enabled = 0;
 #endif
 }
 
@@ -651,6 +653,7 @@ static inline int __ftrace_enabled_save(void)
 #ifdef CONFIG_FUNCTION_TRACER
 	int saved_ftrace_enabled = ftrace_enabled;
 	ftrace_enabled = 0;
+	sysctl_ftrace_enabled = 0;
 	return saved_ftrace_enabled;
 #else
 	return 0;
@@ -661,6 +664,7 @@ static inline void __ftrace_enabled_restore(int enabled)
 {
 #ifdef CONFIG_FUNCTION_TRACER
 	ftrace_enabled = enabled;
+	sysctl_ftrace_enabled = enabled;
 #endif
 }
 
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 00fcea236eba..773fdfc6c645 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -648,7 +648,7 @@ static struct ctl_table kern_table[] = {
 #ifdef CONFIG_FUNCTION_TRACER
 	{
 		.procname	= "ftrace_enabled",
-		.data		= &ftrace_enabled,
+		.data		= &sysctl_ftrace_enabled,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= ftrace_enable_sysctl,
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index dacb8b50a263..b55c9a4e2b5b 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -88,7 +88,7 @@ struct ftrace_ops ftrace_list_end __read_mostly = {
 
 /* ftrace_enabled is a method to turn ftrace on or off */
 int ftrace_enabled __read_mostly;
-static int last_ftrace_enabled;
+int sysctl_ftrace_enabled __read_mostly;
 
 /* Current function tracing op */
 struct ftrace_ops *function_trace_op __read_mostly = &ftrace_list_end;
@@ -6221,7 +6221,7 @@ void __init ftrace_init(void)
 	pr_info("ftrace: allocating %ld entries in %ld pages\n",
 		count, count / ENTRIES_PER_PAGE + 1);
 
-	last_ftrace_enabled = ftrace_enabled = 1;
+	sysctl_ftrace_enabled = ftrace_enabled = 1;
 
 	ret = ftrace_process_locs(NULL,
 				  __start_mcount_loc,
@@ -6265,6 +6265,7 @@ struct ftrace_ops global_ops = {
 static int __init ftrace_nodyn_init(void)
 {
 	ftrace_enabled = 1;
+	sysctl_ftrace_enabled = 1;
 	return 0;
 }
 core_initcall(ftrace_nodyn_init);
@@ -6714,6 +6715,7 @@ void ftrace_kill(void)
 {
 	ftrace_disabled = 1;
 	ftrace_enabled = 0;
+	sysctl_ftrace_enabled = 0;
 	ftrace_trace_function = ftrace_stub;
 }
 
@@ -6796,10 +6798,12 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
 
 	ret = proc_dointvec(table, write, buffer, lenp, ppos);
 
-	if (ret || !write || (last_ftrace_enabled == !!ftrace_enabled))
+	if (ret || !write || (ftrace_enabled == !!sysctl_ftrace_enabled))
 		goto out;
 
-	if (ftrace_enabled) {
+	if (sysctl_ftrace_enabled) {
+
+		ftrace_enabled = true;
 
 		/* we are starting ftrace again */
 		if (rcu_dereference_protected(ftrace_ops_list,
@@ -6810,19 +6814,21 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
 
 	} else {
 		if (is_permanent_ops_registered()) {
-			ftrace_enabled = true;
 			ret = -EBUSY;
 			goto out;
 		}
 
+		ftrace_enabled = false;
+
 		/* stopping ftrace calls (just send to ftrace_stub) */
 		ftrace_trace_function = ftrace_stub;
 
 		ftrace_shutdown_sysctl();
 	}
 
-	last_ftrace_enabled = !!ftrace_enabled;
  out:
+	sysctl_ftrace_enabled = ftrace_enabled;
+
 	mutex_unlock(&ftrace_lock);
 	return ret;
 }