linux-kernel - Re: [PATCH] tracing/profile: Fix profile_disable vs module

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Message-ID: <alpine.DEB.2.00.0908261012020.23885@gandalf.stny.rr.com>
Date:	Wed, 26 Aug 2009 10:37:11 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Li Zefan <lizf@...fujitsu.com>
cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload


On Wed, 26 Aug 2009, Li Zefan wrote:

> Peter Zijlstra wrote:
> > On Wed, 2009-08-26 at 08:46 +0200, Peter Zijlstra wrote:
> > 
> >> Aahh, I see the bug, its only ftrace that knows about the module, not
> >> tracepoints themselves, _that_ needs fixing.
> > 
> > You could possibly do something like:
> > 
> >  struct module *tp_mod = __module_address(&some_tp_symbol);
> >  struct module *cb_mod = __module_text_address(func);
> > 
> >  if (tp_mod && tp_mod != cb_mod) {
> > 	ret = try_get_module(tp_mod);
> > 	if (ret)
> > 		goto fail;
> >  }
> > 
> > in register_trace_##name() or thereabout.
> > 
> 
> Actually I tried it, but it didn't work. As I said, You can't find
> any tp symbol when registering tp callback. The same example again:
> 
> 	In module bar, we have register_trace_foo()
> 	In module foo, we have DEFINE_TRACE(foo) and trace_foo().
> 
> bar doesn't know any symbol of foo, so it can't bump foo's refcnt,
> 
> *Note: you can load module bar without loading module foo*

WTF!!!!

We can register a trace point that is defined in another module without 
having that module?? How is that possible? That looks totally busted, and 
that is not a case that I think we need to worry about, except to prevent 
it from ever happening.

As for ref counts, would something like this work?

 (untested)

-- Steve

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0341f2e..055275b 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -109,8 +109,9 @@ struct tracepoint {
 #define EXPORT_TRACEPOINT_SYMBOL(name)					\
 	EXPORT_SYMBOL(__tracepoint_##name)
 
-extern void tracepoint_update_probe_range(struct tracepoint *begin,
-	struct tracepoint *end);
+extern void tracepoint_update_probe_range(struct module *,
+					  struct tracepoint *begin,
+					  struct tracepoint *end);
 
 #else /* !CONFIG_TRACEPOINTS */
 #define DECLARE_TRACE_WITH_CALLBACK(name, proto, args, reg, unreg)	\
diff --git a/kernel/module.c b/kernel/module.c
index b182143..a8e69fa 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2974,7 +2974,7 @@ void module_update_tracepoints(void)
 	mutex_lock(&module_mutex);
 	list_for_each_entry(mod, &modules, list)
 		if (!mod->taints)
-			tracepoint_update_probe_range(mod->tracepoints,
+			tracepoint_update_probe_range(mod, mod->tracepoints,
 				mod->tracepoints + mod->num_tracepoints);
 	mutex_unlock(&module_mutex);
 }
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 06f165a..b150255 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -274,7 +274,8 @@ static void disable_tracepoint(struct tracepoint *elem)
  * Updates the probe callback corresponding to a range of tracepoints.
  */
 void
-tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
+tracepoint_update_probe_range(struct module *mod,
+			      struct tracepoint *begin, struct tracepoint *end)
 {
 	struct tracepoint *iter;
 	struct tracepoint_entry *mark_entry;
@@ -286,9 +287,15 @@ tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
 	for (iter = begin; iter < end; iter++) {
 		mark_entry = get_tracepoint(iter->name);
 		if (mark_entry) {
+			if (mod) {
+				if (!try_module_get(mod))
+					continue;
+			}
 			set_tracepoint(&mark_entry, iter,
 					!!mark_entry->refcount);
 		} else {
+			if (mod)
+				module_put(mod);
 			disable_tracepoint(iter);
 		}
 	}
@@ -301,7 +308,7 @@ tracepoint_update_probe_range(struct tracepoint *begin, struct tracepoint *end)
 static void tracepoint_update_probes(void)
 {
 	/* Core kernel tracepoints */
-	tracepoint_update_probe_range(__start___tracepoints,
+	tracepoint_update_probe_range(NULL, __start___tracepoints,
 		__stop___tracepoints);
 	/* tracepoints in modules. */
 	module_update_tracepoints();
@@ -556,7 +563,7 @@ int tracepoint_module_notify(struct notifier_block *self,
 	switch (val) {
 	case MODULE_STATE_COMING:
 	case MODULE_STATE_GOING:
-		tracepoint_update_probe_range(mod->tracepoints,
+		tracepoint_update_probe_range(mod, mod->tracepoints,
 			mod->tracepoints + mod->num_tracepoints);
 		break;
 	}
--
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