On Fri, Dec 23, 2016 at 11:18:03AM +0100, Petr Mladek wrote:

On Fri 2016-12-23 10:24:35, Miroslav Benes wrote:

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diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index 5efa262..e79ebb5 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c

@@ -29,6 +29,7 @@
 #include <linux/bug.h>
 #include <linux/printk.h>
 #include "patch.h"
+#include "transition.h"
 
 static LIST_HEAD(klp_ops);
 

@@ -54,15 +55,53 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 {
 	struct klp_ops *ops;
 	struct klp_func *func;
+	int patch_state;
 
 	ops = container_of(fops, struct klp_ops, fops);
 
 	rcu_read_lock();
+
 	func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
 				      stack_node);
-	if (WARN_ON_ONCE(!func))
+
+	if (!func)
 		goto unlock;

Why do you removed the WARN_ON_ONCE(), please?

We still add the function on the stack before registering
the ftrace handler. Also we unregister the ftrace handler
before removing the the last entry from the stack.

AFAIK, unregister_ftrace_function() calls rcu_synchronize()'
to make sure that no-one is inside the handler once finished.
Mirek knows more about it.

Hm, this is news to me.  Mirek, please share :-)

Well, I think the whole thing is well described in emails I exchanged with 
Steven few months ago. See [1].

[1] http://lkml.kernel.org/r/alpine.LNX.2.00.1608081041060.10833@pobox.suse.cz
 

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If this is not true, we have a problem. For example,
we call kfree(ops) after unregister_ftrace_function();

Agreed.

TL;DR - we should be ok as long as we do not do crazy things in the 
handler, deliberate sleeping for example.

WARN_ON_ONCE() may be crazy too. I think we discussed it long ago and we 
came to an agreement to remove it.

There are definitely situations where this might hurt. For example,
when we redirect a function called under logbuf_lock.

On the other hand, there is a work in progress[1][2] that will mitigate
this risk a lot. Also this warning would be printed only when
something goes wrong. IMHO, it is worth the risk. It will succeed
in 99,999% cases and it might save us some headache when debugging
random crashes of the system.

Anyway, if there is a reason to remove the warning, it should be
described. And if it is not strictly related to this patch, it should
be handled separately.

[1] https://lkml.kernel.org/r/20161221143605.2272-1-sergey.senozhatsky@gmail.com
[2] https://lkml.kernel.org/r/1461333180-2897-1-git-send-email-sergey.senozhatsky@gmail.com

Yeah, I'm thinking we should keep the warning to catch any bugs in case
any of our ftrace assumptions change.  Maybe I should add a comment:

	/*
	 * func can never be NULL because preemption should be disabled
	 * here and unregister_ftrace_function() does the equivalent of
	 * a synchronize_sched() before the func_stack removal.
	 */
	if (WARN_ON_ONCE(!func))
		goto unlock;

-- 
Josh