On Wed 2017-02-08 10:46:36, Josh Poimboeuf wrote:

On Wed, Feb 08, 2017 at 04:47:50PM +0100, Petr Mladek wrote:

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Notice in this case that klp_target_state is KLP_PATCHED.  Which means
that klp_complete_transition() would not call synchronize_rcu() at the
right time, nor would it call module_put().  It can be fixed with:

@@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch)
 			pr_warn("failed to enable patch '%s'\n",
 				patch->mod->name);
 
-			klp_unpatch_objects(patch);
+			klp_target_state = KLP_UNPATCHED;
 			klp_complete_transition();
 
 			return ret;

Great catch! Looks good to me.

As it turns out, klp_target_state isn't accessible from this file, so
I'll make a klp_cancel_transition() helper function which reverses
klp_target_state and calls klp_complete_transition().

Sound good to me.

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This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in
klp_try_complete_transition() gets moved to klp_complete_transition() as
you suggested.

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diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index 5efa262..1a77f05 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 +55,58 @@ 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);
+
+	/*
+	 * func should 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;
+
+	/*
+	 * Enforce the order of the ops->func_stack and func->transition reads.
+	 * The corresponding write barrier is in __klp_enable_patch().
+	 */
+	smp_rmb();

I was curious why the comment did not mention __klp_disable_patch().
It was related to the hours of thinking. I would like to avoid this
in the future and add a comment like.

	 * This barrier probably is not needed when the patch is being
	 * disabled. The patch is removed from the stack in
	 * klp_try_complete_transition() and there we need to call
	 * rcu_synchronize() to prevent seeing the patch on the stack
	 * at all.
	 *
	 * Well, it still might be needed to see func->transition
	 * when the patch is removed and the task is migrated. See
	 * the write barrier in __klp_disable_patch().

Agreed, though as you mentioned earlier, there's also the implicit
barrier in klp_update_patch_state(), which would execute first in such a
scenario.  So I think I'll update the barrier comments in
klp_update_patch_state().

You inspired me to a scenario with 3 CPUs:

CPU0			CPU1			CPU2

__klp_disable_patch()

  klp_init_transition()

    func->transition = true

  smp_wmb()

  klp_start_transition()

    set TIF_PATCH_PATCHPENDING

			klp_update_patch_state()

			  task->patch_state
			     = KLP_UNPATCHED

			  smp_mb()

						klp_ftrace_handler()
						  func = list_...

						  smp_rmb() /*needed?*/

						  if (func->transition)

I think this isn't possible.  Remember the comment I added to
klp_update_patch_state():

 * NOTE: If task is not 'current', the caller must ensure the task is inactive.
 * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value.

Right now klp_update_patch_state() is only called for current.
klp_ftrace_handler() on CPU2 would be running in the context of a
different task.

I agree that it is impossible with the current code. In fact, I cannot
imagine a way to migrate the task where the barrier would be needed.
The question if we could/should somehow document it. Something like

	* The barrier is not really needed when the patch is being
	* disabled. The value of func->transition would change
	* the result of this handled only after the task is migrated.
	* But the conditions for the migration are very limited
	* and practically include a full barrier, see
	* klp_update_patch_state().

Well, I'd like to avoid over-commenting this issue.  For v5 I've added
the following comment to klp_update_patch_state() -- see #2:

	/*
	 * This test_and_clear_tsk_thread_flag() call also serves as a read
	 * barrier (smp_rmb) for two cases:
	 *
	 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
	 *    klp_target_state read.  The corresponding write barrier is in
	 *    klp_init_transition().
	 *
	 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
	 *    of func->transition, if klp_ftrace_handler() is called later on
	 *    the same CPU.  See __klp_disable_patch().
	 */

Sounds good.

Is that sufficient?  If not, I could maybe add another related comment
in klp_ftrace_handler() which refers to this comment.

It would be nice. I do not want over-commenting either. On the other
hand, the code is really complex and it is not easy to understand
all the relations. The comments should safe us a lot of pain in
the long term.

Thanks a lot for working on it

Best Regards,
Petr