Re: [Xen-devel] [PATCH 2/2] libxl: fix stale timeout event callback race

[Jim Fehlig <jfehlig@xxxxxxxx>]

Ian Jackson wrote:
> Jim Fehlig writes ("Re: [Xen-devel] [PATCH 2/2] libxl: fix stale timeout 
> event callback race"):
>   
>> Ian Jackson wrote:
>>     
>>> This is a forwards-compatible change for applications using the libxl
>>> API, and will hopefully eliminate these races in callback-supplying
>>> applications (such as libvirt) without the need for corresponding
>>> changes to the application.
>>>       
>
> When I wrote this of course I forgot to mention that previously libxl
> would never call libxl_osevent_hooks->timeout_modify and now it never
> calls ->timeout_deregister.
>
> So this change can expose bugs in the application's implementation of
> ->timeout_modify.
>
>   
>> I'm not sure how to avoid changing the application (libvirt). Currently,
>> the libvirt libxl driver removes the timeout from the libvirt event loop
>> in the timeout_deregister() function. This function is never called now
>> and hence the timeout is never removed. I had to make the following
>> change in the libxl driver to use your patches
>>     
>
> I think this is because of a bug of the kind I describe above.
>
>   
>> - gettimeofday(&now, NULL);
>> - timeout = (abs_t.tv_usec - now.tv_usec) / 1000;
>> - timeout += (abs_t.tv_sec - now.tv_sec) * 1000;
>>     
>
> Specifically, this code has an integer arithmetic overflow.
>   

Well, this patch is removing that buggy code :).  After again reading
libxl_event.h, I'm considering the below patch in the libvirt libxl
driver.  The change is primarily inspired by this comment for
libxl_osevent_occurred_timeout:

/* Implicitly, on entry to this function the timeout has been
 * deregistered.  If _occurred_timeout is called, libxl will not
 * call timeout_deregister; if it wants to requeue the timeout it
 * will call timeout_register again.
 */

Regards,
Jim

diff --git a/src/libxl/libxl_driver.c b/src/libxl/libxl_driver.c
index 302f81c..d4055be 100644
--- a/src/libxl/libxl_driver.c
+++ b/src/libxl/libxl_driver.c
@@ -184,6 +184,8 @@ static void libxlTimerCallback(int timer
ATTRIBUTE_UNUSED, void *timer_v)
 {
     struct libxlOSEventHookTimerInfo *timer_info = timer_v;
 
+    virEventRemoveTimeout(timer_info->id);
+    timer_info->id = -1;
     libxl_osevent_occurred_timeout(timer_info->priv->ctx,
timer_info->xl_priv);
 }
 
@@ -225,16 +227,12 @@ static int libxlTimeoutRegisterEventHook(void *priv,
 
 static int libxlTimeoutModifyEventHook(void *priv ATTRIBUTE_UNUSED,
                                        void **hndp,
-                                       struct timeval abs_t)
+                                       struct timeval abs_t
ATTRIBUTE_UNUSED)
 {
-    struct timeval now;
-    int timeout;
     struct libxlOSEventHookTimerInfo *timer_info = *hndp;
 
-    gettimeofday(&now, NULL);
-    timeout = (abs_t.tv_usec - now.tv_usec) / 1000;
-    timeout += (abs_t.tv_sec - now.tv_sec) * 1000;
-    virEventUpdateTimeout(timer_info->id, timeout);
+    if (timer_info->id > 0)
+        virEventUpdateTimeout(timer_info->id, 0);
     return 0;
 }
 

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