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[Xen-changelog] [linux-2.6.18-xen] pv-on-hvm/kexec: prevent crash in xenwatch_thread() when stale watch events arrive

# HG changeset patch
# User Olaf Hering <olaf@xxxxxxxxx>
# Date 1317893088 -7200
# Node ID 40e33b2fce948cbdcc0ec27404b58b1acc9b7fc8
# Parent  92c7972042d2b20a975acfeae9a469284523caac
pv-on-hvm/kexec: prevent crash in xenwatch_thread() when stale watch events 

commit c4c303c7c5679b4b368e12f41124aee29c325b76

During repeated kexec boots xenwatch_thread() can crash because
xenbus_watch->callback is cleared by xenbus_watch_path() if a node/token
combo for a new watch happens to match an already registered watch from
an old kernel.  In this case xs_watch returns -EEXISTS, then
register_xenbus_watch() does not remove the to-be-registered watch from
the list of active watches but returns the -EEXISTS to the caller

Because the watch is still active in xenstored it will cause an event
which will arrive in the new kernel. process_msg() will find the
encapsulated struct xenbus_watch in its list of registered watches and
puts the "empty" watch handle in the queue for xenwatch_thread().
xenwatch_thread() then calls ->callback which was cleared earlier by

To prevent that crash in a guest running on an old xen toolstack remove
the special -EEXIST handling.

 - remove the EEXIST handing in register_xenbus_watch() instead of
   checking for ->callback in process_msg()

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
Committed-by: Jan Beulich <jbeulich@xxxxxxxx>

diff -r 92c7972042d2 -r 40e33b2fce94 drivers/xen/xenbus/xenbus_xs.c
--- a/drivers/xen/xenbus/xenbus_xs.c    Thu Oct 06 09:47:57 2011 +0200
+++ b/drivers/xen/xenbus/xenbus_xs.c    Thu Oct 06 11:24:48 2011 +0200
@@ -656,8 +656,7 @@
        err = xs_watch(watch->node, token);
-       /* Ignore errors due to multiple registration. */
-       if ((err != 0) && (err != -EEXIST)) {
+       if (err) {

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