mergesort: Avoid accidental set element reordering

In corner cases, expr_msort_cmp() may return 0 for two non-identical
elements. An example are ORed tcp flags: 'syn' and 'syn | ack' are
considered the same value since expr_msort_value() reduces the latter to
its LHS.

Keeping the above in mind and looking at how list_expr_sort() works: The
list in 'head' is cut in half, the first half put into the temporary
list 'list' and finally 'list' is merged back into 'head' considering
each element's position. Shall expr_msort_cmp() return 0 for two
elements, the one from 'list' ends up after the one in 'head', thus
reverting their previous ordering.

The practical implication is that output never matches input for the
sample set '{ syn, syn | ack }' as the sorting after delinearization in
netlink_list_setelems() keeps swapping the elements. Out of coincidence,
the commit this fixes itself illustrates the use-case this breaks,
namely tracking a ruleset in git: Each ruleset reload will trigger an
update to the stored dump.

This change breaks interval set element deletion because __set_delete()
implicitly relies upon this reordering of duplicate entries by inserting
a clone of the one to delete into the start (via list_move()) and after
sorting assumes the clone will end up right behind the original. Fix
this by calling list_move_tail() instead.

Fixes: 14ee0a979b622 ("src: sort set elements in netlink_get_setelems()")
Signed-off-by: Phil Sutter <phil@nwl.cc>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/intervals.c b/src/intervals.c
index 68728349..6c3f36fe 100644
--- a/src/intervals.c
+++ b/src/intervals.c
@@ -466,7 +466,7 @@ static int __set_delete(struct list_head *msgs, struct expr *i,	struct set *set,
 			unsigned int debug_mask)
 {
 	i->flags |= EXPR_F_REMOVE;
-	list_move(&i->list, &existing_set->init->expressions);
+	list_move_tail(&i->list, &existing_set->init->expressions);
 	list_expr_sort(&existing_set->init->expressions);
 
 	return setelem_delete(msgs, set, init, existing_set->init, debug_mask);