ipv6: do not overwrite inetpeer metrics prematurely

If an IPv6 host route with metrics exists, an attempt to add a
new route for the same target with different metrics fails but
rewrites the metrics anyway:

12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1000
12sp0:~ # ip -6 route show
fe80::/64 dev eth0  proto kernel  metric 256
fec0::1 dev eth0  metric 1024  rto_min lock 1s
12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1500
RTNETLINK answers: File exists
12sp0:~ # ip -6 route show
fe80::/64 dev eth0  proto kernel  metric 256
fec0::1 dev eth0  metric 1024  rto_min lock 1.5s

This is caused by all IPv6 host routes using the metrics in
their inetpeer (or the shared default). This also holds for the
new route created in ip6_route_add() which shares the metrics
with the already existing route and thus ip6_route_add()
rewrites the metrics even if the new route ends up not being
used at all.

Another problem is that old metrics in inetpeer can reappear
unexpectedly for a new route, e.g.

12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1000
12sp0:~ # ip route del fec0::1
12sp0:~ # ip route add fec0::1 dev eth0
12sp0:~ # ip route change fec0::1 dev eth0 hoplimit 10
12sp0:~ # ip -6 route show
fe80::/64 dev eth0  proto kernel  metric 256
fec0::1 dev eth0  metric 1024  hoplimit 10 rto_min lock 1s

Resolve the first problem by moving the setting of metrics down
into fib6_add_rt2node() to the point we are sure we are
inserting the new route into the tree. Second problem is
addressed by introducing new flag DST_METRICS_FORCE_OVERWRITE
which is set for a new host route in ip6_route_add() and makes
ipv6_cow_metrics() always overwrite the metrics in inetpeer
(even if they are not "new"); it is reset after that.

v5: use a flag in _metrics member rather than one in flags

v4: fix a typo making a condition always true (thanks to Hannes
Frederic Sowa)

v3: rewritten based on David Miller's idea to move setting the
metrics (and allocation in non-host case) down to the point we
already know the route is to be inserted. Also rebased to
net-next as it is quite late in the cycle.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 2 deletions

diff --git a/include/net/dst.h b/include/net/dst.h
index e01a826f2a9c..46ed958e0c6e 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -108,9 +108,11 @@ struct dst_entry {
 u32 *dst_cow_metrics_generic(struct dst_entry *dst, unsigned long old);
 extern const u32 dst_default_metrics[];
 
-#define DST_METRICS_READ_ONLY	0x1UL
+#define DST_METRICS_READ_ONLY		0x1UL
+#define DST_METRICS_FORCE_OVERWRITE	0x2UL
+#define DST_METRICS_FLAGS		0x3UL
 #define __DST_METRICS_PTR(Y)	\
-	((u32 *)((Y) & ~DST_METRICS_READ_ONLY))
+	((u32 *)((Y) & ~DST_METRICS_FLAGS))
 #define DST_METRICS_PTR(X)	__DST_METRICS_PTR((X)->_metrics)
 
 static inline bool dst_metrics_read_only(const struct dst_entry *dst)
@@ -118,6 +120,11 @@ static inline bool dst_metrics_read_only(const struct dst_entry *dst)
 	return dst->_metrics & DST_METRICS_READ_ONLY;
 }
 
+static inline void dst_metrics_set_force_overwrite(struct dst_entry *dst)
+{
+	dst->_metrics |= DST_METRICS_FORCE_OVERWRITE;
+}
+
 void __dst_destroy_metrics_generic(struct dst_entry *dst, unsigned long old);
 
 static inline void dst_destroy_metrics_generic(struct dst_entry *dst)