net: ipv4: update fnhe_pmtu when first hop’s MTU changes [Linux 3.16.62]

This Linux kernel change "net: ipv4: update fnhe_pmtu when first hop’s MTU changes" is included in the Linux 3.16.62 release. This change is authored by Sabrina Dubroca <sd [at] queasysnail.net> on Tue Oct 9 17:48:14 2018 +0200. The commit for this change in Linux stable tree is b427832 (patch) which is from upstream commit af7d6cc. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream af7d6cc.

net: ipv4: update fnhe_pmtu when first hop's MTU changes

commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 upstream.

Since commit 5aad1de5ea2c ("ipv4: use separate genid for next hop
exceptions"), exceptions get deprecated separately from cached
routes. In particular, administrative changes don't clear PMTU anymore.

As Stefano described in commit e9fa1495d738 ("ipv6: Reflect MTU changes
on PMTU of exceptions for MTU-less routes"), the PMTU discovered before
the local MTU change can become stale:
 - if the local MTU is now lower than the PMTU, that PMTU is now
   incorrect
 - if the local MTU was the lowest value in the path, and is increased,
   we might discover a higher PMTU

Similarly to what commit e9fa1495d738 did for IPv6, update PMTU in those
cases.

If the exception was locked, the discovered PMTU was smaller than the
minimal accepted PMTU. In that case, if the new local MTU is smaller
than the current PMTU, let PMTU discovery figure out if locking of the
exception is still needed.

To do this, we need to know the old link MTU in the NETDEV_CHANGEMTU
notifier. By the time the notifier is called, dev->mtu has been
changed. This patch adds the old MTU as additional information in the
notifier structure, and a new call_netdevice_notifiers_u32() function.

Fixes: 5aad1de5ea2c ("ipv4: use separate genid for next hop exceptions")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16:
 - Pass net_device argument to call_netdevice_notifiers_info()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

There are 90 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 include/linux/netdevice.h |  7 +++++++
 include/net/ip_fib.h      |  1 +
 net/core/dev.c            | 28 ++++++++++++++++++++++++--
 net/ipv4/fib_frontend.c   |  4 ++++
 net/ipv4/fib_semantics.c  | 50 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 88 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 1d2c259..5d74c84 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1867,6 +1867,13 @@ struct netdev_notifier_info {
    struct net_device *dev;
 };

+struct netdev_notifier_info_ext {
+   struct netdev_notifier_info info; /* must be first */
+   union {
+       u32 mtu;
+   } ext;
+};
+
 struct netdev_notifier_change_info {
    struct netdev_notifier_info info; /* must be first */
    unsigned int flags_changed;
diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index 905cd10..68d6df7 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h
@@ -293,6 +293,7 @@ static inline int fib_num_tclassid_users(struct net *net)
 int fib_sync_down_dev(struct net_device *dev, int force);
 int fib_sync_down_addr(struct net *net, __be32 local);
 int fib_sync_up(struct net_device *dev);
+void fib_sync_mtu(struct net_device *dev, u32 orig_mtu);
 void fib_select_multipath(struct fib_result *res);

 /* Exported by fib_trie.c */
diff --git a/net/core/dev.c b/net/core/dev.c
index 24afa72..0a32a1d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1598,6 +1598,28 @@ int call_netdevice_notifiers(unsigned long val, struct net_device *dev)
 }
 EXPORT_SYMBOL(call_netdevice_notifiers);

+/**
+ * call_netdevice_notifiers_mtu - call all network notifier blocks
+ * @val: value passed unmodified to notifier function
+ * @dev: net_device pointer passed unmodified to notifier function
+ * @arg: additional u32 argument passed to the notifier function
+ *
+ * Call all network notifier blocks.  Parameters and return value
+ * are as for raw_notifier_call_chain().
+ */
+static int call_netdevice_notifiers_mtu(unsigned long val,
+                   struct net_device *dev, u32 arg)
+{
+   struct netdev_notifier_info_ext info = {
+       .info.dev = dev,
+       .ext.mtu = arg,
+   };
+
+   BUILD_BUG_ON(offsetof(struct netdev_notifier_info_ext, info) != 0);
+
+   return call_netdevice_notifiers_info(val, dev, &info.info);
+}
+
 static struct static_key netstamp_needed __read_mostly;
 #ifdef HAVE_JUMP_LABEL
 static atomic_t netstamp_needed_deferred;
@@ -5688,14 +5710,16 @@ int dev_set_mtu(struct net_device *dev, int new_mtu)
    err = __dev_set_mtu(dev, new_mtu);

    if (!err) {
-       err = call_netdevice_notifiers(NETDEV_CHANGEMTU, dev);
+       err = call_netdevice_notifiers_mtu(NETDEV_CHANGEMTU, dev,
+                          orig_mtu);
        err = notifier_to_errno(err);
        if (err) {
            /* setting mtu back and notifying everyone again,
             * so that they have a chance to revert changes.
             */
            __dev_set_mtu(dev, orig_mtu);
-           call_netdevice_notifiers(NETDEV_CHANGEMTU, dev);
+           call_netdevice_notifiers_mtu(NETDEV_CHANGEMTU, dev,
+                            new_mtu);
        }
    }
    return err;
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 2020abf..c8c7e1a 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1046,6 +1046,7 @@ static int fib_inetaddr_event(struct notifier_block *this, unsigned long event,
 static int fib_netdev_event(struct notifier_block *this, unsigned long event, void *ptr)
 {
    struct net_device *dev = netdev_notifier_info_to_dev(ptr);
+   struct netdev_notifier_info_ext *info_ext = ptr;
    struct in_device *in_dev;
    struct net *net = dev_net(dev);

@@ -1074,6 +1075,9 @@ static int fib_netdev_event(struct notifier_block *this, unsigned long event, vo
        fib_disable_ip(dev, 0);
        break;
    case NETDEV_CHANGEMTU:
+       fib_sync_mtu(dev, info_ext->ext.mtu);
+       rt_cache_flush(net);
+       break;
    case NETDEV_CHANGE:
        rt_cache_flush(net);
        break;
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 5b4580d..6f44569 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1122,6 +1122,56 @@ int fib_sync_down_addr(struct net *net, __be32 local)
    return ret;
 }

+/* Update the PMTU of exceptions when:
+ * - the new MTU of the first hop becomes smaller than the PMTU
+ * - the old MTU was the same as the PMTU, and it limited discovery of
+ *   larger MTUs on the path. With that limit raised, we can now
+ *   discover larger MTUs
+ * A special case is locked exceptions, for which the PMTU is smaller
+ * than the minimal accepted PMTU:
+ * - if the new MTU is greater than the PMTU, don't make any change
+ * - otherwise, unlock and set PMTU
+ */
+static void nh_update_mtu(struct fib_nh *nh, u32 new, u32 orig)
+{
+   struct fnhe_hash_bucket *bucket;
+   int i;
+
+   bucket = rcu_dereference_protected(nh->nh_exceptions, 1);
+   if (!bucket)
+       return;
+
+   for (i = 0; i < FNHE_HASH_SIZE; i++) {
+       struct fib_nh_exception *fnhe;
+
+       for (fnhe = rcu_dereference_protected(bucket[i].chain, 1);
+            fnhe;
+            fnhe = rcu_dereference_protected(fnhe->fnhe_next, 1)) {
+           if (fnhe->fnhe_mtu_locked) {
+               if (new <= fnhe->fnhe_pmtu) {
+                   fnhe->fnhe_pmtu = new;
+                   fnhe->fnhe_mtu_locked = false;
+               }
+           } else if (new < fnhe->fnhe_pmtu ||
+                  orig == fnhe->fnhe_pmtu) {
+               fnhe->fnhe_pmtu = new;
+           }
+       }
+   }
+}
+
+void fib_sync_mtu(struct net_device *dev, u32 orig_mtu)
+{
+   unsigned int hash = fib_devindex_hashfn(dev->ifindex);
+   struct hlist_head *head = &fib_info_devhash[hash];
+   struct fib_nh *nh;
+
+   hlist_for_each_entry(nh, head, nh_hash) {
+       if (nh->nh_dev == dev)
+           nh_update_mtu(nh, dev->mtu, orig_mtu);
+   }
+}
+
 int fib_sync_down_dev(struct net_device *dev, int force)
 {
    int ret = 0;

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