[PATCH v3 net 4/5] ipv6: Avoid double dst_free

Martin KaFai Lau
2015-09-11T18:07:39Z

It is a prep work to get dst freeing from fib tree undergo
a rcu grace period.

The following is a common paradigm:
if (ip6_del_rt(rt))
dst_free(rt)

which means, if rt cannot be deleted from the fib tree, dst_free(rt) now.
1. We don't know the ip6_del_rt(rt) failure is because it
was not managed by fib tree (e.g. DST_NOCACHE) or it had already been
removed from the fib tree.
2. If rt had been managed by the fib tree, ip6_del_rt(rt) failure means
dst_free(rt) has been called already. A second
dst_free(rt) is not always obviously safe. The rt may have
been destroyed already.
3. If rt is a DST_NOCACHE, dst_free(rt) should not be called.
4. It is a stopper to make dst freeing from fib tree undergo a
rcu grace period.

This patch is to use a DST_NOCACHE flag to indicate a rt is
not managed by the fib tree.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
net/ipv6/addrconf.c | 7 +++----
net/ipv6/ip6_fib.c | 11 +++++++++--
net/ipv6/route.c | 7 ++++---
3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index 030fefd..9001133 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -5127,13 +5127,12 @@ static void __ipv6_ifa_notify(int event, struct inet6_ifaddr *ifp)

rt = addrconf_get_prefix_route(&ifp->peer_addr, 128,
ifp->idev->dev, 0, 0);
- if (rt && ip6_del_rt(rt))
- dst_free(&rt->dst);
+ if (rt)
+ ip6_del_rt(rt);
}
dst_hold(&ifp->rt->dst);

- if (ip6_del_rt(ifp->rt))
- dst_free(&ifp->rt->dst);
+ ip6_del_rt(ifp->rt);

rt_genid_bump_ipv6(net);
break;
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 418d982..e68350b 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -933,6 +933,10 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
int replace_required = 0;
int sernum = fib6_new_sernum(info->nl_net);

+ if (WARN_ON_ONCE((rt->dst.flags & DST_NOCACHE) &&
+ !atomic_read(&rt->dst.__refcnt)))
+ return -EINVAL;
+
if (info->nlh) {
if (!(info->nlh->nlmsg_flags & NLM_F_CREATE))
allow_create = 0;
@@ -1025,6 +1029,7 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
fib6_start_gc(info->nl_net, rt);
if (!(rt->rt6i_flags & RTF_CACHE))
fib6_prune_clones(info->nl_net, pn);
+ rt->dst.flags &= ~DST_NOCACHE;
}

out:
@@ -1049,7 +1054,8 @@ out:
atomic_inc(&pn->leaf->rt6i_ref);
}
#endif
- dst_free(&rt->dst);
+ if (!(rt->dst.flags & DST_NOCACHE))
+ dst_free(&rt->dst);
}
return err;

@@ -1060,7 +1066,8 @@ out:
st_failure:
if (fn && !(fn->fn_flags & (RTN_RTINFO|RTN_ROOT)))
fib6_repair_tree(info->nl_net, fn);
- dst_free(&rt->dst);
+ if (!(rt->dst.flags & DST_NOCACHE))
+ dst_free(&rt->dst);
return err;
#endif
}
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 53617d7..3d3c1b2 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1322,8 +1322,7 @@ static void ip6_link_failure(struct sk_buff *skb)
if (rt) {
if (rt->rt6i_flags & RTF_CACHE) {
dst_hold(&rt->dst);
- if (ip6_del_rt(rt))
- dst_free(&rt->dst);
+ ip6_del_rt(rt);
} else if (rt->rt6i_node && (rt->rt6i_flags & RTF_DEFAULT)) {
rt->rt6i_node->fn_sernum = -1;
}
@@ -2028,7 +2027,8 @@ static int __ip6_del_rt(struct rt6_info *rt, struct nl_info *info)
struct fib6_table *table;
struct net *net = dev_net(rt->dst.dev);

- if (rt == net->ipv6.ip6_null_entry) {
+ if (rt == net->ipv6.ip6_null_entry ||
+ rt->dst.flags & DST_NOCACHE) {
err = -ENOENT;
goto out;
}
@@ -2515,6 +2515,7 @@ struct rt6_info *addrconf_dst_alloc(struct inet6_dev *idev,
rt->rt6i_dst.addr = *addr;
rt->rt6i_dst.plen = 128;
rt->rt6i_table = fib6_get_table(net, RT6_TABLE_LOCAL);
+ rt->dst.flags |= DST_NOCACHE;

atomic_set(&rt->dst.__refcnt, 1);

--
2.5.1