[PATCH][net-next] ipv6: drop container_of when convert dst to rt6_info

[Li RongQing]

we can save container_of computation and return dst directly,
since dst is always first member of struct rt6_info

Add a BUILD_BUG_ON() to catch any change that could break this
assertion.

Signed-off-by: Li RongQing <lirongq...@baidu.com>
---
 include/net/ip6_route.h | 4 +++-
 net/ipv6/route.c        | 6 +++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 7b9c82de11cc..1f09298634cb 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -194,8 +194,10 @@ static inline const struct rt6_info *skb_rt6_info(const 
struct sk_buff *skb)
        const struct dst_entry *dst = skb_dst(skb);
        const struct rt6_info *rt6 = NULL;
 
+       BUILD_BUG_ON(offsetof(struct rt6_info, dst) != 0);
+
        if (dst)
-               rt6 = container_of(dst, struct rt6_info, dst);
+               rt6 = (struct rt6_info *)dst;
 
        return rt6;
 }
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index d28f83e01593..3fb8034fc2d0 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -217,7 +217,7 @@ static struct neighbour *ip6_dst_neigh_lookup(const struct 
dst_entry *dst,
                                              struct sk_buff *skb,
                                              const void *daddr)
 {
-       const struct rt6_info *rt = container_of(dst, struct rt6_info, dst);
+       const struct rt6_info *rt = (struct rt6_info *)dst;
 
        return ip6_neigh_lookup(&rt->rt6i_gateway, dst->dev, skb, daddr);
 }
@@ -2187,7 +2187,7 @@ static struct dst_entry *ip6_dst_check(struct dst_entry 
*dst, u32 cookie)
        struct fib6_info *from;
        struct rt6_info *rt;
 
-       rt = container_of(dst, struct rt6_info, dst);
+       rt = (struct rt6_info *)dst;
 
        rcu_read_lock();
 
@@ -4911,7 +4911,7 @@ static int inet6_rtm_getroute(struct sk_buff *in_skb, 
struct nlmsghdr *nlh,
        }
 
 
-       rt = container_of(dst, struct rt6_info, dst);
+       rt = (struct rt6_info *)dst;
        if (rt->dst.error) {
                err = rt->dst.error;
                ip6_rt_put(rt);
-- 
2.16.2