ndisc: Remove unused space at tail of skb for ndisc messages. (TAKE 3)

Currently, the size of skb allocated for NDISC is MAX_HEADER + LL_RESERVED_SPACE(dev) + packet length + dev->needed_tailroom, but only LL_RESERVED_SPACE(dev) bytes is "reserved" for headers. As a result, the skb looks like this (after construction of the message):

head data tail end

+--------------------------------------------------------------+
+           |                      |          |                |
+--------------------------------------------------------------+
|<-hlen---->|<---ipv6 packet------>|<--tlen-->|<--MAX_HEADER-->|
    =LL_                               = dev
     RESERVED_                           ->needed_
     SPACE(dev)                            tailroom

As the name implies, "MAX_HEADER" is used for headers, and should be "reserved" in prior to packet construction. Or, if some space is really required at the tail of ther skb, it should be explicitly documented.

We have several option after construction of NDISC message:

Option 1:

head data tail end

+---------------------------------------------+
+           |                      |          |
+---------------------------------------------+
|<-hlen---->|<---ipv6 packet------>|<--tlen-->|
   =LL_                                = dev
    RESERVED_                           ->needed_
    SPACE(dev)                            tailroom

Option 2:

head data tail end

+--------------------------------------------------+
+                |                      |          |
+--------------------------------------------------+
|<--MAX_HEADER-->|<---ipv6 packet------>|<--tlen-->|
                                            = dev
                                             ->needed_
                                               tailroom

Option 3:

head data tail end

+--------------------------------------------------------------+
+                |           |                      |          |
+--------------------------------------------------------------+
|<--MAX_HEADER-->|<-hlen---->|<---ipv6 packet------>|<--tlen-->|
                    =LL_                                = dev
                     RESERVED_                          ->needed_
                     SPACE(dev)                           tailroom

Our tunnel drivers try expanding headroom and the space for tunnel encapsulation was not a mandatory space -- so we are not seeing bugs here --, but just for optimization for performance critial situations.

Since NDISC messages are not performance critical unlike TCP, and as we know outgoing device, LL_RESERVED_SPACE(dev) should be just enough for the device in most (if not all) cases:
LL_RESERVED_SPACE(dev) <= LL_MAX_HEADER <= MAX_HEADER
Note that LL_RESERVED_SPACE(dev) is also enough for NDISC over SIT (e.g., ISATAP).

So, I think Option 1 is just fine here.

---
 net/ipv6/ndisc.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 6574175..4c4ccf7 100644
@@ -395,7 +395,7 @@ static struct sk_buff *ndisc_build_skb(struct net_device *dev,
 		len += ndisc_opt_addr_space(dev);
 
 	skb = sock_alloc_send_skb(sk,
-				  (MAX_HEADER + sizeof(struct ipv6hdr) +
+				  (sizeof(struct ipv6hdr) +
 				   len + hlen + tlen),
 				  1, &err);
 	if (!skb) {

@@ -1439,7 +1439,7 @@ void ndisc_send_redirect(struct sk_buff *skb, const struct in6_addr *target)
 	hlen = LL_RESERVED_SPACE(dev);
 	tlen = dev->needed_tailroom;
 	buff = sock_alloc_send_skb(sk,
-				   (MAX_HEADER + sizeof(struct ipv6hdr) +
+				   (sizeof(struct ipv6hdr) +
 				    len + hlen + tlen),
 				   1, &err);
 	if (buff == NULL) {

1.7.9.5

Thanks !

From: Eric Dumazet <[email protected]>
Date: Fri, 04 Jan 2013 08:00:19 -0800

Applied, thanks for your persistence.