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.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
b7dc8c3959