Tomasz Grobelny wrote:
Dnia Sunday 20 of April 2008, Arnaldo Carvalho de Melo napisał:
Em Sat, Apr 19, 2008 at 10:42:32PM +0200, Tomasz Grobelny escreveu:
When the patch failed to compile I thought about those alternatives.
Trying to extend the dccp_skb_cb over and above what is in there will
be messy, since the IPv4/v6 parameters are required by other
subsystems.
If inet{,6}_skb_parm is used only outside DCCP code then why at all
should it be placed in struct dccp_skb_cb taking up quite a lot of
valuable space? Why not put it directly in struct sk_buff? Especially
that it is present in struct udp_skb_cb, struct tcp_skb_cb as well.
Because all this is used in skb->cb[], a scratchpad for protocols to
use, we can go back to what we had before, that is to not reserve use
for inet6?_skb_parm but be sure to zero it before passing it to IP, as
we don't want IP to be confused with things being non zero there. Then
we can use all its space.
Several questions regarding this case:
1. What about SCTP? It doesn't have inet6?_skb_parm in it's structure that is
stored in skb->cb. So does it contain a potential bug (that is to be fixed)
or is it not needed there or what?
Judging by a quick grep, SCTP only uses the CB on input and
appears to be fine.
2. If the sole purpose of this change was to keep skb->cb zeroed then it
doesn't seem to me like the right solution. Wasting about 20 bytes instead of
zeroing them when needed I would consider at least weird. I understand that
TCP and UDP may have enough space left but it just turned out that DCCP
doesn't.
It was the safest solution that late in a release. It also
avoids to memset the cb unnecessarily. If the room is not
enough anymore, its easy to go back to using memset.
3. If it's IP layer that needs zeroes then why not clear skb->cb in IP layer?
That would certainly work, but it adds unnecessary costs for
the other protocols that don't need this.
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