Andrew Thompson wrote:
On Fri, Aug 29, 2008 at 06:41:45PM +0200, Luigi Rizzo wrote:
On Fri, Aug 29, 2008 at 09:32:10AM -0700, Sam Leffler wrote:
Luigi Rizzo wrote:
...
and to be more explicit - the result of m_pullup is that
the number of bytes specified as m_pullup argument are in
a private piece of memory -- the 'data' region within the mbuf -- so
you can freely play with them without trouble.
That is why i suggested to just increase the argument to m_pullup
by the size of the udp header so one can overwrite the checksum
within the mbuf without touching the shared part in the cluster
(if any).
Hmm, never considered the m_pullup guaranteed a private copy (but I see
it in the code). The original semantics were just that the data was
contiguous.
funny, i thought the guarantee of a writable copy was also part
of the original semantics :)
The bridge code does a deep copy of the packet for each interface it
broadcasts on due the firewall code modifying the headers. It sounds
like this should just be a copy+pullup instead.
I'd not do that. I think there are paths that assume the deep copy.
Right now the network code is very poor honoring read-only-ness of mbuf
chains. To get this right we need to do a good audit. I know I hit
issues when doing some tricks w/ marking rx buffers read-only to avoid
cache flushes. netbsd trys to be more pedantic but still has problems too.
Sam
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