RE: Antw: RE: [lwip-users] pbuf_header fails in etharp_output

[Goldschmidt Simon]

> > >> lwIP is a little bit cheeky and reuses the pbuf with the ICMP 
> > >> request to form the ICMP reply.  It is therefore assuming that 
> > >> there is room at the front for the necessary headers.  I suppose
if 
> > >> your ethernet driver was being very efficient and had stripped
off 
> > >> the ethernet header before creating the pbuf, this might not be
the 
> > >> case.  Can you show how you're
> > 
> > Yes, that's what I am doing.
> 
> In that case it's technically the fault of lwIP and we should 
> perhaps change our ICMP to not do this (or at least check the 
> pbuf to see if it's OK before it does).  If you'd rather not 
> work around it, please file a bug so that it will eventually 
> get fixed.

Yes, please file a bug report!
I'd change etharp.c to allocate a temporary pbuf  for the MAC header
as do all other protocolsm but we can discuss that on lwip-devel.

> >  If a ethernet frame with type IP arrives I strip the 14 byte
ethernet 
> > header. But not because I want to save memory (RAM is not the big 
> > problem in my system) but because I need the IP header aligned at a
4 
> > byte boundary.
> 
> The normal way to achieve the IP header aligned on a 4-byte 
> boundary is to get your MAC to deliver the packet to a 2-byte 
> offset.  Then, if you have a 14-byte ethernet header, the IP 
> header should be aligned as you require.  Not all MACs 
> support this though I suppose, so your way should also be supported.

If your "MAC" supports copying only the IP data, I suppose you copy
it using memcpy(). Then you can copy to a 2-byte aligned pointer, too.
I think normal way to achieve the 2-byte alignment is shown in
ethernetif.c
(using ETH_PAD_SIZE).

> An easy workaround would be in this case to call pbuf_header 
> to reserve
> 16 bytes before you copy in the packet data.  This would 
> ensure there is space in the pbuf to put the outgoing 
> ethernet header, but is really a bit of a hack and the right 
> thing to do is fix it in lwIP's ICMP code.

That would make ICMP dependent of the link layer!


Simon