Re: [PATCH] net: fec: fix regression on i.MX28 introduced by rx_copybreak support

[David Miller <davem@davemloft.net>]

From: Lothar Waßmann <LW@KARO-electronics.de>
Date: Tue,  7 Oct 2014 15:19:37 +0200

> commit 1b7bde6d659d ("net: fec: implement rx_copybreak to improve rx performance")
> introduced a regression for i.MX28. The swap_buffer() function doing
> the endian conversion of the received data on i.MX28 may access memory
> beyond the actual packet size in the DMA buffer. fec_enet_copybreak()
> does not copy those bytes, so that the last bytes of a packet may be
> filled with invalid data after swapping.
> This will likely lead to checksum errors on received packets.
> E.g. when trying to mount an NFS rootfs:
> UDP: bad checksum. From 192.168.1.225:111 to 192.168.100.73:44662 ulen 36
> 
> Do the byte swapping and copying to the new skb in one go if
> necessary.
> 
> Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>

Why don't you just round up the length fec_enet_copybreak() uses when
need_swap is true?  Then you will end up mimicking the original behavior
and not require this new helper function.

And in any case I agree with Sergei that if you do retain your approach,
the new 'swap' argument to fec_enet_copybreak() should be a 'bool'.

I'm really surprised there isn't a control register bit to adjust the
endianness of the data DMA'd to/from the network.