linux-kernel - RE: [PATCH] net: fec: fix regression on i.MX28 introduced by rx

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D174C60B1@AcuExch.aculab.com>
Date:	Tue, 7 Oct 2014 14:23:02 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Lothar Waßmann' <LW@...O-electronics.de>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Frank Li <Frank.Li@...escale.com>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] net: fec: fix regression on i.MX28 introduced by
 rx_copybreak support

From: Lothar
> David Laight wrote:
> > From: Lothar Waßmann
> > > 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.
> >
> > ISTM that if you need to do the 'swap' you should copy the data regardless
> > of the length.
> >
> The swap function has to look at at most 3 bytes beyond the actual
> packet length. That is what the original swap_buffer() function does and
> what the new function swap_buffer2(), that does the endian swapping
> while copying to the new buffer, also does.

I understood the bug.

The point I was making is that if you have to do a read-write of the received
data (to byteswap it) then you might as well always copy it into a new skb that
is just big enough for the actual receive frame.

	David