netdev - RE: [PATCH] lan78xx: fix ip header misalignment

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Date:   Sun, 27 Jan 2019 13:14:04 +0000
From:   <RaghuramChary.Jallipalli@...rochip.com>
To:     <yumkam@...il.com>, <netdev@...r.kernel.org>
CC:     <Woojung.Huh@...rochip.com>, <UNGLinuxDriver@...rochip.com>
Subject: RE: [PATCH] lan78xx: fix ip header misalignment

Hi Yuiry,
Thanks for the patch.
Changes look good to me.

Thanks,
-Raghu

> -----Original Message-----
> From: Yuriy M. Kaminskiy <yumkam@...il.com>
> Sent: Monday, January 21, 2019 6:18 PM
> To: netdev@...r.kernel.org
> Cc: Woojung Huh - C21699 <Woojung.Huh@...rochip.com>;
> UNGLinuxDriver <UNGLinuxDriver@...rochip.com>
> Subject: [PATCH] lan78xx: fix ip header misalignment
> 
> lan78xx.c:rx_submit() allocates space for frame-to-be-received with
> netdev_alloc_skb_ip_align(), which misalign start of buffer by 2 bytes in
> expectation that frame will start from 14-byte ethernet header, then ip
> header; if start of buffer misaligned by 2 bytes, ip header will be 16-byte
> aligned.
> 
> Unfortunately, usb frame that is sent by lan78xx starts with another 10-byte
> header (lan78xx_rx(): rx_cmd_a/rx_cmd_b/rx_cmd_c), *then* follows
> ethernet header, and *then* ip header (which ends up being misaligned).
> 
> This issue was observed on arm platform (where misaligned 32-bit word
> access triggers exception and leaves traces in /proc/cpu/alignment, see
> https://github.com/raspberrypi/linux/issues/2599 ; for me, about any
> ipv6 traffic that hits machine - `ping -I eth0 ip6-allnodes`, tcp/udp packets, etc
> triggered increase in this counter, with ip6_datagram_recv_common_ctl,
> icmpv6_echo_reply, etc as culprit).
> 
> If we just allocate skb data without any misalignment tricks, ip header will
> end up and at offset 24 (8-byte aligned).
> 
> Patch attached; runtime-tested with raspbian fork of stable/4.14.y [4.14.92]
> on Raspberry pi 3B+ (it is slightly different from mainline, but patch should
> not have any conflicts, all affected code is pretty same).
> 
> P.S. I'm not subscribed, please CC me on reply.