[OpenWrt-Devel] AA on brcm47xx: Unhandled kernel unaligned access

Rafał Miłecki zajec5 at gmail.com
Mon Jul 14 17:35:19 EDT 2014

On 14 July 2014 18:44, Jonas Gorski <jogo at openwrt.org> wrote:
> On Mon, Jul 14, 2014 at 6:23 PM, Felix Fietkau <nbd at openwrt.org> wrote:
>> It looks to me like the hardware is overwriting the skb shared info (at
>> the end of the skb data buffer), possibly because the configured maximum
>> frame length may be too big for the buffer.
>> If I were to speculate wildly, I would guess that B44_RXMAXLEN refers to
>> the maximum frame length, not the maximum buffer length - and in the
>> code, it's being fed with the maximum buffer length.
>> This would allow the hardware to receive slightly oversized frames which
>> can corrupt the skb.
> Since there is a public datasheet[1], this is easily verifiable, and
> it looks you are right:
> "Receive Maximum Length Register (RcvLength, Offset 0x404):
> The value stored in this register specifies the largest valid Ethernet
> Frame to be received."
> The same is true for the XmtMaxLength register, which is also set "too
> large" (it defaults to 1518).

I wonder what's the point of that register if we set length per-skb
anyway (b44_alloc_rx_skb):
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