Subject: Re: [net PATCH] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get_headlen

From: Sowmini Varadhan <[email-masked]>
Date: Sat, 30 Jan 2016 11:17:02 -0500

On (01/29/16 19:23), Eric Dumazet wrote:
> BTW, even a memcpy(&key_addrs->v4addrs, &iph->saddr, 8) could crash, as
> the compiler can certainly assume src and dst are 4 bytes aligned, and
> could use word accesses when inlining memcpy() even on Sparc.
> 
> Apparently the compiler used by Sowmini is gentle.

One more subtlety that I missed until now.. 

eth_get_headlen passes in flow_keys_buf_dissector (NOT flow_keys_dissector!)

So FLOW_DISSECTOR_KEY_IPV4_ADDRS is not set, and this helps to dodge
the unaligned iph->saddr access.

But as others have pointed out, much of this code is brittle
because it's accessing the data before the driver has had a chance
to align things. The page_offset initialization of NET_IP_ALIGN,
with all its weaknesses, at least matches (in principle) the prescription
used for the xmit path.

--Sowmini