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From: Sowmini Varadhan <sowmini.varadhan@oracle.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Alexander Duyck <aduyck@mirantis.com>,
	netdev@vger.kernel.org, davem@davemloft.net,
	alexander.duyck@gmail.com, tom@herbertland.com
Subject: Re: [net PATCH] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get_headlen
Date: Sat, 30 Jan 2016 11:17:02 -0500	[thread overview]
Message-ID: <20160130161702.GA11601@oracle.com> (raw)
In-Reply-To: <1454124234.7627.119.camel@edumazet-glaptop2.roam.corp.google.com>

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

  parent reply	other threads:[~2016-01-30 16:17 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-29 18:06 [RFC] Kernel unaligned access at __skb_flow_dissect Sowmini Varadhan
2016-01-29 18:33 ` Eric Dumazet
2016-01-29 18:40   ` Eric Dumazet
2016-01-29 19:08     ` David Miller
2016-01-29 19:39       ` Eric Dumazet
2016-01-29 18:54   ` Eric Dumazet
2016-01-29 19:22     ` Sowmini Varadhan
2016-01-29 21:00     ` Tom Herbert
2016-01-29 21:09       ` Sowmini Varadhan
2016-01-29 21:33         ` Tom Herbert
2016-01-29 23:00         ` Tom Herbert
2016-01-29 23:04           ` Sowmini Varadhan
2016-01-29 23:31             ` Tom Herbert
2016-01-29 23:58               ` Sowmini Varadhan
2016-01-30  0:47                 ` Tom Herbert
2016-01-30  1:18                   ` David Miller
2016-01-30  2:15                   ` Eric Dumazet
2016-01-31 22:13                     ` Tom Herbert
2016-02-01  0:04                       ` Eric Dumazet
2016-01-29 19:06   ` David Miller
2016-01-29 19:37     ` Eric Dumazet
2016-01-29 19:41       ` David Miller
2016-01-29 19:44       ` Sowmini Varadhan
2016-01-29 20:01         ` Eric Dumazet
2016-01-29 20:06           ` Eric Dumazet
2016-01-29 21:16           ` Alexander Duyck
2016-01-29 21:33             ` Eric Dumazet
2016-01-29 22:08               ` Alexander Duyck
2016-01-29 22:28                 ` Eric Dumazet
2016-01-29 23:00                   ` Alexander Duyck
2016-02-01 16:55                   ` David Laight
2016-02-01 17:25                     ` Eric Dumazet
2016-01-29 19:04 ` David Miller
2016-01-30  2:49 ` [net PATCH] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get_headlen Alexander Duyck
2016-01-30  3:23   ` Eric Dumazet
2016-01-30  3:35     ` David Miller
2016-01-30  4:46       ` Alexander Duyck
2016-01-30 16:17     ` Sowmini Varadhan [this message]
2016-01-30 17:43       ` Tom Herbert
2016-01-30 19:12         ` Sowmini Varadhan
2016-01-30 18:36       ` Alexander Duyck
2016-01-30 19:26         ` Eric Dumazet
2016-01-31  1:13           ` Alexander Duyck
2016-01-31  3:45             ` David Miller
2016-01-31  9:12               ` Alexander Duyck
2016-01-31 19:17                 ` David Miller
2016-01-31 21:23                   ` Sowmini Varadhan
2016-02-01 16:50               ` David Laight
2016-02-01 18:18                 ` Alexander Duyck
2016-02-02  9:47                   ` David Laight

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