netdev - Re: [PATCH net 1/1] tipc: conditionally expand buffer headroom over udp tunnel

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Message-Id: <20151021.191423.791280620669017040.davem@davemloft.net>
Date:	Wed, 21 Oct 2015 19:14:23 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jon.maloy@...csson.com
Cc:	netdev@...r.kernel.org, paul.gortmaker@...driver.com,
	parthasarathy.xx.bhuvaragan@...csson.com,
	richard.alpe@...csson.com, ying.xue@...driver.com,
	maloy@...jonn.com, tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net 1/1] tipc: conditionally expand buffer headroom
 over udp tunnel

From: Jon Maloy <jon.maloy@...csson.com>
Date: Mon, 19 Oct 2015 11:43:11 -0400

> In commit d999297c3dbbe ("tipc: reduce locking scope during packet reception")
> we altered the packet retransmission function. Since then, when
> restransmitting packets, we create a clone of the original buffer
> using __pskb_copy(skb, MIN_H_SIZE), where MIN_H_SIZE is the size of
> the area we want to have copied, but also the smallest possible TIPC
> packet size. The value of MIN_H_SIZE is 24.
> 
> Unfortunately, __pskb_copy() also has the effect that the headroom
> of the cloned buffer takes the size MIN_H_SIZE. This is too small
> for carrying the packet over the UDP tunnel bearer, which requires
> a minimum headroom of 28 bytes. A change to just use pskb_copy()
> lets the clone inherit the original headroom of 80 bytes, but also
> assumes that the copied data area is of at least that size, something
> that is not always the case. So that is not a viable solution.
> 
> We now fix this by adding a check for sufficient headroom in the
> transmit function of udp_media.c, and expanding it when necessary.
> 
> Fixes: commit d999297c3dbbe ("tipc: reduce locking scope during packet reception")
> Signed-off-by: Jon Maloy <jon.maloy@...csson.com>

Applied.
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