netdev - Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg

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Date:   Thu, 20 Oct 2016 17:40:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Jon Maloy <jon.maloy@...csson.com>, Ying Xue <ying.xue0@...il.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Qian Zhang <zhangqian-c@....cn>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()

On Thu, 2016-10-20 at 14:51 +0000, Jon Maloy wrote:
[...]
> > At this point we're about to copy INT_H_SIZE + mhsz bytes into the
> > first fragment.  If that's already limited to be less than or equal to
> > MAX_H_SIZE, comparing with MAX_H_SIZE would be fine.  But if MAX_H_SIZE
> > is the maximum value of mhsz, that won't be good enough.
> 
> 
> MAX_H_SIZE is 60 bytes, but in practice you will never see an mhsz larger than the biggest header we are actually using, which is MCAST_H_SIZE (==44 bytes).
> INT_H_SIZE is 40 bytes, so you are in reality testing for whether we have an mtu < 84 bytes.
> You won't find any interfaces or protocols that come even close to this limitation, so to me this test is redundant.

But I can easily create such an interface:

$ unshare -n -U -r
# ip l set lo mtu 1

Ben.

-- 
Ben Hutchings
Never put off till tomorrow what you can avoid all together.


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