netdev - Re: [PATCH v2 net-next] net: adjust skb->truesize in pskb_expand

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Message-Id: <20170127.120344.536515545735368429.davem@davemloft.net>
Date:   Fri, 27 Jan 2017 12:03:44 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, slavash@...lanox.com
Subject: Re: [PATCH v2 net-next] net: adjust skb->truesize in
 pskb_expand_head()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 27 Jan 2017 07:11:27 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Slava Shwartsman reported a warning in skb_try_coalesce(), when we
> detect skb->truesize is completely wrong.
> 
> In his case, issue came from IPv6 reassembly coping with malicious
> datagrams, that forced various pskb_may_pull() to reallocate a bigger
> skb->head than the one allocated by NIC driver before entering GRO
> layer.
> 
> Current code does not change skb->truesize, leaving this burden to
> callers if they care enough.
> 
> Blindly changing skb->truesize in pskb_expand_head() is not
> easy, as some producers might track skb->truesize, for example
> in xmit path for back pressure feedback (sk->sk_wmem_alloc)
> 
> We can detect the cases where it should be safe to change
> skb->truesize :
> 
> 1) skb is not attached to a socket.
> 2) If it is attached to a socket, destructor is sock_edemux()
> 
> My audit gave only two callers doing their own skb->truesize
> manipulation.
> 
> I had to remove skb parameter in sock_edemux macro when
> CONFIG_INET is not set to avoid a compile error.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Slava Shwartsman <slavash@...lanox.com>

Looks good, applied, thanks Eric.