netdev - Re: [PATCH net] packet: fix reserve calculation

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Message-Id: <20180524.215600.843239504709621574.davem@davemloft.net>
Date:   Thu, 24 May 2018 21:56:00 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, willemb@...gle.com
Subject: Re: [PATCH net] packet: fix reserve calculation

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Thu, 24 May 2018 18:10:30 -0400

> From: Willem de Bruijn <willemb@...gle.com>
> 
> Commit b84bbaf7a6c8 ("packet: in packet_snd start writing at link
> layer allocation") ensures that packet_snd always starts writing
> the link layer header in reserved headroom allocated for this
> purpose.
> 
> This is needed because packets may be shorter than hard_header_len,
> in which case the space up to hard_header_len may be zeroed. But
> that necessary padding is not accounted for in skb->len.
> 
> The fix, however, is buggy. It calls skb_push, which grows skb->len
> when moving skb->data back. But in this case packet length should not
> change.
> 
> Instead, call skb_reserve, which moves both skb->data and skb->tail
> back, without changing length.
> 
> Fixes: b84bbaf7a6c8 ("packet: in packet_snd start writing at link layer allocation")
> Reported-by: Tariq Toukan <tariqt@...lanox.com>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Applied and queued up for -stable.