Re: [PATCH] drivers/net/wan: lapb: Corrected the usage of skb_cow

[David Miller]

From: Xie He <xie.he.0141@xxxxxxxxx>
Date: Fri, 24 Jul 2020 09:33:47 -0700

> This patch fixed 2 issues with the usage of skb_cow in LAPB drivers
> "lapbether" and "hdlc_x25":
> 
> 1) After skb_cow fails, kfree_skb should be called to drop a reference
> to the skb. But in both drivers, kfree_skb is not called.
> 
> 2) skb_cow should be called before skb_push so that is can ensure the
> safety of skb_push. But in "lapbether", it is incorrectly called after
> skb_push.
> 
> More details about these 2 issues:
> 
> 1) The behavior of calling kfree_skb on failure is also the behavior of
> netif_rx, which is called by this function with "return netif_rx(skb);".
> So this function should follow this behavior, too.
> 
> 2) In "lapbether", skb_cow is called after skb_push. This results in 2
> logical issues:
>    a) skb_push is not protected by skb_cow;
>    b) An extra headroom of 1 byte is ensured after skb_push. This extra
>       headroom has no use in this function. It also has no use in the
>       upper-layer function that this function passes the skb to
>       (x25_lapb_receive_frame in net/x25/x25_dev.c).
> So logically skb_cow should instead be called before skb_push.
> 
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Martin Schiller <ms@xxxxxxxxxx>
> Signed-off-by: Xie He <xie.he.0141@xxxxxxxxx>

Applied, thank you.