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Date:   Fri, 24 Jul 2020 20:17:59 -0700 (PDT)
From:   David Miller <>
Subject: Re: [PATCH] drivers/net/wan: lapb: Corrected the usage of skb_cow

From: Xie He <>
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 <>
> Cc: Martin Schiller <>
> Signed-off-by: Xie He <>

Applied, thank you.

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