Re: [PATCH net-next] drivers/net/wan/hdlc_fr: Correctly handle special skb->protocol values

[David Miller]

From: Xie He <xie.he.0141@xxxxxxxxx>
Date: Mon, 28 Sep 2020 05:56:43 -0700

> The fr_hard_header function is used to prepend the header to skbs before
> transmission. It is used in 3 situations:
> 1) When a control packet is generated internally in this driver;
> 2) When a user sends an skb on an Ethernet-emulating PVC device;
> 3) When a user sends an skb on a normal PVC device.
> 
> These 3 situations need to be handled differently by fr_hard_header.
> Different headers should be prepended to the skb in different situations.
> 
> Currently fr_hard_header distinguishes these 3 situations using
> skb->protocol. For situation 1 and 2, a special skb->protocol value
> will be assigned before calling fr_hard_header, so that it can recognize
> these 2 situations. All skb->protocol values other than these special ones
> are treated by fr_hard_header as situation 3.
> 
> However, it is possible that in situation 3, the user sends an skb with
> one of the special skb->protocol values. In this case, fr_hard_header
> would incorrectly treat it as situation 1 or 2.
> 
> This patch tries to solve this issue by using skb->dev instead of
> skb->protocol to distinguish between these 3 situations. For situation
> 1, skb->dev would be NULL; for situation 2, skb->dev->type would be
> ARPHRD_ETHER; and for situation 3, skb->dev->type would be ARPHRD_DLCI.
> 
> This way fr_hard_header would be able to distinguish these 3 situations
> correctly regardless what skb->protocol value the user tries to use in
> situation 3.
> 
> Cc: Krzysztof Halasa <khc@xxxxxxxxx>
> Signed-off-by: Xie He <xie.he.0141@xxxxxxxxx>

Applied, thank you.