Re: [PATCH net v1] lan743x: fix ethernet frame cutoff issue

[Julian Wiedmann]

On 09.04.21 02:39, Sven Van Asbroeck wrote:
> From: Sven Van Asbroeck <thesven73@xxxxxxxxx>
> 
> The ethernet frame length is calculated incorrectly. Depending on
> the value of RX_HEAD_PADDING, this may result in ethernet frames
> that are too short (cut off at the end), or too long (garbage added
> to the end).
> 
> Fix by calculating the ethernet frame length correctly. For added
> clarity, use the ETH_FCS_LEN constant in the calculation.
> 
> Many thanks to Heiner Kallweit for suggesting this solution. 
> 
> Fixes: 3e21a10fdea3 ("lan743x: trim all 4 bytes of the FCS; not just 2")
> Link: https://lore.kernel.org/lkml/20210408172353.21143-1-TheSven73@xxxxxxxxx/
> Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
> ---
> 
> Tree: git://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git # 864db232dc70
> 
> To: Bryan Whitehead <bryan.whitehead@xxxxxxxxxxxxx>
> To: "David S. Miller" <davem@xxxxxxxxxxxxx>
> To: Jakub Kicinski <kuba@xxxxxxxxxx>
> To: George McCollister <george.mccollister@xxxxxxxxx>
> Cc: Heiner Kallweit <hkallweit1@xxxxxxxxx>
> Cc: Andrew Lunn <andrew@xxxxxxx>
> Cc: UNGLinuxDriver@xxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> 
>  drivers/net/ethernet/microchip/lan743x_main.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.c b/drivers/net/ethernet/microchip/lan743x_main.c
> index 1c3e204d727c..7b6794aa8ea9 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.c
> +++ b/drivers/net/ethernet/microchip/lan743x_main.c
> @@ -885,8 +885,8 @@ static int lan743x_mac_set_mtu(struct lan743x_adapter *adapter, int new_mtu)
>  	}
>  
>  	mac_rx &= ~(MAC_RX_MAX_SIZE_MASK_);
> -	mac_rx |= (((new_mtu + ETH_HLEN + 4) << MAC_RX_MAX_SIZE_SHIFT_) &
> -		  MAC_RX_MAX_SIZE_MASK_);
> +	mac_rx |= (((new_mtu + ETH_HLEN + ETH_FCS_LEN)
> +		  << MAC_RX_MAX_SIZE_SHIFT_) & MAC_RX_MAX_SIZE_MASK_);
>  	lan743x_csr_write(adapter, MAC_RX, mac_rx);
>  
>  	if (enabled) {
> @@ -1944,7 +1944,7 @@ static int lan743x_rx_init_ring_element(struct lan743x_rx *rx, int index)
>  	struct sk_buff *skb;
>  	dma_addr_t dma_ptr;
>  
> -	buffer_length = netdev->mtu + ETH_HLEN + 4 + RX_HEAD_PADDING;
> +	buffer_length = netdev->mtu + ETH_HLEN + ETH_FCS_LEN + RX_HEAD_PADDING;
>  

On a cursory glance, using __netdev_alloc_skb_ip_align() here should
allow you to get rid of all the RX_HEAD_PADDING gymnastics.

And also avoid the need for setting RX_CFG_B_RX_PAD_2_, as the
NET_IP_ALIGN part would no longer get dma-mapped.

>  	descriptor = &rx->ring_cpu_ptr[index];
>  	buffer_info = &rx->buffer_info[index];
> @@ -2040,7 +2040,7 @@ lan743x_rx_trim_skb(struct sk_buff *skb, int frame_length)
>  		dev_kfree_skb_irq(skb);
>  		return NULL;
>  	}
> -	frame_length = max_t(int, 0, frame_length - RX_HEAD_PADDING - 4);
> +	frame_length = max_t(int, 0, frame_length - ETH_FCS_LEN);
>  	if (skb->len > frame_length) {
>  		skb->tail -= skb->len - frame_length;
>  		skb->len = frame_length;
>