netdev - Re: [PATCH] net: ethernet: use ip_hdrlen() instead of bit shift

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Message-ID: <20240802141534.GA2504122@kernel.org>
Date: Fri, 2 Aug 2024 15:15:34 +0100
From: Simon Horman <horms@...nel.org>
To: Moon Yeounsu <yyyynoom@...il.com>
Cc: cooldavid@...ldavid.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: ethernet: use ip_hdrlen() instead of bit shift

On Fri, Aug 02, 2024 at 02:44:21PM +0900, Moon Yeounsu wrote:
> `ip_hdr(skb)->ihl << 2` are the same as `ip_hdrlen(skb)`
> Therefore, we should use a well-defined function not a bit shift
> to find the header length.
> 
> It also compress two lines at a single line.
> 
> Signed-off-by: Moon Yeounsu <yyyynoom@...il.com>

Firstly, I think this clean-up is both correct and safe.  Safe because
ip_hdrlen() only relies on ip_hdr(), which is already used in the same code
path. And correct because ip_hdrlen multiplies ihl by 4, which is clearly
equivalent to a left shift of 2 bits.

However, I do wonder about the value of clean-ups for what appears to be a
very old driver, which hasn't received a new feature for quite sometime

And further, I wonder if we should update this driver from "Maintained" to
"Odd Fixes" as the maintainer, "Guo-Fu Tseng" <cooldavid@...ldavid.org>,
doesn't seem to have been seen by lore since early 2020.

https://lore.kernel.org/netdev/20200219034801.M31679@cooldavid.org/

> ---
>  drivers/net/ethernet/jme.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/jme.c b/drivers/net/ethernet/jme.c
> index b06e24562973..83b185c995df 100644
> --- a/drivers/net/ethernet/jme.c
> +++ b/drivers/net/ethernet/jme.c
> @@ -946,15 +946,13 @@ jme_udpsum(struct sk_buff *skb)
>  	if (skb->protocol != htons(ETH_P_IP))
>  		return csum;
>  	skb_set_network_header(skb, ETH_HLEN);
> +
>  	if ((ip_hdr(skb)->protocol != IPPROTO_UDP) ||
> -	    (skb->len < (ETH_HLEN +
> -			(ip_hdr(skb)->ihl << 2) +
> -			sizeof(struct udphdr)))) {
> +	    (skb->len < (ETH_HLEN + (ip_hdrlen(skb)) + sizeof(struct udphdr)))) {

The parentheses around the call to ip_hdrlen are unnecessary.
And this line is now too long: networking codes till prefers
code to be 80 columns wide or less.

>  		skb_reset_network_header(skb);
>  		return csum;
>  	}
> -	skb_set_transport_header(skb,
> -			ETH_HLEN + (ip_hdr(skb)->ihl << 2));
> +	skb_set_transport_header(skb, ETH_HLEN + (ip_hdrlen(skb)));

Unnecessary parentheses here too.

>  	csum = udp_hdr(skb)->check;
>  	skb_reset_transport_header(skb);
>  	skb_reset_network_header(skb);

-- 
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