netdev - Re: [PATCH v1 1/1] l2tp: Use existing macros instead of literal number

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Date:   Thu, 18 Aug 2016 20:41:39 +0200
From:   Guillaume Nault <g.nault@...halink.fr>
To:     fgao@...ai8.com
Cc:     jchapman@...alix.com, davem@...emloft.net, netdev@...r.kernel.org,
        gfree.wind@...il.com
Subject: Re: [PATCH v1 1/1] l2tp: Use existing macros instead of literal
 number

On Thu, Aug 18, 2016 at 03:05:19PM +0800, fgao@...ai8.com wrote:
> From: Gao Feng <fgao@...ai8.com>
> 
> 1. Use PPP_ALLSTATIONS/PPP_UI instead of literal 0xff/0x03;
> 2. Use one static const global fixed_ppphdr instead of two same
> static variable ppph in two different functions;
> 3. Use SEND_SHUTDOWN instead of literal 2;
> 
> Signed-off-by: Gao Feng <fgao@...ai8.com>
> ---
>  v1: Initial patch
No need to send 'v1' for the initial series.

> --- a/net/l2tp/l2tp_ppp.c
> +++ b/net/l2tp/l2tp_ppp.c
> @@ -138,6 +138,8 @@ static const struct ppp_channel_ops pppol2tp_chan_ops = {
>  
>  static const struct proto_ops pppol2tp_ops;
>  
> +static const unsigned char fixed_ppphdr[2] = {PPP_ALLSTATIONS, PPP_UI};
> +
>  /* Helpers to obtain tunnel/session contexts from sockets.
>   */
>  static inline struct l2tp_session *pppol2tp_sock_to_session(struct sock *sk)
> @@ -174,11 +176,11 @@ static int pppol2tp_recv_payload_hook(struct sk_buff *skb)
>  	 * Note that skb->data[] isn't dereferenced from a u16 ptr here since
>  	 * the field may be unaligned.
>  	 */
> -	if (!pskb_may_pull(skb, 2))
> +	if (!pskb_may_pull(skb, sizeof(fixed_ppphdr)))
>  		return 1;
>  
> -	if ((skb->data[0] == 0xff) && (skb->data[1] == 0x03))
> -		skb_pull(skb, 2);
> +	if ((PPP_ADDRESS(skb->data) == PPP_ALLSTATIONS) && (PPP_CONTROL(skb->data) == PPP_UI))
> +		skb_pull(skb, sizeof(fixed_ppphdr));
>
Sorry, but I find the original code clearer. It's important to be
explicit about what's done with the sk_buff. Hiding skb->data[x] behind
macros certainly doesn't help.

Same thing for the use of sizeof(fixed_ppphdr) in pskb_may_pull(). The
size of fixed_ppphdr isn't used aftewards, so it's unclear why its size
was pulled. 2 was not a magic number here, it was directly related with
the operations done on the skb (i.e. accessing skb->data[0] and
skb->data[1]). So pskb_may_pull(skb, 2) makes perfect sense.

OTOH, replacing 0xff and 0x03 with PPP_ALLSTATIONS and PPP_UI is fine.

> @@ -312,7 +313,7 @@ static int pppol2tp_sendmsg(struct socket *sock, struct msghdr *m,
>  	error = -ENOMEM;
>  	skb = sock_wmalloc(sk, NET_SKB_PAD + sizeof(struct iphdr) +
>  			   uhlen + session->hdr_len +
> -			   sizeof(ppph) + total_len,
> +			   sizeof(fixed_ppphdr) + total_len,
>  			   0, GFP_KERNEL);
>  	if (!skb)
>  		goto error_put_sess_tun;
> @@ -325,9 +326,9 @@ static int pppol2tp_sendmsg(struct socket *sock, struct msghdr *m,
>  	skb_reserve(skb, uhlen);
>  
>  	/* Add PPP header */
> -	skb->data[0] = ppph[0];
> -	skb->data[1] = ppph[1];
> -	skb_put(skb, 2);
> +	PPP_ADDRESS(skb->data) = fixed_ppphdr[0];
> +	PPP_CONTROL(skb->data) = fixed_ppphdr[1];
> +	skb_put(skb, sizeof(fixed_ppphdr));
>
Same here. What about
+ skb->data[0] = PPP_ALLSTATIONS;
+ skb->data[1] = PPP_UI;
+ skb_put(skb, 2);
and removing ppph entirely?

>  	/* Copy user data into skb */
>  	error = memcpy_from_msg(skb_put(skb, total_len), m, total_len);
> @@ -369,7 +370,6 @@ error:
>   */
>  static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
>  {
> -	static const u8 ppph[2] = { 0xff, 0x03 };
>  	struct sock *sk = (struct sock *) chan->private;
>  	struct sock *sk_tun;
>  	struct l2tp_session *session;
> @@ -398,14 +398,14 @@ static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
>  		   sizeof(struct iphdr) + /* IP header */
>  		   uhlen +		/* UDP header (if L2TP_ENCAPTYPE_UDP) */
>  		   session->hdr_len +	/* L2TP header */
> -		   sizeof(ppph);	/* PPP header */
> +		   sizeof(fixed_ppphdr); /* PPP header */
>  	if (skb_cow_head(skb, headroom))
>  		goto abort_put_sess_tun;
>  
>  	/* Setup PPP header */
> -	__skb_push(skb, sizeof(ppph));
> -	skb->data[0] = ppph[0];
> -	skb->data[1] = ppph[1];
> +	__skb_push(skb, sizeof(fixed_ppphdr));
> +	skb->data[0] = fixed_ppphdr[0];
> +	skb->data[1] = fixed_ppphdr[1];
> 
Same as for pppol2tp_sendmsg().

> @@ -440,7 +440,7 @@ static void pppol2tp_session_close(struct l2tp_session *session)
>  	BUG_ON(session->magic != L2TP_SESSION_MAGIC);
>  
>  	if (sock) {
> -		inet_shutdown(sock, 2);
> +		inet_shutdown(sock, SEND_SHUTDOWN);
> 
Ok.