netdev - Re: [PATCH 10/14] Phonet: Phonet datagram transport protocol

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Date:	Fri, 19 Sep 2008 09:33:33 +0300
From:	"Rémi Denis-Courmont" 
	<remi.denis-courmont@...ia.com>
To:	"ext Arnaldo Carvalho de Melo" <acme@...hat.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 10/14] Phonet: Phonet datagram transport protocol

On Tuesday 16 September 2008 20:06:44 ext Arnaldo Carvalho de Melo, you wrote:
> > +/*
> > + * Prepends an ISI header and sends a datagram.
> > + */
> > +static int pn_send(struct sk_buff *skb, struct net_device *dev,
> > +			u16 dst, u16 src, u8 res)
> > +{
> > +	struct phonethdr *ph;
> > +	int err;
> > +
> > +	if (skb->len > 0xfffd) {
>
> wow, what a magic number! :-)

Taking into account that we push sizeof(*ph) in-between, this protects against 
protocol overflow a few lines down:
    ph->length = __cpu_to_be16(skb->len + 2 - sizeof(*ph));

I guess it deserves a /* comment */.

By the way, is the protocol stack or the device driver responsible for not 
transmitting above the MTU?

> > +	skb_reset_transport_header(skb);
> > +	WARN_ON(skb_headroom(skb) & 1); /* HW assumes word alignment */
> > +	ph = (struct phonethdr *)skb_push(skb, sizeof(struct phonethdr));
> > +	skb_reset_network_header(skb);
>
> Here you could just use:
>
> 	ph = (struct phonethdr *)skb_network_header(skb);
>
> 	or even just:
>
> 	ph = pn_hdr(skb);
>
> 	as other protocols such as IP do (see ip_hdr())

Right.

> > +	ph->rdev = pn_dev(dst);
> > +	ph->sdev = pn_dev(src);
> > +	ph->function = res;
> > +	ph->length = __cpu_to_be16(skb->len + 2 - sizeof(*ph));
> > +	ph->robj = pn_obj(dst);
> > +	ph->sobj = pn_obj(src);
> > +
> > +	skb->protocol = __constant_htons(ETH_P_PHONET);
>
> No need for __constant_htons(CONSTANT), htons will do the right thing
> and the code will be shorter, clearer.

Ok. There are quite many of these in net/ though... is that reserved for 
switch/case only?

(...)

> > +static int pn_ioctl(struct sock *sk, int cmd, unsigned long arg)
> > +{
> > +	struct sk_buff *skb;
> > +	int answ;
> > +
> > +	switch (cmd) {
> > +	case SIOCINQ:
> > +		spin_lock_bh(&sk->sk_receive_queue.lock);
> > +		skb = skb_peek(&sk->sk_receive_queue);
> > +		answ = skb ? skb->len : 0;
> > +		spin_unlock_bh(&sk->sk_receive_queue.lock);
> > +		return put_user(answ, (int __user *)arg);
>
> Why not use lock_sock/release_sock as tcp?

The socket lock was not taken on the RX path, as I was not using 
sk_receive_skb(). I believe no state needs protection from softirq in the 
datagram protocol, other than the receive queue itself.

Of course, with sk_receive_skb(), I guess I could use lock_sock().

Thanks many times for the review,

-- 
Rémi Denis-Courmont
Maemo Software, Nokia Devices R&D
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