netdev - Re: [PATCH net] netpoll: fix rx_hook() interface by passing the skb

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Date:	Wed, 23 Oct 2013 12:28:48 +0200
From:	Antonio Quartulli <antonio@...hcoding.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net] netpoll: fix rx_hook() interface by passing the skb

On Wed, Oct 23, 2013 at 09:33:49AM +0100, David Laight wrote:
> ...
> > > I can't remember which value you passed as 'offset' (and my mailer makes
> > > it hard to find), but to ease the code changes the offset of the udp data
> > > would make sense.
> > > In that case you still need to pass the source port.
> > 
> > I decided not to pass the source port because if the user is really interested
> > in it, it is still possible to get the udp_hdr from the skb and read its value.
> 
> It just seemed that there was no need to require that the hook re-parse
> the ip header just to find the source port.
> (ok it could assume that the udp header is just before the data)

Also David (M.) pointed out the same. I will keep the port as argument for
rx_hook.

>  
> > > If you do rx_hook(np, source_port, skb, offset) then if anyone manages to
> > > load an old module (or code that casts the assignement to rx_poll)
> > > at least it won't go 'bang'.
> > > Renaming the structure member will guarantee to generate compile errors.
> > 
> > so you suggest to rename rx_hook to something else to warn people about the
> > change?
> 
> Yes.
> 

mh..what about rx_skb_hook ? this way we also make it easy to notice the
difference (both in arguments and behaviour).

> > If we go for the "no udp port" approach they will get an error any way because
> > of the mismatching arguments.
> 
> No - you only get a warning when you assign a function pointer of the wrong type.
> And that is true even if you just change the type of the pointer.
> However code might already have a cast on the function pointer (eg because the
> hook has 'unsigned char *') - so you won't even get a warning.
> You then get an OOPS when the hook tries to read the buffer.
> 
> It is a really bad interface...
> There isn't even a flags/options (etc) word that can be used
> to detect enhancements.
> 


agreed. But I am not sure about what I could do to fix that.

My idea is to use the following API:

rx_skb_hook(struct netpoll *np, int source, struct sk_buff *skb, int len);


Any suggestion or objection?


Regards,


-- 
Antonio Quartulli

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