netdev - Re: [PATCH 4/7] net/packet: Ask driver for protocol if not provided by user

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Message-ID: <CAF=yD-KtPvZuRUf5KwBf2i=6B0Dj2KPvFPKfabetpc5wNYG-Nw@mail.gmail.com>
Date:   Thu, 17 Jan 2019 10:41:18 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Maxim Mikityanskiy <maximmi@...lanox.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jason Wang <jasowang@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [PATCH 4/7] net/packet: Ask driver for protocol if not provided
 by user

On Thu, Jan 17, 2019 at 4:10 AM Maxim Mikityanskiy <maximmi@...lanox.com> wrote:
>
> > > > +static void packet_parse_headers(struct sk_buff *skb, struct socket
> > *sock)
> > > > +{
> > > > +       if (!skb->protocol && sock->type == SOCK_RAW) {
> > > > +               skb_reset_mac_header(skb);
> > > > +               skb->protocol = dev_parse_header_protocol(skb);
> > > > +       }
> > > > +
> > > > +       skb_try_probe_transport_header(skb);
> > > > +}
> > >
> > >
> > > In relation to the discussion at
> > >
> > >   af_packet: fix raw sockets over 6in4 tunnel
> > >   http://patchwork.ozlabs.org/patch/1023623/
> > >
> > > if adding a new header_ops callback to parse link layer headers,
> > > please have it return both protocol and link layer header length.
>
> Sorry, I miss the point here, can you elaborate more? If all you need is
> to have some header_ops callback that returns the L2 header length,
> there is one already, it's called parse. Or do you have a specific
> reason why you want my callback to also return the header length?

The main reason is to avoid multiple indirect function calls, both
essentially doing the same: parsing the ll header. But admittedly the
instances where dev->header_ops->validate are called are rare.

> > This could just be an extension of existing header_ops->validate.
>
> If you suggest extending an existing function, parse looks more
> suitable, but I decided not to touch the existing ones for two reasons:
>
> 1. I don't want to break the existing code that uses the parse function
> and will need to be modified to pass an extra parameter.
>
> 2. I don't want to spend machine time on copying the destination MAC
> when I only need the protocol, and vice versa.
>
> I'm looking forward to hearing your thoughts about it.

header_ops.parse is also a good candidate. As a matter of fact, parse
and validate could (eventually) probably be combined.

The only direct caller to header_ops.parse appears to be
dev_parse_header, so modifying its interface should be fairly
straightforward. Allowing a NULL haddr could avoid the address copy
cost if not needed. This does require modifying all implementations.
But from a quick scan, there appear to be only 8. And only 1 for
validate. So changing the implementation is quite acceptable. Another
issue, though, would be what to return as protocol if a header does
not encode that.

Given these non-trivial changes, if you prefer to just add the
dedicated new callback, that's fine. We can see independently whether
deduplication makes sense. With three ll header parse functions, I
think that it will be. But even if so, it is better to do so as a
stand-alone noop patch than combining refactoring and new features,
anyway.

Long story short, sounds good. Thanks.