Getting at the UDP headers from ->data_ready()

Getting at the UDP headers from ->data_ready()

[David Howells]

Hi Willem,

With regards to:

    commit 4d0fc73ebe94ac984a187f21fbf4f3a1ac846f5a
    Author: Willem de Bruijn <willemb@google.com>
    Date:   Thu Apr 7 11:44:59 2016 -0400

    rxrpc: do not pull udp headers on receive
    
    Commit e6afc8ace6dd modified the udp receive path by pulling the udp
    header before queuing an skbuff onto the receive queue.
 
Does that mean that I can no longer access the UDP header via udp_hdr(skb)
from with the ->data_ready() handler?

I'm guessing that's not actually the case since ip_hdr(skb) seems to work - or
is that something I shouldn't be using since the part of the buffer containing
the IP header might've been discarded?

David

Re: Getting at the UDP headers from ->data_ready()

[Willem de Bruijn]

On Mon, Apr 11, 2016 at 5:04 PM, David Howells <dhowells@redhat.com> wrote:
> Hi Willem,
>
> With regards to:
>
>     commit 4d0fc73ebe94ac984a187f21fbf4f3a1ac846f5a
>     Author: Willem de Bruijn <willemb@google.com>
>     Date:   Thu Apr 7 11:44:59 2016 -0400
>
>     rxrpc: do not pull udp headers on receive
>
>     Commit e6afc8ace6dd modified the udp receive path by pulling the udp
>     header before queuing an skbuff onto the receive queue.
>
> Does that mean that I can no longer access the UDP header via udp_hdr(skb)
> from with the ->data_ready() handler?
>
> I'm guessing that's not actually the case since ip_hdr(skb) seems to work - or
> is that something I shouldn't be using since the part of the buffer containing
> the IP header might've been discarded?

The network and transport header pointers are still valid. Commit e6afc8ace6dd
only  changes where skb->data points to. It does not discard the data between
skb->head and skb->data. This rxrpc follow-up patch fixes some offset
arithmetic to the payload, which is computed relative to skb->data.

There are other uses of ip_hdr and udp_hdr in udp_recvmsg and similar
recvmsg handlers for other protocols. For instance, in the source address
processing for recvfrom ("if (sin) { .. }")

Re: Getting at the UDP headers from ->data_ready()

[David Howells]

Willem de Bruijn <willemdebruijn.kernel@gmail.com> wrote:

> The network and transport header pointers are still valid. Commit e6afc8ace6dd
> only  changes where skb->data points to. It does not discard the data between
> skb->head and skb->data. This rxrpc follow-up patch fixes some offset
> arithmetic to the payload, which is computed relative to skb->data.
> 
> There are other uses of ip_hdr and udp_hdr in udp_recvmsg and similar
> recvmsg handlers for other protocols. For instance, in the source address
> processing for recvfrom ("if (sin) { .. }")

Excellent, thanks!

David