Re: nfqueue: detect when packet has already been checksummed?

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Florian,

On Sun, May 26, 2013 at 10:48:26PM +0200, Florian Westphal wrote:
> Hi.
> 
> When using nfqueue, userspace currently has no way
> to tell wheter queued packets have a bad checksum, i.e.
> applications that need data integrity must do full checksum
> validation in userspace (except maybe when only queueing in OUTPUT).
> 
> However, there are several places where incoming packets are already
> checksummed in kernel, before packet hits nfqueue, e.g. via nic rx
> csum offload, or in conntrack.
> 
> So I think it would be nice to provide a hint that kernel already did
> checksumming.
> 
> The SKB_INFO attribute added in -net for GRO support seems like a
> candidate.  However, since 'already checksummed' is the common case this
> would mean adding that attribute most of the time.
> 
> Unless we would do the opposite hint, i.e. tell userspace when
> checksumming has NOT been performed yet.
> 
> Such change would however need to go into -net, else userspace can't tell
> 'checksum ok' from 'kernel too old to provide flag in SKB_INFO attribute'.

User-space has no way to know what info flags are available. If we add
more info flags in the future, we'll have the same problem that we have
now. I mean, currently an unset info flag from userspace may mean:

* It's actually unset.
* It is not available in this kernel.

So I think we need to pass a mask of available info flags, so
user-space knows how to interpret what 'unset' means. The mask would
also help to tell user-space that some info flag is not available.

Regards.