Re: [PATCH 0/4 net-next] flow_offload: update mangle action representation

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

On Thu, Aug 29, 2019 at 06:54:48PM -0700, Jakub Kicinski wrote:
> On Fri, 30 Aug 2019 02:53:32 +0200, Pablo Neira Ayuso wrote:
> > * Offsets do not need to be on the 32-bits boundaries anymore. This
> >   patchset adds front-end code to adjust the offset and length coming
> >   from the tc pedit representation, so drivers get an exact header field
> >   offset and length.
> 
> But drivers use offsetof(start of field) to match headers, and I don't
> see you changing that. So how does this work then?

Drivers can only use offsetof() for fields that are on the 32-bits
boundary.

Before this patchset, if you want to mangle the destination port, then
the driver needs to refer to the source port offset and the length is
4 bytes, so the mask is telling what needs to be mangled.

After this patchset, the offset is set to the destination port, the
length is set to 2-bytes, and the mask is telling what bytes of the
destination port field you specifically want to update.

It's just 100 LOC of preprocessing that is simplifying driver
codebase.

> Say - I want to change the second byte of an IPv4 address.

Then, the front-end sets the offset to IPv4 address header field, and
the mask tells what to update.

> > * The front-end coalesces consecutive pedit actions into one single
> >   word, so drivers can mangle IPv6 and ethernet address fields in one
> >   single go.
> 
> You still only coalesce up to 16 bytes, no?

You only have to rise FLOW_ACTION_MANGLE_MAXLEN coming in this patch
if you need more. I don't know of any packet field larger than 16
bytes. If there is a use-case for this, it should be easy to rise that
definition.

> As I said previously drivers will continue to implement mangle action
> merge code if that's the case. It'd be nice if core did the coalescing,
> and mark down first and last action, in case there is a setup cost for
> rewrite group.

In this patchset, the core front-end is doing the coalescing.