Re: [libnftnl RFC] data_reg: Improve data reg value printing

[Phil Sutter <phil@nwl.cc>]

Hi Pablo,

On Mon, Sep 15, 2025 at 11:19:27PM +0200, Pablo Neira Ayuso wrote:
> On Thu, Sep 11, 2025 at 04:11:45PM +0200, Phil Sutter wrote:
> > The old code printing each field with data as u32 value is problematic
> > in two ways:
> > 
> > A) Field values are printed in host byte order which may not be correct
> >    and output for identical data will divert between machines of
> >    different Endianness.
> > 
> > B) The actual data length is not clearly readable from given output.
> > 
> > This patch won't entirely fix for (A) given that data may be in host
> > byte order but it solves for the common case of matching against packet
> > data.
> > 
> > Fixing for (B) is crucial to see what's happening beneath the bonnet.
> > The new output will show exactly what is used e.g. by a cmp expression.
> 
> Could you fix this from libnftables? ie. add print functions that have
> access to the byteorder, so print can do accordingly.

You mean replacing nftnl_expr_fprintf() and all per-expr callbacks
entirely? I guess this exposes lots of libnftnl internals to
libnftables.

IIRC, the last approach from 2020 was to communicate LHS byteorder to
RHS in libnftnl internally and in addition to that annotate sets with
element byteorder info (which is not entirely trivial due to
concatenations of data with varying byteorder.

I don't get the code in my old branches anymore, though. Maybe if I read
up on our past mails. Or maybe we just retry from scratch with a fresh
mind. :)

Cheers, Phil