Re: [iptables PATCH] nft: Eliminate table list from cache

[Phil Sutter <phil@nwl.cc>]

Hi Pablo,

On Thu, Jul 30, 2020 at 09:25:54PM +0200, Pablo Neira Ayuso wrote:
> On Thu, Jul 30, 2020 at 03:57:10PM +0200, Phil Sutter wrote:
> > The full list of tables in kernel is not relevant, only those used by
> > iptables-nft and for those, knowing if they exist or not is sufficient.
> > For holding that information, the already existing 'table' array in
> > nft_cache suits well.
> > 
> > Consequently, nft_table_find() merely checks if the new 'exists' boolean
> > is true or not and nft_for_each_table() iterates over the builtin_table
> > array in nft_handle, additionally checking the boolean in cache for
> > whether to skip the entry or not.
> > 
> > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > ---
> >  iptables/nft-cache.c | 73 +++++++++++---------------------------------
> >  iptables/nft-cache.h |  9 ------
> >  iptables/nft.c       | 55 +++++++++------------------------
> >  iptables/nft.h       |  2 +-
> >  4 files changed, 34 insertions(+), 105 deletions(-)
> 
> This diffstat looks interesting :-)

As promised, I wanted to leverage your change for further optimization,
but ended up optimizing your code out along with the old one. :D

> One question:
> 
>         c->table[i].exists = true;
> 
> then we assume this table is still in the kernel and we don't recheck?

Upon each COMMIT line, nft_action() calls nft_release_cache(). This will
also reset the 'exists' value to false.

> I mean, if you pipe command to an open process running
> iptables-restore (which has been the recommended interface for years
> to avoid of the overhead of system() invocation and to ensure atomic
> updates), is there any cache this new approach might get out of sync?

This is not just a problem of iptables-restore running in a pipe -
restoring a large ruleset (or just pure coincidence) could lead to the
same result.

Playing with 'iptables-nft-restore --noflush' reading from stdin and
calling 'nft flush ruleset' in a second shell right before entering
'COMMIT' leads to funny errors. This is not related to the table list
elimination though. I'll investigate.

Cheers, Phil