Re: [nf-next PATCH v2] netfilter: nf_tables: Introduce NFTA_RULE_ACTUAL_EXPR

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

Hi Phil,

On Wed, Jan 18, 2023 at 02:48:20PM +0100, Phil Sutter wrote:
[...]
> The crucial aspect of this implementation is to provide a compatible
> rule representation for old software which is not aware of it. This is
> only possible by dumping the compat representation in the well-known
> NFTA_RULE_EXPRESSIONS attribute.

OK, so NFTA_RULE_EXPRESSIONS contains the xt expressions.

Then, _ACTUAL_EXPR is taken if kernel supports it and these are
expressions that run from datapath, if present.

> This means what is contained in NFTA_RULE_EXPRESSIONS may not be what
> the kernel actually executes. To make this less scary, the kernel should
> dump the actual rule in a second attribute for the sake of verification
> in user space.
>
> While rule dumps are pretty much fixed given the above, there is
> flexibility when it comes to loading the rule:
> 
> A) Submit the compat representation as additional attribute
> 
> This was my initial approach, but Florian objected because the changing
> content of NFTA_RULE_EXPRESSIONS attribute may be confusing:

It is indeed.

> On input, NFTA_RULE_EXPRESSIONS contains the new rule representation, on
> output it contains the compat one. The extra attribute I introduced
> behaves identical, i.e. on input it holds the compat representation
> while on output it holds the new one.
> 
> B) Submit the new representation as additional attribute
> 
> This is the current approach: If the additional attribute is present,
> the kernel will use it to build the rule and leave NFTA_RULE_EXPRESSIONS
> alone (actually: store it for dumps). Otherwise it will "fall back" to
> using NFTA_RULE_EXPRESSIONS just as usual.
>
> When dumping, if a stored NFTA_RULE_EXPRESSIONS content is present, it
> will dump that as-is and serialize the active rule into an additional
> attribute. Otherwise the active rule will go into NFTA_RULE_EXPRESSIONS
> just as usual.

So this is not swapping things, right? Probably I am still getting
confused but the initial approach described in A.

When, dumping back to userspace, NFTA_RULE_EXPRESSIONS still stores
the xt compat representation and NFTA_RULE_ACTUAL_EXPRS the one that
runs from kernel datapath (if the kernel supports this attribute).

[...]
> I am swapping things around in libnftnl - it uses NFTA_RULE_ACTUAL_EXPRS
> if present and puts NFTA_RULE_EXPRESSIONS into a second list for
> verification only. In iptables, I parse both lists separately into
> iptables_command_state objects and compare them. If not identical,
> there's a bug.

Old kernels would simply discard the ACTUAL_ attribute. Maybe _ALT_
standing by alternative is a better name?

Sorry, this is a bit confusing but I understand something like this is
required as you explained during the NFWS.