Hi Pablo, On Thu, Feb 02, 2023 at 10:31:58PM +0100, Pablo Neira Ayuso wrote: > 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. Yes, this is indeed somewhat of a downside of this approach: a kernel which doesn't support the new attribute will use the compatible version of the rule instead of the improved one. But apart from that, everything just works. > > 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. Sorry. :( > > 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. No swap: The kernel will dump in NFTA_RULE_EXPRESSIONS exactly what it got in that attribute, same for the new one. > 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). Yes, exactly. And old user space or nft will put the "new" representation into NFTA_RULE_EXPRESSIONS, not attach NFTA_RULE_ACTUAL_EXPRS and thus the kernel will use the former in its data path. > [...] > > 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? Fine with me! "ACTUAL" was suggested by Florian, probably to point out that it's what should take precedence if present. In my understanding, "ALT" means "as good as". > Sorry, this is a bit confusing but I understand something like this is > required as you explained during the NFWS. Thanks. Irrespective of the "crazy container people" mixing iptables versions and variants like mad, I believe it will allow us to make more drastic changes in future. Cheers, Phil
