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.
