On Tue, Jun 9, 2020 at 10:18 PM Tobias Brunner <tob...@strongswan.org> wrote: > > Hi Xin, > > >> I guess we could workaround this issue in strongSwan by installing > >> policies that share the same mark and selector with the same priority, > >> so only one instance is ever installed in the kernel. But the inability > >> to address the exact policy when querying/deleting still looks like a > >> problem to me in general. > >> > > For deleting, yes, but for querying, I think it makes sense not to pass > > the priority, and always get the policy with the highest priority. > > While I agree it's less of a problem (at least for strongSwan), it > should be possible to query the exact policy one wants. Because as far > as I understand, the whole point of Steffen's original patch was that > all duplicate policies could get used concurrently, depending on the > marks and masks on them and the traffic, so all of them must be queryable. > > But I actually think the previous check that viewed policies with the > exact same mark and value as duplicates made sense, because those will > never be used concurrently. It would at least fix the default behavior > with strongSwan (users have to configure marks/masks manually). > > > We can separate the deleting path from the querying path when > > XFRMA_PRIORITY attribute is set. > > > > Is that enough for your case to only fix for the policy deleting? > > While such an attribute could be part of a solution, it does not fix the > regression your patch created. The kernel behavior changed and a > userland modification is required to get back to something resembling > the previous behavior (without an additional kernel patch we'll actually > not be able to restore the previous behavior, where we separated > different types of policies into priority classes). That is, current > and old strongSwan versions could create lots of duplicate/lingering > policies, which is not good. > > A problem with such an attribute is how userland would learn when to use > it. We could query the kernel version, but patches might get > backported. So how can we know the kernel will create duplicates when > we update a policy and change the priority, which we then have to delete > (or even can delete with such a new attribute)? Do we have to do a > runtime check (e.g. install two duplicate policies with different > priorities and delete twice to see if the second attempt results in an > error)? With marks it's relatively easy as users have to configure them > explicitly and they work or they don't depending on the kernel version. > But here it's not so easy as the IKE daemon uses priorities extensively > already. > > Like the marks it might work somehow if the new attribute also had to be > passed in the message that creates a policy (marks have to be passed > with every message, including querying them). While that's not super > ideal as we'd have two priority values in these messages (and have to > keep track of them in the kernel state), there is some precedent with > the anti-replay config for SAs (which can be passed via xfrm_usersa_info > struct or as separate attribute with more options for ESN). Userland > would still have to learn somehow that the kernel understands the new > attribute and duplicate policies with different priorities are possible. > But if there was any advantage in using this, we could perhaps later > add an option for users to enable it. At least the current behavior > would not change (i.e. older strongSwan versions would continue to run > on newer kernels without modifications). > Now I can see some about how userland is using "priority". We probably need to revert both this patch and 7cb8a93968e3 ("xfrm: Allow inserting policies with matching mark and different priorities").
Thanks for the explanation, I will think more about it tomorrow.