Lennart Poettering
acf7f253de
This new kernel 4.15 flag permits that multiple BPF programs can be executed for each packet processed: multiple per cgroup plus all programs defined up the tree on all parent cgroups. We can use this for two features: 1. Finally provide per-slice IP accounting (which was previously unavailable) 2. Permit delegation of BPF programs to services (i.e. leaf nodes). This patch beefs up PID1's handling of BPF to enable both. Note two special items to keep in mind: a. Our inner-node BPF programs (i.e. the ones we attach to slices) do not enforce IP access lists, that's done exclsuively in the leaf-node BPF programs. That's a good thing, since that way rules in leaf nodes can cancel out rules further up (i.e. for example to implement a logic of "disallow everything except httpd.service"). Inner node BPF programs to accounting however if that's requested. This is beneficial for performance reasons: it means in order to provide per-slice IP accounting we don't have to add up all child unit's data. b. When this code is run on pre-4.15 kernel (i.e. where BPF_F_ALLOW_MULTI is not available) we'll make IP acocunting on slice units unavailable (i.e. revert to behaviour from before this commit). For leaf nodes we'll fallback to non-ALLOW_MULTI mode however, which means that BPF delegation is not available there at all, if IP fw/acct is turned on for the unit. This is a change from earlier behaviour, where we use the BPF_F_ALLOW_OVERRIDE flag, so that our fw/acct would lose its effect as soon as delegation was turned on and some client made use of that. I think the new behaviour is the safer choice in this case, as silent bypassing of our fw rules is not possible anymore. And if people want proper delegation then the way out is a more modern kernel or turning off IP firewalling/acct for the unit algother.
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