Mindaugas Rasiukevicius wrote:
Hi, OK, to summarise what has been discussed: - Problem There is a need to perform more complex operations from the BPF program. Currently, there is no (practical) way to do that from the byte-code. Such functionality is useful for the packet filters or other components, which could integrate with BPF. For example, while most of the packet inspection logic can stay in the byte-code, such operations as looking up an IP address in some container or walking the IPv6 headers and returning some offsets have to be done externally. The first existing user of such capability would be NPF in NetBSD.
I'd argue that the IPv6 problem is of such a generic nature that it deserves its own instruction/s. We may look at IPv6 today and think nobody uses it much but over time that is going to change. Thus there will be an outcome not possible with co-processor approach if an instruction is created for that purpose and is common across all platforms through libpcap. Unless the IPv6 problem is too complex for a single instruction (this has not been demonstrated.) In that case maybe BPF itself needs to evolve such that it can support more complex instructions. The current implementation of BPF makes it very hard to expand the instruction set without impinging on the ability to make future changes due to the way in which instructions are codified into 32bits. Whilst the method of supporting a co-processor gets around that, it does so in such a generic fashion that it becomes too easy to use it as a bit-bucket for anything you think might be a good idea if BPF could do without really evaluating if it should do. When it comes to looking up addresses in tables, I don't see the advantage in adding this to BPF to support NPF. My suspicioun is that the goal is to support expressing the entire rule as just BPF byte code. For rules, it makes no sense as the expensive operation (table lookup) could just as easily be done after the rest of the packet is matched with BPF. Or is there something else here at play that I'm missing?