On Tue, May 21, 2019 at 04:47:58PM +0100, Lorenz Bauer wrote: > On Fri, 17 May 2019 at 00:38, Nitin Hande <nitin.hande@xxxxxxxxx> wrote: > > > > On Thu, May 16, 2019 at 2:57 PM Alexei Starovoitov > > <alexei.starovoitov@xxxxxxxxx> wrote: > > > > > > On Thu, May 16, 2019 at 09:41:34AM +0100, Lorenz Bauer wrote: > > > > On Wed, 15 May 2019 at 18:16, Joe Stringer <joe@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On Wed, May 15, 2019 at 8:11 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > In the BPF-based TPROXY session with Joe Stringer [1], I mentioned > > > > > > that the sk_lookup_* helpers currently return inconsistent results if > > > > > > SK_REUSEPORT programs are in play. > > > > > > > > > > > > SK_REUSEPORT programs are a hook point in inet_lookup. They get access > > > > > > to the full packet > > > > > > that triggered the look up. To support this, inet_lookup gained a new > > > > > > skb argument to provide such context. If skb is NULL, the SK_REUSEPORT > > > > > > program is skipped and instead the socket is selected by its hash. > > > > > > > > > > > > The first problem is that not all callers to inet_lookup from BPF have > > > > > > an skb, e.g. XDP. This means that a look up from XDP gives an > > > > > > incorrect result. For now that is not a huge problem. However, once we > > > > > > get sk_assign as proposed by Joe, we can end up circumventing > > > > > > SK_REUSEPORT. > > > > > > > > > > To clarify a bit, the reason this is a problem is that a > > > > > straightforward implementation may just consider passing the skb > > > > > context into the sk_lookup_*() and through to the inet_lookup() so > > > > > that it would run the SK_REUSEPORT BPF program for socket selection on > > > > > the skb when the packet-path BPF program performs the socket lookup. > > > > > However, as this paragraph describes, the skb context is not always > > > > > available. > > > > > > > > > > > At the conference, someone suggested using a similar approach to the > > > > > > work done on the flow dissector by Stanislav: create a dedicated > > > > > > context sk_reuseport which can either take an skb or a plain pointer. > > > > > > Patch up load_bytes to deal with both. Pass the context to > > > > > > inet_lookup. > > > > > > > > > > > > This is when we hit the second problem: using the skb or XDP context > > > > > > directly is incorrect, because it assumes that the relevant protocol > > > > > > headers are at the start of the buffer. In our use case, the correct > > > > > > headers are at an offset since we're inspecting encapsulated packets. > > > > > > > > > > > > The best solution I've come up with is to steal 17 bits from the flags > > > > > > argument to sk_lookup_*, 1 bit for BPF_F_HEADERS_AT_OFFSET, 16bit for > > > > > > the offset itself. > > > > > > > > > > FYI there's also the upper 32 bits of the netns_id parameter, another > > > > > option would be to steal 16 bits from there. > > > > > > > > Or len, which is only 16 bits realistically. The offset doesn't really fit into > > > > either of them very well, using flags seemed the cleanest to me. > > > > Is there some best practice around this? > > > > > > > > > > > > > > > Thoughts? > > > > > > > > > > Internally with skbs, we use `skb_pull()` to manage header offsets, > > > > > could we do something similar with `bpf_xdp_adjust_head()` prior to > > > > > the call to `bpf_sk_lookup_*()`? > > > > > > > > That would only work if it retained the contents of the skipped > > > > buffer, and if there > > > > was a way to undo the adjustment later. We're doing the sk_lookup to > > > > decide whether to > > > > accept or forward the packet, so at the point of the call we might still need > > > > that data. Is that feasible with skb / XDP ctx? > > > > > > While discussing the solution for reuseport I propose to use > > > progs/test_select_reuseport_kern.c as an example of realistic program. > > > It reads tcp/udp header directly via ctx->data or via bpf_skb_load_bytes() > > > including payload after the header. > > > It also uses bpf_skb_load_bytes_relative() to fetch IP. > > > I think if we're fixing the sk_lookup from XDP the above program > > > would need to work. > > > > > > And I think we can make it work by adding new requirement that > > > 'struct bpf_sock_tuple *' argument to bpf_sk_lookup_* must be > > > a pointer to the packet and not a pointer to bpf program stack. > > > Then helper can construct a fake skb and assign > > > fake_skb->data = &bpf_sock_tuple_arg.sport > > > It can check that struct bpf_sock_tuple * pointer is within 100-ish bytes > > > from xdp->data and within xdp->data_end > > > This way the reuseport program's assumption that ctx->data points to tcp/udp > > > will be preserved and it can access it all including payload. > > > > > > This approach doesn't need to mess with xdp_adjust_head and adjust uapi to pass length. > > > Existing progs/test_sk_lookup_kern.c will magically start working with XDP > > > even when reuseport prog is attached. > > > Thoughts? > > > > I like this approach. A fake_skb approach will normalize the bpf_sk_lookup_*() > > API peering into the kernel API between TC and XDP invocation. Just one question > > that comes, I remember one of the comments I received during my XDP commit > > was the stateless nature of XDP services and providing a fake_skb may bring > > some potential side-effects to the desire of statelessness. Is that > > still a possibility? > > How do we guard against it? > > To follow up on this, I'm also not sure how to tackle a "fake skb". If > I remember this > came up during the flow dissector series, and wasn't met with > enthusiasm. Granted, > replacing the skb argument to the lookup functions seems even harder, so maybe > this is the lesser evil? flow_dissector pretends to have 'skb' as bpf prog argument. It doesn't allocate actual 'struct sk_buff' on the kernel side. The verifier rewrites __sk_buff->foo accesses into 'struct bpf_flow_dissector'->bar access. I was hoping similar idea can apply here. BPF_PROG_TYPE_SK_REUSEPORT type takes 'struct sk_reuseport_md *' so we don't need real skb there anyway. But for older socket_filter based so_reuseport progs it's more difficult, since at the verification time they're generic socket_filter progs. Right now I don't have a better idea than to do static per_cpu struct sk_buff.
