Re: tc_classid access in skb bpf context

[Matthew Cover]

On 05/22/2019 01:52 AM, Matthew Cover wrote:
> > __sk_buff has a member tc_classid which I'm interested in accessing from the skb bpf context.
> > 
> > A bpf program which accesses skb->tc_classid compiles, but fails verification; the specific failure is "invalid bpf_context access".
> > 
> > if (skb->tc_classid != 0)
> >  return 1;
> > return 0;
> > 
> > Some of the tests in tools/testing/selftests/bpf/verifier/ (those on tc_classid) further confirm that this is, in all likelihood, intentional behavior.
> > 
> > The very similar bpf program which instead accesses skb->mark works as desired.
> > 
> > if (skb->mark != 0)
> >  return 1;
> > return 0;
> 
> You should be able to access skb->tc_classid, perhaps you're using the wrong program
> type? BPF_PROG_TYPE_SCHED_CLS is supposed to work (if not we'd have a regression).
> 

I am in fact using BPF_PROG_TYPE_SOCKET_FILTER and using the program as PACKET_FANOUT_DATA with PACKET_FANOUT_EBPF.

I have been working on a series of utils which leverage PACKET_FANOUT to provide various per-socket-fd (per-cpu, per-queue, per-rx-flow-hash-indirection-table-idx) statistics and pcap files. While playing with PACKET_FANOUT_EBPF, I realized that I could use the bpf program to categorize packets in ways packet-filter(7) does not provide.

As a concrete example, I plan to build a util `rxtxmark` which could be passed something like `--mark-list 42,88`. This would be translated to a bpf program where the return code is the ordinality of the mark in the list.

if (skb->mark == 42)
 return 1;
if (skb->mark == 88)
 return 2;
return 0;

Packets enqueued to fd0 are simply ignored. Packets enqueued to the other fds are processed into pcaps and statistics.

While I may build a util for tc_classid which does per-user-requested-classid pcaps and statistics like `rxtxmark` does for marks, I'm also interested in using tc_classid as a simple way to capture tx packets from a long running program on the fly.

The program under inspection would simply be added to a net_cls cgroup which has a unique classid defined. A bpf program would be attached to map packets with that classid to fd1. While I can do this already by using iptables to translate the tc_classid to a mark, that complicates the implementation greatly since the firewall has to be touched (which is probably overreaching for a packet capture util and would most likely be left to the user to configure).

> > I built a kernel (v5.1) with 4 instances of the following line removed from net/core/filter.c to test the behavior when the instructions pass verification.
> > 
> >     switch (off) {
> > -    case bpf_ctx_range(struct __sk_buff, tc_classid):
> > ...
> >         return false;
> > 
> > It appears skb->tc_classid is always zero within my bpf program, even when I verify by other means (e.g. netfilter) that the value is set non-zero.
> > 
> > I gather that sk_buff proper sometimes (i.e. at some layers) has qdisc_skb_cb stored in skb->cb, but not always.
> > 
> > I suspect that the tc_classid is available at l3 (and therefore to utils like netfilter, ip route, tc), but not at l2 (and not to AF_PACKET).
> 
> From tc/BPF context you can use it; it's been long time, but I think back then
> we mapped it into cb[] so it can be used within the BPF context to pass skb data
> around e.g. between tail calls, and cls_bpf_classify() when in direct-action mode
> which likely everyone is/should-be using then maps that skb->tc_classid u16 cb[]
> value to res->classid on program return which then in either sch_handle_ingress()
> or sch_handle_egress() is transferred into the skb->tc_index.
> 

It sounds like just before the start of a BPF_PROG_TYPE_SCHED_CLS bpf program tc_classid id placed in skb->cb. The missing plumbing to support my use case is probably the same thing, but for BPF_PROG_TYPE_SOCKET_FILTER.

I'll see about familiarizing myself with both as time permits and perhaps I can get tc_classid working for a BPF_PROG_TYPE_SOCKET_FILTER program; it certainly sounds like it's doable.

> > Is it impractical to make skb->tc_classid available in this bpf context or is there just some plumbing which hasn't been connected yet?
> > 
> > Is my suspicion that skb->cb no longer contains qdisc_skb_cb due to crossing a layer boundary well founded?
> > 
> > I'm willing to look into hooking things together as time permits if it's a feasible task.
> > 
> > It's trivial to have iptables match on tc_classid and set a mark which is available to bpf at l2, but I'd like to better understand this.
> > 
> > Thanks,
> > Matt C.
> >