__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;
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).
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.
