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From:   Matthew Cover <matthew.cover@stackpath.com>
To:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        "David S. Miller" <davem@davemloft.net>,
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        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Matthew Cover <werekraken@gmail.com>
Subject: tc_classid access in skb bpf context
Thread-Topic: tc_classid access in skb bpf context
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Date:   Tue, 21 May 2019 23:52:43 +0000
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__sk_buff has a member tc_classid which I'm interested in accessing from th=
e skb bpf context.

A bpf program which accesses skb->tc_classid compiles, but fails verificati=
on; the specific failure is "invalid bpf_context access".

if (skb->tc_classid !=3D 0)
 return 1;
return 0;

Some of the tests in tools/testing/selftests/bpf/verifier/ (those on tc_cla=
ssid) further confirm that this is, in all likelihood, intentional behavior=
.

The very similar bpf program which instead accesses skb->mark works as desi=
red.

if (skb->mark !=3D 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 verifica=
tion.

    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 li=
ke 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 crossin=
g 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 a=
vailable to bpf at l2, but I'd like to better understand this.

Thanks,
Matt C.=