bpf: fix cls_bpf on filter replace

Running the following sequence is currently broken:

  # tc qdisc add dev foo clsact
  # tc filter replace dev foo ingress prio 1 handle 1 bpf da obj bar.o
  # tc filter replace dev foo ingress prio 1 handle 1 bpf da obj bar.o
  RTNETLINK answers: Invalid argument

The normal expectation on kernel side is that the second command
succeeds replacing the existing program. However, what happens is
in cls_bpf_change(), we bail out with err in the second run in
cls_bpf_offload(). The EINVAL comes directly in cls_bpf_offload()
when comparing prog vs oldprog's gen_flags. In case of above
replace the new prog's gen_flags are 0, but the old ones are 8,
which means TCA_CLS_FLAGS_NOT_IN_HW is set (e.g. drivers not having
cls_bpf offload).

Fix 102740bd94 ("cls_bpf: fix offload assumptions after callback
conversion") in the following way: gen_flags from user space passed
down via netlink cannot include status flags like TCA_CLS_FLAGS_IN_HW
or TCA_CLS_FLAGS_NOT_IN_HW as opposed to oldprog that we previously
loaded. Therefore, it doesn't make any sense to include them in the
gen_flags comparison with the new prog before we even attempt to
offload. Thus, lets fix this before 4.15 goes out.

Fixes: 102740bd94 ("cls_bpf: fix offload assumptions after callback conversion")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_bpf.c

@@ -183,10 +183,17 @@ static int cls_bpf_offload_cmd(struct tcf_proto *tp, struct cls_bpf_prog *prog,
 	return 0;
 }
 
+static u32 cls_bpf_flags(u32 flags)
+{
+	return flags & CLS_BPF_SUPPORTED_GEN_FLAGS;
+}
+
 static int cls_bpf_offload(struct tcf_proto *tp, struct cls_bpf_prog *prog,
 			   struct cls_bpf_prog *oldprog)
 {
-	if (prog && oldprog && prog->gen_flags != oldprog->gen_flags)
+	if (prog && oldprog &&
+	    cls_bpf_flags(prog->gen_flags) !=
+	    cls_bpf_flags(oldprog->gen_flags))
 		return -EINVAL;
 
 	if (prog && tc_skip_hw(prog->gen_flags))