On 4/29/18 7:13 PM, David Ahern wrote:
>
> The idea here is to fast pass packets that fit a supported profile and
> are to be forwarded. Everything else should continue up the stack as it
> has wider capabilities. The helper and XDP programs should make no
> assumptions on what the broader kernel and userspace might be monitoring
> or want to do with packets that can not be forwarded in the fast path.
> This is very similar to hardware forwarding when it punts packets to the
> CPU for control plane assistance.
>
Thinking about this some more and how to return more information to the
bpf program about the FIB lookup.
bpf_fib_lookup struct is 64-bytes. It can not be expanded without
hurting performance. I could do another union on an input parameter and
return flags indicating why the returned index is 0. Something like this:
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 360a1168c353..75591522444c 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2314,6 +2314,12 @@ struct bpf_raw_tracepoint_args {
#define BPF_FIB_LOOKUP_DIRECT BIT(0)
#define BPF_FIB_LOOKUP_OUTPUT BIT(1)
+#define BPF_FIB_LKUP_RET_NO_FWD BIT(0) /* pkt is not fwded */
+#define BPF_FIB_LKUP_RET_UNSUPP_LWT BIT(1) /* fwd requires unsupp
encap */
+#define BPF_FIB_LKUP_RET_NO_NHDEV BIT(2) /* nh device does not exist */
+#define BPF_FIB_LKUP_RET_NO_NEIGH BIT(3) /* no neigh entry for nh */
+#define BPF_FIB_LKUP_RET_FRAG_NEEDED BIT(4) /* pkt too big to fwd */
+
struct bpf_fib_lookup {
/* input */
__u8 family; /* network family, AF_INET, AF_INET6, AF_MPLS */
@@ -2325,7 +2331,11 @@ struct bpf_fib_lookup {
/* total length of packet from network header - used for MTU
check */
__u16 tot_len;
- __u32 ifindex; /* L3 device index for lookup */
+
+ union {
+ __u32 ifindex; /* in: L3 device index for lookup */
+ __u32 ret_flags; /* out: BPF_FIB_LOOKUP_RET flags */
+ }
union {
/* inputs to lookup */
Similarly for the fib result, it could be returned with a union on say
family:
union {
__u8 family; /* in: network family, AF_INET, AF_INET6, AF_MPLS */
__u8 rt_type; /* out: FIB lookup route type */
};
Then if the fib result is -EINVAL/-EHOSTUNREACH/-EACCES, rt_type is set
to RTN_BLACKHOLE/RTN_UNREACHABLE/RTN_PROHIBIT allowing the XDP program
to make an informed decision on dropping the packet.
To avoid performance hits on the forwarding path, these return values
would *only* set if the ifindex returned is 0.
