Re: [RFC v2 bpf-next 8/9] bpf: Provide helper to do lookups in kernel FIB table

[David Ahern <dsahern@gmail.com>]

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.