samples/bpf: Decrement ttl in fib forwarding example

Only consider forwarding packets if ttl in received packet is > 1 and
decrement ttl before handing off to bpf_redirect_map.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index cdf4fc3..4a6be0f 100644 (file)
--- a/samples/bpf/xdp_fwd_kern.c
+++ b/samples/bpf/xdp_fwd_kern.c
@@ -30,12 +30,24 @@ struct bpf_map_def SEC("maps") tx_port = {
        .max_entries = 64,
 };
 
+/* from include/net/ip.h */
+static __always_inline int ip_decrease_ttl(struct iphdr *iph)
+{
+       u32 check = (__force u32)iph->check;
+
+       check += (__force u32)htons(0x0100);
+       iph->check = (__force __sum16)(check + (check >= 0xFFFF));
+       return --iph->ttl;
+}
+
 static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
 {
        void *data_end = (void *)(long)ctx->data_end;
        void *data = (void *)(long)ctx->data;
        struct bpf_fib_lookup fib_params;
        struct ethhdr *eth = data;
+       struct ipv6hdr *ip6h;
+       struct iphdr *iph;
        int out_index;
        u16 h_proto;
        u64 nh_off;
@@ -48,11 +60,14 @@ static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
 
        h_proto = eth->h_proto;
        if (h_proto == htons(ETH_P_IP)) {
-               struct iphdr *iph = data + nh_off;
+               iph = data + nh_off;
 
                if (iph + 1 > data_end)
                        return XDP_DROP;
 
+               if (iph->ttl <= 1)
+                       return XDP_PASS;
+
                fib_params.family       = AF_INET;
                fib_params.tos          = iph->tos;
                fib_params.l4_protocol  = iph->protocol;
@@ -64,19 +79,22 @@ static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
        } else if (h_proto == htons(ETH_P_IPV6)) {
                struct in6_addr *src = (struct in6_addr *) fib_params.ipv6_src;
                struct in6_addr *dst = (struct in6_addr *) fib_params.ipv6_dst;
-               struct ipv6hdr *iph = data + nh_off;
 
-               if (iph + 1 > data_end)
+               ip6h = data + nh_off;
+               if (ip6h + 1 > data_end)
                        return XDP_DROP;
 
+               if (ip6h->hop_limit <= 1)
+                       return XDP_PASS;
+
                fib_params.family       = AF_INET6;
-               fib_params.flowlabel    = *(__be32 *)iph & IPV6_FLOWINFO_MASK;
-               fib_params.l4_protocol  = iph->nexthdr;
+               fib_params.flowlabel    = *(__be32 *)ip6h & IPV6_FLOWINFO_MASK;
+               fib_params.l4_protocol  = ip6h->nexthdr;
                fib_params.sport        = 0;
                fib_params.dport        = 0;
-               fib_params.tot_len      = ntohs(iph->payload_len);
-               *src                    = iph->saddr;
-               *dst                    = iph->daddr;
+               fib_params.tot_len      = ntohs(ip6h->payload_len);
+               *src                    = ip6h->saddr;
+               *dst                    = ip6h->daddr;
        } else {
                return XDP_PASS;
        }
@@ -92,6 +110,11 @@ static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
         *       forwarding packets are dropped.
         */
        if (out_index > 0) {
+               if (h_proto == htons(ETH_P_IP))
+                       ip_decrease_ttl(iph);
+               else if (h_proto == htons(ETH_P_IPV6))
+                       ip6h->hop_limit--;
+
                memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
                memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
                return bpf_redirect_map(&tx_port, out_index, 0);