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>

1 files changed, 31 insertions, 8 deletions

diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index cdf4fc383cc9..4a6be0f87505 100644
--- 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);