Re: [PATCH bpf-next V8 3/8] bpf: bpf_fib_lookup return MTU value as output when looked up

Daniel Borkmann <daniel@xxxxxxxxxxxxx> · Wed, 2 Dec 2020 23:23:51 +0100

On 11/27/20 7:06 PM, Jesper Dangaard Brouer wrote:
The BPF-helpers for FIB lookup (bpf_xdp_fib_lookup and bpf_skb_fib_lookup)
can perform MTU check and return BPF_FIB_LKUP_RET_FRAG_NEEDED.  The BPF-prog
don't know the MTU value that caused this rejection.

If the BPF-prog wants to implement PMTU (Path MTU Discovery) (rfc1191) it
need to know this MTU value for the ICMP packet.

Patch change lookup and result struct bpf_fib_lookup, to contain this MTU
value as output via a union with 'tot_len' as this is the value used for
the MTU lookup.

V5:
  - Fixed uninit value spotted by Dan Carpenter.
  - Name struct output member mtu_result

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
  include/uapi/linux/bpf.h       |   11 +++++++++--
  net/core/filter.c              |   22 +++++++++++++++-------
  tools/include/uapi/linux/bpf.h |   11 +++++++++--
  3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index c3458ec1f30a..848398bd5a54 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2220,6 +2220,9 @@ union bpf_attr {
   *		* > 0 one of **BPF_FIB_LKUP_RET_** codes explaining why the
   *		  packet is not forwarded or needs assist from full stack
   *
+ *		If lookup fails with BPF_FIB_LKUP_RET_FRAG_NEEDED, then the MTU
+ *		was exceeded and output params->mtu_result contains the MTU.
+ *
   * long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
   *	Description
   *		Add an entry to, or update a sockhash *map* referencing sockets.
@@ -4961,9 +4964,13 @@ struct bpf_fib_lookup {
  	__be16	sport;
  	__be16	dport;
  
-	/* total length of packet from network header - used for MTU check */
-	__u16	tot_len;
+	union {	/* used for MTU check */
+		/* input to lookup */
+		__u16	tot_len; /* L3 length from network hdr (iph->tot_len) */
  
+		/* output: MTU value */
+		__u16	mtu_result;
+	};
  	/* input: L3 device index for lookup
  	 * output: device index from FIB lookup
  	 */
diff --git a/net/core/filter.c b/net/core/filter.c
index 84d77c425fbe..25b137ffdced 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5265,12 +5265,14 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
  #if IS_ENABLED(CONFIG_INET) || IS_ENABLED(CONFIG_IPV6)
  static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
  				  const struct neighbour *neigh,
-				  const struct net_device *dev)
+				  const struct net_device *dev, u32 mtu)
  {
  	memcpy(params->dmac, neigh->ha, ETH_ALEN);
  	memcpy(params->smac, dev->dev_addr, ETH_ALEN);
  	params->h_vlan_TCI = 0;
  	params->h_vlan_proto = 0;
+	if (mtu)
+		params->mtu_result = mtu; /* union with tot_len */
  
  	return 0;
  }
@@ -5286,8 +5288,8 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  	struct net_device *dev;
  	struct fib_result res;
  	struct flowi4 fl4;
+	u32 mtu = 0;
  	int err;
-	u32 mtu;
  
  	dev = dev_get_by_index_rcu(net, params->ifindex);
  	if (unlikely(!dev))
@@ -5354,8 +5356,10 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  
  	if (check_mtu) {
  		mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
  			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
+		}
  	}
  
  	nhc = res.nhc;
@@ -5389,7 +5393,7 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  	if (!neigh)
  		return BPF_FIB_LKUP_RET_NO_NEIGH;
  
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
  }
  #endif
  
@@ -5406,7 +5410,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  	struct flowi6 fl6;
  	int strict = 0;
  	int oif, err;
-	u32 mtu;
+	u32 mtu = 0;
  
  	/* link local addresses are never forwarded */
  	if (rt6_need_strict(dst) || rt6_need_strict(src))
@@ -5481,8 +5485,10 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  
  	if (check_mtu) {
  		mtu = ipv6_stub->ip6_mtu_from_fib6(&res, dst, src);
-		if (params->tot_len > mtu)
+		if (params->tot_len > mtu) {
+			params->mtu_result = mtu; /* union with tot_len */
  			return BPF_FIB_LKUP_RET_FRAG_NEEDED;

And with mentioned approach from [0] we also only ever need to set the above,
but we don't need to drag it anywhere else via bpf_fib_set_fwd_params() with
the extra zero check on mtu. So this should simplify this one as well.

  [0] https://lore.kernel.org/bpf/6f9cac4e-a231-ff8d-43a5-828995ca5ec7@xxxxxxxxxxxxx/

+		}
  	}
  
  	if (res.nh->fib_nh_lws)
@@ -5502,7 +5508,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
  	if (!neigh)
  		return BPF_FIB_LKUP_RET_NO_NEIGH;
  
-	return bpf_fib_set_fwd_params(params, neigh, dev);
+	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);






