diff options
authorJason A. Donenfeld <Jason@zx2c4.com>2020-04-27 14:42:08 -0600
committerJason A. Donenfeld <Jason@zx2c4.com>2020-08-15 09:39:05 +0200
commite9027093a9eeb181e4128723acf2bf72924b9f75 (patch)
parentnet: xdp: pull ethernet header off packet after computing skb->protocol (diff)
net: xdp: account for layer 3 packets in generic skb handlerjd/xdp-l3
A user reported that packets from wireguard were possibly ignored by XDP [1]. Another user reported that modifying packets from layer 3 interfaces results in impossible to diagnose drops. Apparently, the generic skb xdp handler path seems to assume that packets will always have an ethernet header, which really isn't always the case for layer 3 packets, which are produced by multiple drivers. This patch fixes the oversight. If the mac_len is 0 and so is hard_header_len, then we know that the skb is a layer 3 packet, and in that case prepend a pseudo ethhdr to the packet whose h_proto is copied from skb->protocol, which will have the appropriate v4 or v6 ethertype. This allows us to keep XDP programs' assumption correct about packets always having that ethernet header, so that existing code doesn't break, while still allowing layer 3 devices to use the generic XDP handler. We push on the ethernet header and then pull it right off and set mac_len to the ethernet header size, so that the rest of the XDP code does not need any changes. That is, it makes it so that the skb has its ethernet header just before the data pointer, of size ETH_HLEN. Previous discussions have included the point that maybe XDP should just be intentionally broken on layer 3 interfaces, by design, and that layer 3 people should be using cls_bpf. However, I think there are good grounds to reconsider this perspective: - Complicated deployments wind up applying XDP modifications to a variety of different devices on a given host, some of which are using specialized ethernet cards and other ones using virtual layer 3 interfaces, such as WireGuard. Being able to apply one codebase to each of these winds up being essential. - cls_bpf does not support the same feature set as XDP, and operates at a slightly different stage in the networking stack. You may reply, "then add all the features you want to cls_bpf", but that seems to be missing the point, and would still result in there being two ways to do everything, which is not desirable for anyone actually _using_ this code. - While XDP was originally made for hardware offloading, and while many look disdainfully upon the generic mode, it nevertheless remains a highly useful and popular way of adding bespoke packet transformations, and from that perspective, a difference between layer 2 and layer 3 packets is immaterial if the user is primarily concerned with transformations to layer 3 and beyond. - It's not impossible to imagine layer 3 hardware (e.g. a WireGuard PCIe card) including eBPF/XDP functionality built-in. In that case, why limit XDP as a technology to only layer 2? Then, having generic XDP work for layer 3 would naturally fit as well. [1] https://lore.kernel.org/wireguard/M5WzVK5--3-2@tuta.io/ Reported-by: Thomas Ptacek <thomas@sockpuppet.org> Reported-by: Adhipati Blambangan <adhipati@tuta.io> Cc: David Ahern <dsahern@gmail.com> Cc: Toke Høiland-Jørgensen <toke@redhat.com> Cc: Jakub Kicinski <kuba@kernel.org> Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com> Cc: Jesper Dangaard Brouer <brouer@redhat.com> Cc: John Fastabend <john.fastabend@gmail.com> Cc: Daniel Borkmann <daniel@iogearbox.net> Cc: David S. Miller <davem@davemloft.net> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
1 files changed, 12 insertions, 0 deletions
diff --git a/net/core/dev.c b/net/core/dev.c
index 151f1651439f..79c15f4244e6 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4630,6 +4630,18 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
* header.
mac_len = skb->data - skb_mac_header(skb);
+ if (!mac_len && !skb->dev->hard_header_len) {
+ /* For l3 packets, we push on a fake mac header, and then
+ * pull it off again, so that it has the same skb geometry
+ * as for the l2 case.
+ */
+ eth = skb_push(skb, ETH_HLEN);
+ eth_zero_addr(eth->h_source);
+ eth_zero_addr(eth->h_dest);
+ eth->h_proto = skb->protocol;
+ __skb_pull(skb, ETH_HLEN);
+ mac_len = ETH_HLEN;
+ }
hlen = skb_headlen(skb) + mac_len;
xdp->data = skb->data - mac_len;
xdp->data_meta = xdp->data;