| Message ID | 20200814073048.30291-1-Jason@zx2c4.com |
|---|---|
| State | New |
| Headers | show |
| Series | [net,v5] net: xdp: account for layer 3 packets in generic skb handler | expand |
From: "Jason A. Donenfeld" <Jason@zx2c4.com> Date: Fri, 14 Aug 2020 09:30:48 +0200 > @@ -4676,6 +4688,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb, > (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) { > __skb_push(skb, ETH_HLEN); > skb->protocol = eth_type_trans(skb, skb->dev); > + __skb_pull(skb, ETH_HLEN); > } > > switch (act) { This bug fix is separate from your other changes. Please do not combine them.
On 8/14/20, David Miller <davem@davemloft.net> wrote: > From: "Jason A. Donenfeld" <Jason@zx2c4.com> > Date: Fri, 14 Aug 2020 09:30:48 +0200 > >> @@ -4676,6 +4688,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff >> *skb, >> (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) { >> __skb_push(skb, ETH_HLEN); >> skb->protocol = eth_type_trans(skb, skb->dev); >> + __skb_pull(skb, ETH_HLEN); >> } >> >> switch (act) { > > This bug fix is separate from your other changes. Please do not combine > them. > No problem. I'll split this out and resend tomorrow morning. Jason
diff --git a/net/core/dev.c b/net/core/dev.c index 7df6c9617321..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; @@ -4676,6 +4688,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb, (orig_bcast != is_multicast_ether_addr_64bits(eth->h_dest))) { __skb_push(skb, ETH_HLEN); skb->protocol = eth_type_trans(skb, skb->dev); + __skb_pull(skb, ETH_HLEN); } switch (act) {
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. While we're at it, this also fixes a small inconsistency from the prior code, in which an XDP program that changes skb->protocol would wind up pushing the ethernet header back on but would forget to take it back off following the h_proto parsing. 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. [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> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> --- I had originally dropped this patch, but the issue kept coming up in user reports, so here's a v4 of it. Testing of it is still rather slim, but hopefully that will change in the coming days. Changes v4->v5: - Rather than tracking in a messy manner whether the skb is l3, we just do the check once, and then adjust the skb geometry to be identical to the l2 case. This simplifies the code quite a bit. - Fix a preexisting bug where the l2 header remained attached if skb->protocol was updated. Changes v3->v4: - We now preserve the same logic for XDP_TX/XDP_REDIRECT as before. - hard_header_len is checked in addition to mac_len. net/core/dev.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) -- 2.28.0