Re: [PATCH net] net: Allow flow dissector to handle non 4-byte aligned headers
From: Tom Herbert <hidden>
Date: 2016-02-03 17:07:21
On Mon, Feb 1, 2016 at 4:31 PM, Sowmini Varadhan [off-list ref] wrote:
quoted hunk ↗ jump to hunk
On (01/31/16 13:37), Tom Herbert wrote:quoted
Call get_unaligned_be32 when we access 32-bit fields in __skb_flow_dissect. At the beginning check for unlikely case of 1-byte aligned packet. Note that flow_dissector may be asked to parse packet unaligned fields in two instances:I tested this out with the following setup (hostnames in quotes, network connectivity in the line above) gre-tunnel (eth1) --- (eth1) (eth0) ... {routed} ... (eth0) "sparc44" "sparc45" "x86-1" eth1 connections are point-to-point ethernets. The GRE tunnel is configured with some non-zero gre key. So sparc45 bridges sparc44 to x86-1 and has sparc45# brctl show br0 bridge name bridge id STP enabled interfaces br0 8000.0010e0570711 no eth1 gretap2 I tried various pings etc across the nodes, and made sure that (excluding the mpls change) all of the changes in Tom's patch worked fine. To do this, I had to enable rps and disable rxhash/ntuple in my intel driver. And then I noticed that the iph->saddr access does indeed result in an unaligned access (because this time it gets called with flow_keys_dissector). The other thing that generated unaligned access was the attempt to do this if (proto == htons(ETH_P_TEB)) { : eth = __skb_header_pointer(skb, nhoff, sizeof(_eth), data, hlen, &_eth); : proto = eth->h_proto; } which I fixed thus:@@ -394,7 +407,7 @@ ip_proto_again: data, hlen, &_eth); if (!eth) goto out_bad; - proto = eth->h_proto; + proto = get_unaligned_be16(ð->h_proto); nhoff += sizeof(*eth); }Having said all this, the larger problem remains: There are a number of unaligned access on the xmit path, I've listed a few below. A lot of it is coming of gre_tap_xmit not setting up offsets properly, but there are others. In general, since the actual path taken by the packet (i.e., what driver gets to process this next, what types/sizes of headers will get added, etc) is determined dynamically, it's not enough to just do a one-time skb_reserve (or page_offset) for NET_IP_ALIGN. Kernel unaligned access at TPC[9150dc] ipv4_neigh_lookup+0x38/0x170
Sowmini, This doesn't look like a hard crash to me. Instead of trying to fix all the alignment issues for Sparc, can we just take the trap, fix up the load, and continue without any further fuss? Performance might suffer, but it doesn't seem like the bad alignments are happening in critical paths. Tom
CPU: 3 PID: 0 Comm: swapper/3 Tainted: G E 4.4.0-dissect+ #19 Call Trace: [0000000000432f4c] kernel_unaligned_trap+0xfc/0x5ac [0000000000427ef8] sun4v_do_mna+0x58/0x9c [0000000000406d30] sun4v_mna+0x5c/0x68 [00000000009150dc] ipv4_neigh_lookup+0x38/0x170 [00000000008ead10] neigh_update+0x450/0x5cc [00000000008eb8b8] neigh_event_ns+0x84/0x94 [00000000009518f8] arp_process+0x470/0x854 [0000000000951e78] arp_rcv+0x19c/0x1dc [00000000008d647c] __netif_receive_skb_core+0xa24/0xa58 [00000000008d6510] __netif_receive_skb+0x60/0x74 [00000000008d65dc] process_backlog+0xb8/0x168 [00000000008dc9f8] napi_poll+0x74/0x1c8 [00000000008dcc20] net_rx_action+0xd4/0x1d4 [00000000004633a4] __do_softirq+0x11c/0x28c [000000000042abb0] do_softirq_own_stack+0x34/0x48 [0000000000463050] irq_exit+0x48/0x60 Kernel unaligned access at TPC[103e720c] build_header+0x64/0xc4 [ip_gre] CPU: 44 PID: 0 Comm: swapper/44 Tainted: G E 4.4.0-dissect+ #19 Call Trace: [0000000000432f4c] kernel_unaligned_trap+0xfc/0x5ac [0000000000427ef8] sun4v_do_mna+0x58/0x9c [0000000000406d30] sun4v_mna+0x5c/0x68 [00000000103e720c] build_header+0x64/0xc4 [ip_gre] [00000000103e76a4] __gre_xmit+0x40/0x7c [ip_gre] [00000000103e79e8] gre_tap_xmit+0xfc/0x12c [ip_gre] [00000000008de6dc] dev_hard_start_xmit+0x304/0x3ec [0000000000904d30] sch_direct_xmit+0x74/0x1dc [00000000008df1a8] __dev_queue_xmit+0x568/0x7e8 [00000000008df45c] dev_queue_xmit+0x10/0x24 [0000000010207654] ip6_finish_output2+0x5b4/0x6b0 [ipv6] [0000000010207954] ip6_finish_output+0x204/0x218 [ipv6] [0000000010207afc] ip6_output+0x194/0x1a8 [ipv6] [000000001021f97c] ndisc_send_skb+0x390/0x438 [ipv6] [000000001021ff14] ndisc_send_rs+0xe0/0xfc [ipv6] [000000001020b190] addrconf_rs_timer+0xc4/0x124 [ipv6] Kernel unaligned access at TPC[969ee0] iptunnel_xmit+0xcc/0x1c0 CPU: 0 PID: 0 Comm: swapper/0 Tainted: G E 4.4.0-dissect+ #19 Call Trace: [0000000000432f4c] kernel_unaligned_trap+0xfc/0x5ac [0000000000427ef8] sun4v_do_mna+0x58/0x9c [0000000000406d30] sun4v_mna+0x5c/0x68 [0000000000969ee0] iptunnel_xmit+0xcc/0x1c0 [00000000103dc9dc] ip_tunnel_xmit+0xb78/0xc2c [ip_tunnel] [00000000103e76d0] __gre_xmit+0x6c/0x7c [ip_gre] [00000000103e79e8] gre_tap_xmit+0xfc/0x12c [ip_gre] [00000000008de6dc] dev_hard_start_xmit+0x304/0x3ec [0000000000904d30] sch_direct_xmit+0x74/0x1dc [00000000008df1a8] __dev_queue_xmit+0x568/0x7e8 [00000000008df45c] dev_queue_xmit+0x10/0x24 [00000000104034b8] br_dev_queue_push_xmit+0x1d8/0x1fc [bridge] [000000001040355c] br_forward_finish+0x80/0x94 [bridge] [0000000010403684] __br_forward+0x114/0x124 [bridge] [0000000010403700] br_forward+0x6c/0x94 [bridge]