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SubjectRe: 4.20-rc6: WARNING: CPU: 30 PID: 197360 at net/core/flow_dissector.c:764 __skb_flow_dissect
+Willem

On Thu, Dec 20, 2018 at 08:45:40AM +0100, Christian Borntraeger wrote:
> Folks,
>
> I got this warning today. I cant tell when and why this happened, so I do not know yet how to reproduce.
> Maybe someone has a quick idea.
>
> [85109.572032] WARNING: CPU: 30 PID: 197360 at net/core/flow_dissector.c:764 __skb_flow_dissect+0x1f0/0x1318

I managed to trigger this warning as well the other day, but from a
different call path:

[280155.348526] WARNING: CPU: 1 PID: 24819 at net/core/flow_dissector.c:764 __skb_flow_dissect+0x314/0x16b0
[280155.348529] Modules linked in: dummy vrf intel_powerclamp coretemp kvm_intel kvm irqbypass crct10dif_pclmul leds_mlxreg i2c_mux_reg i2c_mlxcpld crc32_pclmul mlxreg_hotplug mlxreg_io i2c_mux ghash_clmulni_intel iTCO_wdt gpio_ich iTCO_vendor_support mlx_platform aesni_intel aes_x86_64 crypto_simd cryptd glue_helper intel_cstate mac_hid lpc_ich ip_tables x_tables autofs4 mlxsw_spectrum mlxfw vxlan ip6_udp_tunnel udp_tunnel ip6_tunnel tunnel6 objagg psample parman bridge stp llc mlxsw_pci igb ahci mlxsw_core dca i2c_algo_bit libahci devlink i2c_ismt
[280155.348570] CPU: 1 PID: 24819 Comm: ip Not tainted 4.20.0-rc6-nn181213 #1
[280155.348572] Hardware name: Mellanox Technologies Ltd. MSN2100/SA001390, BIOS 5.6.5 06/07/2016
[280155.348576] RIP: 0010:__skb_flow_dissect+0x314/0x16b0
[280155.348579] Code: 85 19 0e 00 00 45 0f b7 6c 24 04 41 0f b7 44 24 06 4d 01 fd 48 85 db 4d 8d 14 07 74 0f 48 8b 43 18 48 85 c0 0f 85 e5 02 00 00 <0f> 0b 41 f6 04 24 40 0f 85 a4 02 00 00 c7 85 30 ff ff ff 00 00 00
[280155.348581] RSP: 0018:ffffa0df41fdf650 EFLAGS: 00010246
[280155.348584] RAX: 0000000000000000 RBX: ffff8bcded232000 RCX: 0000000000000000
[280155.348586] RDX: ffffa0df41fdf7e0 RSI: ffffffff98e415a0 RDI: ffff8bcded232000
[280155.348588] RBP: ffffa0df41fdf760 R08: 0000000000000000 R09: 0000000000000000
[280155.348590] R10: ffffa0df41fdf7e8 R11: ffff8bcdf27a3000 R12: ffffffff98e415a0
[280155.348591] R13: ffffa0df41fdf7e0 R14: ffffffff98dd2980 R15: ffffa0df41fdf7e0
[280155.348594] FS: 00007f46f6897680(0000) GS:ffff8bcdf7a80000(0000) knlGS:0000000000000000
[280155.348596] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[280155.348598] CR2: 000055933e95f9a0 CR3: 000000021e636000 CR4: 00000000001006e0
[280155.348600] Call Trace:
[280155.348610] fib_multipath_hash+0x28c/0x2d0
[280155.348613] ? fib_multipath_hash+0x28c/0x2d0
[280155.348619] fib_select_path+0x241/0x32f
[280155.348622] ? __fib_lookup+0x6a/0xb0
[280155.348626] ip_route_output_key_hash_rcu+0x650/0xa30
[280155.348631] ? __alloc_skb+0x9b/0x1d0
[280155.348634] inet_rtm_getroute+0x3f7/0xb80
[280155.348640] ? __alloc_pages_nodemask+0x11c/0x2c0
[280155.348646] rtnetlink_rcv_msg+0x1d9/0x2f0
[280155.348650] ? rtnl_calcit.isra.24+0x120/0x120
[280155.348654] netlink_rcv_skb+0x54/0x130
[280155.348657] rtnetlink_rcv+0x15/0x20
[280155.348661] netlink_unicast+0x20a/0x2c0
[280155.348664] netlink_sendmsg+0x2d1/0x3d0
[280155.348669] sock_sendmsg+0x39/0x50
[280155.348672] ___sys_sendmsg+0x2a0/0x2f0
[280155.348677] ? filemap_map_pages+0x16b/0x360
[280155.348682] ? __handle_mm_fault+0x108e/0x13d0
[280155.348686] __sys_sendmsg+0x63/0xa0
[280155.348688] ? __sys_sendmsg+0x63/0xa0
[280155.348692] __x64_sys_sendmsg+0x1f/0x30
[280155.348697] do_syscall_64+0x5a/0x120
[280155.348701] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[280155.348704] RIP: 0033:0x7f46f5b80d04
[280155.348707] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b5 0f 1f 80 00 00 00 00 48 8d 05 01 dc 2c 00 8b 00 85 c0 75 13 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 f3 c3 66 90 41 54 55 41 89 d4 53 48 89 f5
[280155.348709] RSP: 002b:00007fff82d62778 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[280155.348712] RAX: ffffffffffffffda RBX: 000000005c1900ae RCX: 00007f46f5b80d04
[280155.348713] RDX: 0000000000000000 RSI: 00007fff82d627e0 RDI: 0000000000000003
[280155.348715] RBP: 00007fff82d628d8 R08: 0000000000000001 R09: 0000000000000000
[280155.348717] R10: 00007f46f5bfccc0 R11: 0000000000000246 R12: 0000000000000001
[280155.348718] R13: 000055933eb90020 R14: 0000000000000000 R15: 00007fff82d63030
[280155.348722] ---[ end trace e14023d76a175374 ]---

Problem is the synthesized skb for output route resolution does not have
skb->dev or skb->sk set. When a multipath route is hit and
net.ipv4.fib_multipath_hash_policy is set the flow dissector is called
with this skb and the warning is triggered.

I plan to fix it by setting skb->dev to net->loopback_dev. I assume we
want to keep this warning to prevent call paths which will otherwise
silently fallback to standard flow dissector instead of the BPF one.

> [85109.572036] Modules linked in: vhost_net vhost macvtap macvlan tap vfio_ap vfio_mdev mdev vfio_iommu_type1 vfio kvm xt_CHECKSUM ipt_MASQUERADE tun bridge stp llc xt_tcpudp ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 ipt_REJECT nf_reject_ipv4 xt_conntrack ip6table_nat nf_nat_ipv6 ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat_ipv4 nf_nat iptable_mangle iptable_raw iptable_security nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 ip_set nfnetlink ip6table_filter ip6_tables iptable_filter dm_service_time rpcrdma sunrpc rdma_ucm rdma_cm configfs iw_cm ib_cm mlx4_ib ib_uverbs ib_core pkey ghash_s390 prng aes_s390 des_s390 des_generic sha512_s390 sha1_s390 zcrypt_cex4 zcrypt rng_core eadm_sch sch_fq_codel ip_tables x_tables mlx4_en mlx4_core sha256_s390 sha_common dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua dm_mirror dm_region_hash dm_log dm_mod autofs4
> [85109.572072] CPU: 30 PID: 197360 Comm: vhost-197330 Not tainted 4.20.0-20181213.rc6.git0.407d079170c1.300.fc29.s390x #1
> [85109.572074] Hardware name: IBM 2964 NC9 712 (LPAR)
> [85109.572075] Krnl PSW : 0704c00180000000 000000000092e320 (__skb_flow_dissect+0x1f0/0x1318)
> [85109.572078] R:0 T:1 IO:1 EX:1 Key:0 M:1 W:0 P:0 AS:3 CC:0 PM:0 RI:0 EA:3
> [85109.572080] Krnl GPRS: 000000000000002a 0000000000000000 000003e0385bfc84 0000000000d91e30
> [85109.572081] 000003e0385bfc84 0000000000000000 0000000000000000 0000000000d91e30
> [85109.572083] 000003e0385bfc84 000000000000000e 0000007e3eb88100 0000007ff3561e00
> [85109.572084] 0000000000000806 0000000000b4f288 000003e0385bfbb8 000003e0385bfab0
> [85109.572115] Krnl Code: 000000000092e312: e310b0180002 ltg %r1,24(%r11)
> 000000000092e318: a7740271 brc 7,92e7fa
> #000000000092e31c: a7f40001 brc 15,92e31e
> >000000000092e320: 91407003 tm 3(%r7),64
> 000000000092e324: a7740257 brc 7,92e7d2
> 000000000092e328: 5810f0b4 l %r1,180(%r15)
> 000000000092e32c: e54cf0c80000 mvhi 200(%r15),0
> 000000000092e332: c01b00000008 nilf %r1,8
> [85109.572129] Call Trace:
> [85109.572130] ([<0000000000000000>] (null))
> [85109.572134] [<000003ff800c81e4>] tap_sendmsg+0x384/0x430 [tap]

I'm not familiar with tap code, so someone else will need to patch this
case, but it looks like:

tap_sendmsg()
tap_get_user()
skb_probe_transport_header()
skb_flow_dissect_flow_keys_basic()
__skb_flow_dissect()

skb->dev is only set later in the code.

> [85109.572137] [<000003ff801acdee>] vhost_tx_batch.isra.10+0x66/0xe0 [vhost_net]
> [85109.572138] [<000003ff801ad61c>] handle_tx_copy+0x18c/0x568 [vhost_net]
> [85109.572140] [<000003ff801adab4>] handle_tx+0xbc/0x100 [vhost_net]
> [85109.572145] [<000003ff8019bbe8>] vhost_worker+0xc8/0x128 [vhost]
> [85109.572148] [<00000000001690b8>] kthread+0x140/0x160
> [85109.572152] [<0000000000a84266>] kernel_thread_starter+0x6/0x10
> [85109.572154] [<0000000000a84260>] kernel_thread_starter+0x0/0x10
> [85109.572155] Last Breaking-Event-Address:
> [85109.572156] [<000000000092e31c>] __skb_flow_dissect+0x1ec/0x1318
> [85109.572158] ---[ end trace 97c040a6691bc000 ]---

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