linux-kernel - Re: [PATCH net-next v5 2/2] L2TP:Adjust intf MTU, add underlay L3, L2 hdrs.

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Date:   Tue, 11 Apr 2017 12:40:48 +0200
From:   Guillaume Nault <g.nault@...halink.fr>
To:     "R. Parameswaran" <parameswaran.r7@...il.com>
Cc:     netdev@...r.kernel.org, jchapman@...alix.com, kleptog@...na.org,
        nprachan@...cade.com, rshearma@...cade.com,
        stephen@...workplumber.org, sdietric@...cade.com,
        ciwillia@...cade.com, lboccass@...cade.com, dfawcus@...cade.com,
        bhong@...cade.com, jblunck@...cade.com, davem@...emloft.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v5 2/2] L2TP:Adjust intf MTU, add underlay L3,
 L2 hdrs.

On Wed, Apr 05, 2017 at 05:00:07PM -0700, R. Parameswaran wrote:
> 
> Change-set here uses the new kernel function, kernel_sock_ip_overhead(),
> to factor the outer IP overhead on the L2TP tunnel socket (including
> IP Options, if any) when calculating the default MTU for an Ethernet
> pseudowire, along with consideration of the inner Ethernet header.
> 
I get the following warning with CONFIG_LOCKDEP when creating a new
session:
# ip l2tp add tunnel local 10.1.8.64 remote 10.1.8.32 udp_sport 1701 udp_dport 1701 tunnel_id 1 peer_tunnel_id 1
# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
...
[   45.524535] ------------[ cut here ]------------
[   45.524570] WARNING: CPU: 3 PID: 732 at ./include/net/sock.h:1509 kernel_sock_ip_overhead+0x54/0x1a1
[   45.524574] Modules linked in: l2tp_eth l2tp_netlink l2tp_core ip6_udp_tunnel udp_tunnel crc32c_intel ghash_clmulni_intel pcbc aesni_intel aes_x86_64 crypto_simd cryptd glue_helper evdev acpi_cpufreq processor button serio_raw ext4 crc16 jbd2 mbcache virtio_blk virtio_net virtio_pci virtio_ring virtio
[   45.524696] CPU: 3 PID: 732 Comm: ip Not tainted 4.11.0-rc5 #1
[   45.524700] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[   45.524704] Call Trace:
[   45.524714]  dump_stack+0x67/0x90
[   45.524725]  __warn+0xfd/0x118
[   45.524739]  warn_slowpath_null+0x18/0x1a
[   45.524747]  kernel_sock_ip_overhead+0x54/0x1a1
[   45.524761]  l2tp_eth_create+0x1eb/0x557 [l2tp_eth]
[   45.524768]  ? __mutex_unlock_slowpath+0xb5/0x2c2
[   45.524787]  ? l2tp_eth_dev_uninit+0xd9/0xd9 [l2tp_eth]
[   45.524800]  l2tp_nl_cmd_session_create+0x521/0x56b [l2tp_netlink]
[   45.524827]  genl_family_rcv_msg+0x445/0x4b3
[   45.524857]  genl_rcv_msg+0x60/0x84
[   45.524867]  ? genl_family_rcv_msg+0x4b3/0x4b3
[   45.524875]  netlink_rcv_skb+0x95/0x102
[   45.524881]  ? down_read+0x41/0x62
[   45.524893]  genl_rcv+0x23/0x32
[   45.524901]  netlink_unicast+0x1b0/0x23b
[   45.524915]  netlink_sendmsg+0x46f/0x48f
[   45.524933]  ? netlink_unicast+0x23b/0x23b
[   45.524942]  sock_sendmsg_nosec+0x41/0x51
[   45.524953]  sock_sendmsg+0x33/0x38
[   45.524962]  ___sys_sendmsg+0x2a0/0x374
[   45.524991]  ? do_raw_spin_unlock+0xc2/0xcc
[   45.525002]  ? _raw_spin_unlock+0x22/0x25
[   45.525014]  ? match_held_lock+0x20/0x113
[   45.525027]  ? __fget_light+0x89/0xae
[   45.525045]  __sys_sendmsg+0x40/0x6b
[   45.525052]  ? __sys_sendmsg+0x40/0x6b
[   45.525075]  SyS_sendmsg+0x9/0xb
[   45.525083]  entry_SYSCALL_64_fastpath+0x18/0xad
[   45.525089] RIP: 0033:0x7fb224391690
[   45.525094] RSP: 002b:00007ffe53943dd8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[   45.525104] RAX: ffffffffffffffda RBX: 00007ffe539480f0 RCX: 00007fb224391690
[   45.525108] RDX: 0000000000000000 RSI: 00007ffe53943e20 RDI: 0000000000000004
[   45.525113] RBP: ffffffff810a1e49 R08: 0000000000000000 R09: 0000000000000005
[   45.525119] R10: 0000000000000000 R11: 0000000000000246 R12: ffff88003436ff98
[   45.525124] R13: 0000000000000046 R14: 00007ffe539486a0 R15: 00007ffe53947ea0
[   45.525136]  ? trace_hardirqs_off_caller+0x121/0x12f
[   45.525157] ---[ end trace 0834023e7b30e761 ]---

I guess you neet to lock_sock(tunnel->socket) before calling
kernel_sock_ip_overhead().