[Previously posted at http://article.gmane.org/gmane.linux.nfs/38949 including a patch]

BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
IP: [<ffffffffa05b5e08>] xs_tcp_setup_socket+0x348/0x4a0 [sunrpc]
PGD 0 
Oops: 0000 [#1] SMP 
last sysfs file: /sys/devices/pci0000:00/0000:00:1c.0/0000:02:00.0/0000:03:00.0/irq
CPU 0 
Modules linked in: netconsole configfs nfs lockd fscache nfs_acl auth_rpcgss ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat bridge stp llc autofs4 sunrpc be2iscsi iscsi_boot_sysfs bnx2i cnic uio cxgb3i libcxgbi cxgb3 ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr ipv6 iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ext2 dm_mirror dm_region_hash dm_log dm_mod vhost_net macvtap macvlan tun kvm_intel kvm uinput bnx2 sg dcdbas serio_raw pcspkr iTCO_wdt iTCO_vendor_support i5k_amb i5000_edac edac_core ioatdma dca sfc mtd mdio shpchp ext3 jbd mbcache sr_mod cdrom sd_mod crc_t10dif pata_acpi ata_generic ata_piix mptsas mptscsih mptbase scsi_transport_sas radeon ttm drm_kms_helper drm hwmon i2c_algo_bit i2c_core [last unloaded: speedstep_lib]
Pid: 10, comm: kworker/0:1 Not tainted 2.6.38-rc5 #3 Dell Inc. PowerEdge 2950/0CX396
RIP: 0010:[<ffffffffa05b5e08>]  [<ffffffffa05b5e08>] xs_tcp_setup_socket+0x348/0x4a0 [sunrpc]
RSP: 0018:ffff880126c11da0  EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8801220ee000 RCX: 0000000100016909
RDX: 000000000000001e RSI: ffff880123065a80 RDI: 0000000000000000
RBP: ffff880126c11df0 R08: f018000000000000 R09: febef9edc98abe03
R10: 0000000000000480 R11: 0000000000000000 R12: ffff8801220ee680
R13: ffffe8ffffc0dd00 R14: 0000000000000000 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8800cf800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000020 CR3: 000000012191f000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process kworker/0:1 (pid: 10, threadinfo ffff880126c10000, task ffff880126c0f560)
Stack:
 0000000000014e80 ffff880126c0f560 ffff880126c0faf0 00000000c17d7567
 ffff880126c0faf8 ffff8801273cbc40 ffff8800cf811040 ffffe8ffffc0dd00
 ffffffffa05b5ac0 0000000000000000 ffff880126c11e50 ffffffff8107b884
Call Trace:
 [<ffffffffa05b5ac0>] ? xs_tcp_setup_socket+0x0/0x4a0 [sunrpc]
 [<ffffffff8107b884>] process_one_work+0x124/0x430
 [<ffffffff8107e1d1>] worker_thread+0x181/0x3c0
 [<ffffffff8107e050>] ? worker_thread+0x0/0x3c0
 [<ffffffff810828c6>] kthread+0x96/0xa0
 [<ffffffff8100cdc4>] kernel_thread_helper+0x4/0x10
 [<ffffffff81082830>] ? kthread+0x0/0xa0
 [<ffffffff8100cdc0>] ? kernel_thread_helper+0x0/0x10
Code: 0f 1f 00 0f 84 3a ff ff ff e9 4b fe ff ff 0f 1f 44 00 00 41 83 fd 91 0f 85 3c fe ff ff 66 0f 1f 44 00 00 e9 1b ff ff ff 0f 1f 00 <4d> 8b 6e 20 4d 8d bd 68 01 00 00 4c 89 ff e8 45 99 f0 e0 49 8b 
RIP  [<ffffffffa05b5e08>] xs_tcp_setup_socket+0x348/0x4a0 [sunrpc]
 RSP <ffff880126c11da0>
CR2: 0000000000000020
---[ end trace 6efc43bb9b1264f8 ]---

The code dump alone is pretty useless as the IP is at the start of a
block, but having disassembled the entire sunrpc module it appears that
it corresponds to:

static int xs_tcp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock)
{
        struct sock_xprt *transport = container_of(xprt, struct sock_xprt, xprt);

        if (!transport->inet) {
                struct sock *sk = sock->sk; /* <-- this line */

                write_lock_bh(&sk->sk_callback_lock);

I think the bug is that xs_create_sock() returns 0 if xs_bind() fails.

This bug appears to have been introduced in 2.6.37 by:

commit b65c0310611af73569f94c526a1e2323d99b380a
Author: Pavel Emelyanov <xemul@parallels.com>
Date:   Mon Oct 4 16:53:46 2010 +0400

    sunrpc: Factor out udp sockets creation

commit 22f793268de3b4dff8abfcd873ba7afc1f34224f
Author: Pavel Emelyanov <xemul@parallels.com>
Date:   Mon Oct 4 16:54:26 2010 +0400

    sunrpc: Factor out v4 sockets creation

commit 22d44a7d8a03456aa6d0a047c051aa28728e6ecd
Author: Pavel Emelyanov <xemul@parallels.com>
Date:   Mon Oct 4 16:54:55 2010 +0400

    sunrpc: Factor out v6 sockets creation