[linux-next] Kernel panic while tetsing criu

[Cyrill Gorcunov <gorcunov@gmail.com>]

Hi Eric! We're regularly running criu on linux-next and today
kernel get panicing.
---
[  753.478579] BUG: unable to handle kernel NULL pointer dereference at 00000000000006a8
[  753.479674] PGD 800000011215f067 P4D 800000011215f067 PUD 1134a8067 PMD 0
[  753.480590] Oops: 0000 [#1] SMP PTI
[  753.481054] CPU: 0 PID: 32493 Comm: file_fown Not tainted 4.18.0-next-20180815-00001-g1532db2f419f-dirty #2
[  753.482329] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
[  753.484096] RIP: 0010:send_sigio_to_task+0x81/0x2c0
[  753.484792] Code: b9 02 00 00 00 31 f6 48 c7 c7 60 35 46 b2 e8 46 9c e3 ff e8 71 94 e5 ff 5a 85 c0 74 0d 80 3d 1e 26 2b 01 00 0f 84 cc 01 00 00 <4d> 8b b4 24 a8 06 00 00 e8 52 94 e5 ff 85 c0 74 0d 80 3d fe 25 2b
[  753.487383] RSP: 0018:ffffbd8440f5bcc0 EFLAGS: 00010202
[  753.488128] RAX: 0000000000000001 RBX: ffff99a75224f7c8 RCX: 00000000133c1702
[  753.489166] RDX: ffffffffb12bd995 RSI: 00000000d1f2807e RDI: 0000000000000246
[  753.490184] RBP: ffffbd8440f5bd78 R08: 0000000000000001 R09: 0000000000000000
[  753.491204] R10: ffffffffb2463560 R11: 0000000000000000 R12: 0000000000000000
[  753.492249] R13: 0000000000000002 R14: 0000000000000005 R15: 0000000000000001
[  753.493273] FS:  00007f01488d04c0(0000) GS:ffff99a77ba00000(0000) knlGS:0000000000000000
[  753.494423] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  753.495244] CR2: 00000000000006a8 CR3: 00000001327b8004 CR4: 00000000003606f0
[  753.496251] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  753.497269] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  753.498276] Call Trace:
[  753.498653]  ? __lock_is_held+0x4f/0x90
[  753.499198]  send_sigio+0x137/0x1c0
[  753.499701]  kill_fasync+0xdd/0x210
[  753.500208]  pipe_read+0x165/0x310
[  753.500703]  __vfs_read+0x133/0x190
[  753.501201]  vfs_read+0x9c/0x150
[  753.501764]  ksys_read+0x52/0xc0
[  753.502229]  do_syscall_64+0x60/0x210
[  753.502756]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  753.503474] RIP: 0033:0x7f01483f5701
[  753.504012] Code: fe ff ff 48 8d 3d af 8f 09 00 48 83 ec 08 e8 96 fe 01 00 66 0f 1f 44 00 00 8b 05 4a f1 2c 00 48 63 ff 85 c0 75 13 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 57 f3 c3 0f 1f 44 00 00 55 53 48 89 d5 48 89
[  753.506667] RSP: 002b:00007ffdbb2d2878 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[  753.507737] RAX: ffffffffffffffda RBX: 0000000000000025 RCX: 00007f01483f5701
[  753.508733] RDX: 000000000000001c RSI: 00007ffdbb2d28a0 RDI: 0000000000000004
[  753.509714] RBP: 00000000004043f0 R08: 0000000000000000 R09: 0000000000000000
[  753.510696] R10: 000000000000038b R11: 0000000000000246 R12: 0000000000000000
[  753.511677] R13: 00007ffdbb2d2a70 R14: 0000000000000000 R15: 0000000000000000
[  753.512662] Modules linked in:
[  753.513095] CR2: 00000000000006a8
[  753.513579] ---[ end trace 2d68e222d9dac4c3 ]---

we suspect it might be due to commit 9c2db007787ef1aac6728c5e03d37b0ae935d122
because oneliner

diff --git a/fs/fcntl.c b/fs/fcntl.c
index a04accf6847f..20e4daf83aab 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -791,6 +791,8 @@ void send_sigio(struct fown_struct *fown, int fd, int band)
       if (type <= PIDTYPE_TGID) {
               rcu_read_lock();
               p = pid_task(pid, PIDTYPE_PID);
+               if (!p)
+                       goto out_unlock_fown;
               send_sigio_to_task(p, fown, fd, band, type);
               rcu_read_unlock();
       } else {

has helped.

Could you please take a look once time permit?

p.s. Andrew noticed the problem and asked me to notify,
also he has been testing this oneliner patch. I'm out
of sources at the moment but I think Andrew will help
to test if needed.

	Cyrill