Comment 8 for bug 1967924

This patch seems to introduce this NULL pointer dereference, that can be triggered systematically running lxc autopkgtest:

BUG: kernel NULL pointer dereference, address: 0000000000000008
[ 447.039738] #PF: supervisor read access in kernel mode
[ 447.040369] #PF: error_code(0x0000) - not-present page
[ 447.041002] PGD 0 P4D 0
[ 447.041325] Oops: 0000 [#1] SMP NOPTI
[ 447.041798] CPU: 0 PID: 73766 Comm: sudo Not tainted 5.15.0-28-generic #29~20.04.1-Ubuntu
[ 447.042800] Hardware name: OpenStack Foundation OpenStack Nova, BIOS Ubuntu-1.8.2-1ubuntu1+esm1 04/01/2014
[ 447.043979] RIP: 0010:aa_file_perm+0x3a/0x470
[ 447.044565] Code: 54 53 48 83 ec 68 48 89 7d 80 89 4d 8c 65 48 8b 04 25 28 00 00 00 48 89 45 d0 31 c0 48 63 05 01 0a 19 01 48 03 82 c0 00 00 00 <4c> 8b 68 08 f6 46 40 02 0f 85 d0 00 00 00 41 f6 45 40 02 0f 85 c5
[ 447.046837] RSP: 0018:ffffaefe80a4bca8 EFLAGS: 00010246
[ 447.047481] RAX: 0000000000000000 RBX: ffff96e4038abd01 RCX: 0000000000000004
[ 447.048351] RDX: ffff96e4038abd00 RSI: ffff96e401215eb8 RDI: ffffffff9c22a2ac
[ 447.049241] RBP: ffffaefe80a4bd38 R08: 0000000000000000 R09: 0000000000000000
[ 447.050121] R10: 0000000000000000 R11: 0000000000000000 R12: ffff96e401215eb8
[ 447.051040] R13: ffff96e4038abd00 R14: ffffffff9c22a2ac R15: 0000000000000004
[ 447.051942] FS: 00007eff3c0f8c80(0000) GS:ffff96e45e400000(0000) knlGS:0000000000000000
[ 447.052981] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 447.053696] CR2: 0000000000000008 CR3: 0000000002be2000 CR4: 00000000003506f0
[ 447.054571] Call Trace:
[ 447.054883] <TASK>
[ 447.055154] ? unlock_page_memcg+0x2f/0x40
[ 447.055668] ? page_remove_rmap+0x4b/0x320
[ 447.056180] common_file_perm+0x72/0x170
[ 447.056669] apparmor_file_permission+0x1c/0x20
[ 447.057237] security_file_permission+0x30/0x1a0
[ 447.057898] rw_verify_area+0x35/0x60
[ 447.058392] vfs_read+0x6d/0x1a0
[ 447.058842] ksys_read+0xb1/0xe0
[ 447.059276] __x64_sys_read+0x1a/0x20
[ 447.059732] do_syscall_64+0x5c/0xc0
[ 447.060183] ? __set_current_blocked+0x3b/0x60
[ 447.060738] ? exit_to_user_mode_prepare+0x3d/0x1c0
[ 447.061434] ? syscall_exit_to_user_mode+0x27/0x50
[ 447.062099] ? do_syscall_64+0x69/0xc0
[ 447.062603] ? irqentry_exit_to_user_mode+0x9/0x20
[ 447.063210] ? irqentry_exit+0x19/0x30
[ 447.063678] ? exc_page_fault+0x89/0x160
[ 447.064165] ? asm_exc_page_fault+0x8/0x30
[ 447.064675] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 447.065298] RIP: 0033:0x7eff3c2cb002

This panic happens only when AUFS is enabled (so it can be seen only in focal with 5.15 at the moment).

I'm going to revert the patch for now, until we figure out a better way to re-apply this change.