syzbot

loop2: detected capacity change from 0 to 4096
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.2.343/10016 is trying to acquire lock:
ffff888028f29d30 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:400

but task is already holding lock:
ffff88813ff76238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x267/0x870 fs/kernfs/dir.c:1898

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #4 (&root->kernfs_rwsem){++++}-{4:4}:
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1590
       kernfs_add_one+0x41/0x5c0 fs/kernfs/dir.c:796
       kernfs_create_dir_ns+0xde/0x130 fs/kernfs/dir.c:1098
       sysfs_create_dir_ns+0x123/0x280 fs/sysfs/dir.c:59
       create_dir lib/kobject.c:73 [inline]
       kobject_add_internal+0x6b1/0xcd0 lib/kobject.c:240
       kobject_add_varg lib/kobject.c:374 [inline]
       kobject_init_and_add+0x125/0x190 lib/kobject.c:457
       btrfs_sysfs_add_qgroups+0x111/0x2b0 fs/btrfs/sysfs.c:2697
       btrfs_quota_enable+0x25d/0x28f0 fs/btrfs/qgroup.c:1002
       btrfs_ioctl_quota_ctl+0x186/0x1c0 fs/btrfs/ioctl.c:3613
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #3 (&fs_info->qgroup_ioctl_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:533 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:552
       btrfs_quota_enable+0x29c/0x28f0 fs/btrfs/qgroup.c:1031
       btrfs_ioctl_quota_ctl+0x186/0x1c0 fs/btrfs/ioctl.c:3613
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #2 (btrfs_trans_num_extwriters){++++}-{0:0}:
       join_transaction+0x1a4/0xc90 fs/btrfs/transaction.c:323
       start_transaction+0x6ba/0x15f0 fs/btrfs/transaction.c:708
       btrfs_ioctl_subvol_setflags+0x461/0x820 fs/btrfs/ioctl.c:1409
       btrfs_ioctl+0x93d/0xd00 fs/btrfs/ioctl.c:5218
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (btrfs_trans_num_writers){++++}-{0:0}:
       __lock_release kernel/locking/lockdep.c:5574 [inline]
       lock_release+0x19a/0x3b0 kernel/locking/lockdep.c:5889
       percpu_up_read include/linux/percpu-rwsem.h:112 [inline]
       __sb_end_write include/linux/fs/super.h:14 [inline]
       sb_end_intwrite+0x26/0x1c0 include/linux/fs/super.h:101
       __btrfs_end_transaction+0x248/0x640 fs/btrfs/transaction.c:1084
       btrfs_dirty_inode+0x14c/0x190 fs/btrfs/inode.c:6346
       inode_update_time fs/inode.c:2167 [inline]
       touch_atime+0x2f9/0x6d0 fs/inode.c:2240
       file_accessed include/linux/fs.h:2254 [inline]
       btrfs_file_mmap_prepare+0x189/0x200 fs/btrfs/file.c:2053
       vfs_mmap_prepare include/linux/fs.h:2058 [inline]
       call_mmap_prepare mm/vma.c:2596 [inline]
       __mmap_region mm/vma.c:2692 [inline]
       mmap_region+0xb4c/0x1d00 mm/vma.c:2786
       do_mmap+0xc23/0x10c0 mm/mmap.c:558
       vm_mmap_pgoff+0x2a9/0x4d0 mm/util.c:581
       ksys_mmap_pgoff+0x4e9/0x720 mm/mmap.c:604
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&mm->mmap_lock){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0x107/0x340 kernel/locking/lockdep.c:5868
       down_read_killable+0x9d/0x220 kernel/locking/rwsem.c:1560
       mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:400
       get_mmap_lock_carefully mm/mmap_lock.c:399 [inline]
       lock_mm_and_find_vma+0x2a8/0x300 mm/mmap_lock.c:450
       do_user_addr_fault+0x331/0x1380 arch/x86/mm/fault.c:1359
       handle_page_fault arch/x86/mm/fault.c:1476 [inline]
       exc_page_fault+0x71/0xd0 arch/x86/mm/fault.c:1532
       asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
       filldir64+0x2ce/0x690 fs/readdir.c:-1
       dir_emit include/linux/fs.h:3527 [inline]
       kernfs_fop_readdir+0x53a/0x870 fs/kernfs/dir.c:1915
       iterate_dir+0x3a5/0x580 fs/readdir.c:108
       __do_sys_getdents64 fs/readdir.c:410 [inline]
       __se_sys_getdents64+0xe4/0x260 fs/readdir.c:396
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &mm->mmap_lock --> &fs_info->qgroup_ioctl_lock --> &root->kernfs_rwsem

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&root->kernfs_rwsem);
                               lock(&fs_info->qgroup_ioctl_lock);
                               lock(&root->kernfs_rwsem);
  rlock(&mm->mmap_lock);

 *** DEADLOCK ***

3 locks held by syz.2.343/10016:
 #0: ffff888030fee528 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x253/0x320 fs/file.c:1255
 #1: ffff88805b309cf8 (&type->i_mutex_dir_key#6){++++}-{4:4}, at: iterate_dir+0x29e/0x580 fs/readdir.c:101
 #2: ffff88813ff76238 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_fop_readdir+0x267/0x870 fs/kernfs/dir.c:1898

stack backtrace:
CPU: 1 UID: 0 PID: 10016 Comm: syz.2.343 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2043
 check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x107/0x340 kernel/locking/lockdep.c:5868
 down_read_killable+0x9d/0x220 kernel/locking/rwsem.c:1560
 mmap_read_lock_killable+0x1d/0x70 include/linux/mmap_lock.h:400
 get_mmap_lock_carefully mm/mmap_lock.c:399 [inline]
 lock_mm_and_find_vma+0x2a8/0x300 mm/mmap_lock.c:450
 do_user_addr_fault+0x331/0x1380 arch/x86/mm/fault.c:1359
 handle_page_fault arch/x86/mm/fault.c:1476 [inline]
 exc_page_fault+0x71/0xd0 arch/x86/mm/fault.c:1532
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0010:filldir64+0x2ce/0x690 fs/readdir.c:381
Code: 4c 24 08 48 8b 44 24 58 48 89 01 48 8b 04 24 8b 54 24 14 49 bc 00 00 00 00 00 fc ff df 66 89 41 10 80 e2 0f 88 51 12 49 63 ee <c6> 44 29 13 00 4c 8d 79 13 bf 07 00 00 00 44 89 f6 e8 4c 8a 8b ff
RSP: 0018:ffffc90004f77c58 EFLAGS: 00050202
RAX: 0000000000000028 RBX: ffffc90004f77e38 RCX: 0000200000001fe8
RDX: 0000000000000008 RSI: 0000200000001fc0 RDI: 0000200000002010
RBP: 000000000000000e R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffed10037783c9 R12: dffffc0000000000
R13: ffff888024300860 R14: 000000000000000e R15: 0000200000002010
 dir_emit include/linux/fs.h:3527 [inline]
 kernfs_fop_readdir+0x53a/0x870 fs/kernfs/dir.c:1915
 iterate_dir+0x3a5/0x580 fs/readdir.c:108
 __do_sys_getdents64 fs/readdir.c:410 [inline]
 __se_sys_getdents64+0xe4/0x260 fs/readdir.c:396
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f1f532af749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f1f514cc038 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f1f53506180 RCX: 00007f1f532af749
RDX: 0000000000001017 RSI: 0000200000001d80 RDI: 0000000000000003
RBP: 00007f1f53333f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f1f53506218 R14: 00007f1f53506180 R15: 00007ffd7920af38
 </TASK>
----------------
Code disassembly (best guess):
   0:	4c 24 08             	rex.WR and $0x8,%al
   3:	48 8b 44 24 58       	mov    0x58(%rsp),%rax
   8:	48 89 01             	mov    %rax,(%rcx)
   b:	48 8b 04 24          	mov    (%rsp),%rax
   f:	8b 54 24 14          	mov    0x14(%rsp),%edx
  13:	49 bc 00 00 00 00 00 	movabs $0xdffffc0000000000,%r12
  1a:	fc ff df
  1d:	66 89 41 10          	mov    %ax,0x10(%rcx)
  21:	80 e2 0f             	and    $0xf,%dl
  24:	88 51 12             	mov    %dl,0x12(%rcx)
  27:	49 63 ee             	movslq %r14d,%rbp
* 2a:	c6 44 29 13 00       	movb   $0x0,0x13(%rcx,%rbp,1) <-- trapping instruction
  2f:	4c 8d 79 13          	lea    0x13(%rcx),%r15
  33:	bf 07 00 00 00       	mov    $0x7,%edi
  38:	44 89 f6             	mov    %r14d,%esi
  3b:	e8 4c 8a 8b ff       	call   0xff8b8a8c