Message-ID: <CAOQ4uxiDyyg=FfJrmWXVHuxQXu0Z6fn=6wth42iLcdhie0Qrsw@mail.gmail.com>
Date: Thu, 4 Sep 2025 20:08:18 +0200
From: Amir Goldstein <amir73il@...il.com>
To: syzbot <syzbot+1090a418b58c19e9a57b@...kaller.appspotmail.com>
Cc: kent.overstreet@...ux.dev, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org, 
	miklos@...redi.hu, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [overlayfs?] [bcachefs?] possible deadlock in vfs_link

On Thu, Sep 4, 2025 at 7:37 PM syzbot
<syzbot+1090a418b58c19e9a57b@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    8f5ae30d69d7 Linux 6.17-rc1
> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> console output: https://syzkaller.appspot.com/x/log.txt?x=15322e62580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8c5ac3d8b8abfcb
> dashboard link: https://syzkaller.appspot.com/bug?extid=1090a418b58c19e9a57b
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> userspace arch: arm64
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=121cea42580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15589e62580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/18a2e4bd0c4a/disk-8f5ae30d.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/3b5395881b25/vmlinux-8f5ae30d.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e875f4e3b7ff/Image-8f5ae30d.gz.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/cf46540aa8ee/mount_0.gz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+1090a418b58c19e9a57b@...kaller.appspotmail.com
>
> bcachefs (loop0): resume_logged_ops... done
> bcachefs (loop0): delete_dead_inodes... done
> bcachefs (loop0): done starting filesystem
> ============================================
> WARNING: possible recursive locking detected
> 6.17.0-rc1-syzkaller-g8f5ae30d69d7 #0 Not tainted
> --------------------------------------------
> syz.0.19/6755 is trying to acquire lock:
> ffff0000ec7c5c68 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> ffff0000ec7c5c68 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: vfs_link+0x2f0/0x590 fs/namei.c:4845
>
> but task is already holding lock:
> ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_temp fs/overlayfs/dir.c:228 [inline]
> ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_over_whiteout fs/overlayfs/dir.c:473 [inline]
> ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_or_link+0x608/0x11b8 fs/overlayfs/dir.c:629
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(&sb->s_type->i_mutex_key#21);
>   lock(&sb->s_type->i_mutex_key#21);
>
>  *** DEADLOCK ***
>
>  May be due to missing lock nesting notation
>
> 6 locks held by syz.0.19/6755:
>  #0: ffff0000dcc7e428 (sb_writers#12){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:557
>  #1: ffff0000ec7b1e50 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:914 [inline]
>  #1: ffff0000ec7b1e50 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: filename_create+0x1ac/0x32c fs/namei.c:4139
>  #2: ffff0000ec7b2418 (&ovl_i_mutex_key[depth]){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
>  #2: ffff0000ec7b2418 (&ovl_i_mutex_key[depth]){+.+.}-{4:4}, at: vfs_link+0x2f0/0x590 fs/namei.c:4845
>  #3: ffff0000ec7b27d0 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:672 [inline]
>  #3: ffff0000ec7b27d0 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_nlink_start+0x2fc/0x6e0 fs/overlayfs/util.c:1176
>  #4: ffff0000d2034428 (sb_writers#11){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:557
>  #5: ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
>  #5: ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_temp fs/overlayfs/dir.c:228 [inline]
>  #5: ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_over_whiteout fs/overlayfs/dir.c:473 [inline]
>  #5: ffff0000ec7c54d0 (&sb->s_type->i_mutex_key#21){++++}-{4:4}, at: ovl_create_or_link+0x608/0x11b8 fs/overlayfs/dir.c:629
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 6755 Comm: syz.0.19 Not tainted 6.17.0-rc1-syzkaller-g8f5ae30d69d7 #0 PREEMPT
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/30/2025
> Call trace:
>  show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
>  __dump_stack+0x30/0x40 lib/dump_stack.c:94
>  dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
>  dump_stack+0x1c/0x28 lib/dump_stack.c:129
>  print_deadlock_bug+0x2e8/0x2f0 kernel/locking/lockdep.c:3041
>  check_deadlock kernel/locking/lockdep.c:3093 [inline]
>  validate_chain kernel/locking/lockdep.c:3895 [inline]
>  __lock_acquire+0x2940/0x30a4 kernel/locking/lockdep.c:5237
>  lock_acquire+0x14c/0x2e0 kernel/locking/lockdep.c:5868
>  down_write+0x50/0xc0 kernel/locking/rwsem.c:1590
>  inode_lock include/linux/fs.h:869 [inline]
>  vfs_link+0x2f0/0x590 fs/namei.c:4845
>  ovl_do_link fs/overlayfs/overlayfs.h:227 [inline]
>  ovl_create_real+0x11c/0x3fc fs/overlayfs/dir.c:179
>  ovl_create_temp fs/overlayfs/dir.c:229 [inline]
>  ovl_create_over_whiteout fs/overlayfs/dir.c:473 [inline]
>  ovl_create_or_link+0x62c/0x11b8 fs/overlayfs/dir.c:629
>  ovl_link+0x1d8/0x258 fs/overlayfs/dir.c:746
>  vfs_link+0x3e0/0x590 fs/namei.c:4854
>  do_linkat+0x224/0x48c fs/namei.c:4924
>  __do_sys_linkat fs/namei.c:4952 [inline]
>  __se_sys_linkat fs/namei.c:4949 [inline]
>  __arm64_sys_linkat+0xdc/0xf8 fs/namei.c:4949
>  __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>  invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
>  el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
>  do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
>  el0_svc+0x58/0x180 arch/arm64/kernel/entry-common.c:879
>  el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:898
>  el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
>
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
>
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
>
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
>
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
>
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report

#syz dup: WARNING in shmem_unlink

Thanks,
Amir.

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