Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq_start

[Hillf Danton]

On Fri, 10 May 2024 07:26:13 +0800 Hillf Danton <hdanton@xxxxxxxx> wrote:
> On Thu, 9 May 2024 17:52:21 +0300 Amir Goldstein <amir73il@xxxxxxxxx>
> > On Thu, May 9, 2024 at 1:49 PM Hillf Danton <hdanton@xxxxxxxx> wrote:
> > >
> > > The correct locking order is
> > >
> > >                 sb_writers
> > 
> > This is sb of overlayfs
> > 
> > >                 inode lock
> > 
> > This is real inode
> > 
> WRT sb_writers the order
> 
> 	lock inode parent
> 	lock inode kid
> 
> becomes
> 	lock inode kid
> 	sb_writers
> 	lock inode parent 
> 
> given call trace
> 
> > -> #2 (sb_writers#4){.+.+}-{0:0}:
> >        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
> >        percpu_down_read include/linux/percpu-rwsem.h:51 [inline]
> >        __sb_start_write include/linux/fs.h:1664 [inline]
> >        sb_start_write+0x4d/0x1c0 include/linux/fs.h:1800
> >        mnt_want_write+0x3f/0x90 fs/namespace.c:409
> >        ovl_create_object+0x13b/0x370 fs/overlayfs/dir.c:629
> >        lookup_open fs/namei.c:3497 [inline]
> >        open_last_lookups fs/namei.c:3566 [inline]
> 
> and code snippet [1]
> 
> 	if (open_flag & O_CREAT)
> 		inode_lock(dir->d_inode);
> 	else
> 		inode_lock_shared(dir->d_inode);
> 	dentry = lookup_open(nd, file, op, got_write);
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/namei.c?id=dccb07f2914c#n3566

JFYI simply cutting off mnt_want_write() in ovl_create_object() survived
the syzpot repro [2], so acquiring sb_writers with inode locked at least
in the lookup path makes trouble.

[2] https://lore.kernel.org/lkml/000000000000975906061817416b@xxxxxxxxxx/