linux-kernel - Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq

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Message-Id: <20240509232613.2459-1-hdanton@sina.com>
Date: Fri, 10 May 2024 07:26:13 +0800
From: Hillf Danton <hdanton@...a.com>
To: Amir Goldstein <amir73il@...il.com>
Cc: syzbot <syzbot+4c493dcd5a68168a94b2@...kaller.appspotmail.com>,
	linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Pavel Machek <pavel@....cz>,
	linux-pm@...r.kernel.org
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq_start

On Thu, 9 May 2024 17:52:21 +0300 Amir Goldstein <amir73il@...il.com>
> On Thu, May 9, 2024 at 1:49 PM Hillf Danton <hdanton@...a.com> 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