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

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Message-ID: <20240406040923.GX538574@ZenIV>
Date: Sat, 6 Apr 2024 05:09:23 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Amir Goldstein <amir73il@...il.com>
Cc: syzbot <syzbot+9a5b0ced8b1bfb238b56@...kaller.appspotmail.com>,
	gregkh@...uxfoundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
	tj@...nel.org, valesini@...dex-team.ru,
	Christoph Hellwig <hch@....de>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek

On Thu, Apr 04, 2024 at 12:33:40PM +0300, Amir Goldstein wrote:

> We do not (anymore) lock ovl inode in ovl_llseek(), see:
> b1f9d3858f72 ovl: use ovl_inode_lock in ovl_llseek()
> but ovl inode is held in operations (e.g. ovl_rename)
> which trigger copy up and call vfs_llseek() on the lower file.

OK, but why do we bother with ovl_inode_lock() there?
Note that serialization on struct file level is provided
on syscall level - see call of fdget_pos() in there.
IOW, which object are you protecting?  If it's struct file
passed your way, you should already have the serialization.
If it's underlying file on disk, that's up to vfs_llseek().
Exclusion with copyup by a different operation?

I'm not saying it's wrong - it's just that the thing is
tricky enough, so some clarification might be a good idea.