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

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Message-ID: <20240406035451.GW538574@ZenIV>
Date: Sat, 6 Apr 2024 04:54:51 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christoph Hellwig <hch@....de>
Cc: Amir Goldstein <amir73il@...il.com>,
	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,
	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 Fri, Apr 05, 2024 at 08:51:35AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 04, 2024 at 12:33:40PM +0300, Amir Goldstein wrote:
> > I don't follow what you are saying.
> > Which code is in non-starter violation?
> > kernfs for calling lookup_bdev() with internal of->mutex held?
> 
> That is a huge problem, and has been causing endless annoying lockdep
> chains in the block layer for us.  If we have some way to kill this
> the whole block layer would benefit.

Specifically of->mutex or having pathwalk done from some ->write_iter()?

Incidentally, losetup happily accepts sysfs files as backing store.
I don't think it adds problems we don't have otherwise, but IMO
that's really bogus...