linux-kernel - Re: [PATCH 7/9] vfs: protect remounting superblock read-only

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Message-Id: <E1P9Gh4-00054y-EA@pomaz-ex.szeredi.hu>
Date:	Fri, 22 Oct 2010 14:25:58 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	miklos@...redi.hu, dave@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9] vfs: protect remounting superblock read-only

On Fri, 22 Oct 2010, Al Viro wrote:
> On Tue, Oct 05, 2010 at 12:31:15PM +0200, Miklos Szeredi wrote:
> > From: Miklos Szeredi <mszeredi@...e.cz>
> > 
> > Currently remouting superblock read-only is racy in a major way.
> > 
> > With the per mount read-only infrastructure it is now possible to
> > prevent most races, which this patch attempts.
> > 
> > Before starting the remount read-only, set MNT_WRITE_HOLD on all
> > mounts so that writes are held off until the remount either succeeds
> > or fails.
> 
> Umm...  What'll happen if your remount will say mnt_want_write() on
> e.g. internal vfsmount over that sb?

Quickly looking through filesystems I didn't find this sort of usage.

>   Or, more subtle, tries to
> update atime on some opened struct file on that sb.

Hmm, load_nls() will apparently do that.  Drat.

Plan B: remount all vfsmounts read-only before ->remount_fs() but
remember which ones were read-write and restore on failure.  Will
result in ugly transient write failures if remount_fs() fails, but I
don't think anybody would care in practice.

Thanks,
Miklos
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