linux-kernel - Re: [PATCH 1/5] initmpfs: replace MS

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Date:	Sat, 29 Jun 2013 18:15:40 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Rob Landley <rob@...dley.net>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 1/5] initmpfs: replace MS_NOUSER in initramfs

Rob Landley <rob@...dley.net> writes:

> From: Rob Landley <rob@...dley.net>
>
> Mounting MS_NOUSER prevents --bind mounts from rootfs. Prevent new rootfs
> mounts with a different mechanism that doesn't affect bind mounts.

I don't see patches 4 and 5 so I don't know if you have covered this
elsewhere but a very important part of the reason for MS_NOUSER is to
prevent unmounting of rootfs.

The entire vfs breaks if you are allowed to unmount rootfs, and it
appears this patch is allowing that.

Eric

> Signed-off-by: Rob Landley <rob@...dley.net>
> ---
>
>  fs/ramfs/inode.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ramfs/inode.c b/fs/ramfs/inode.c
> index c24f1e1..14b9c35 100644
> --- a/fs/ramfs/inode.c
> +++ b/fs/ramfs/inode.c
> @@ -247,7 +247,14 @@ struct dentry *ramfs_mount(struct file_system_type *fs_type,
>  static struct dentry *rootfs_mount(struct file_system_type *fs_type,
>  	int flags, const char *dev_name, void *data)
>  {
> -	return mount_nodev(fs_type, flags|MS_NOUSER, data, ramfs_fill_super);
> +	static int once;
> +
> +	if (once)
> +		return ERR_PTR(-ENODEV);
> +	else
> +		once++;
> +
> +	return mount_nodev(fs_type, flags, data, ramfs_fill_super);
>  }
>  
>  static void ramfs_kill_sb(struct super_block *sb)
--
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