linux-kernel - Re: [PATCH] ovl: Prevent rw remount when it should be ro mount

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Date:	Sun, 4 Jan 2015 10:59:00 +0800
From:	hujianyang <hujianyang@...wei.com>
To:	Seunghun Lee <waydi1@...il.com>
CC:	<miklos@...redi.hu>, <sedat.dilek@...il.com>,
	<richard.weinberger@...il.com>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-unionfs@...r.kernel.org>
Subject: Re: [PATCH] ovl: Prevent rw remount when it should be ro mount

On 2015/1/3 1:26, Seunghun Lee wrote:
> Overlayfs should be mounted read-only when upper-fs is read-only or nonexistent.
> But now it can be remounted read-write and this can cause kernel panic.
> So we should prevent read-write remount when the above situation happens.
> 
> Signed-off-by: Seunghun Lee <waydi1@...il.com>
> ---
>  fs/overlayfs/super.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 84f3144..8944651 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -522,10 +522,21 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
>  	return 0;
>  }
>  
> +static int ovl_remount(struct super_block *sb, int *flags, char *data)
> +{
> +	struct ovl_fs *ufs = sb->s_fs_info;
> +
> +	if (!ufs->upper_mnt || (ufs->upper_mnt->mnt_sb->s_flags & MS_RDONLY))
> +		*flags |= MS_RDONLY;
> +
> +	return 0;
> +}
> +
>  static const struct super_operations ovl_super_operations = {
>  	.put_super	= ovl_put_super,
>  	.statfs		= ovl_statfs,
>  	.show_options	= ovl_show_options,
> +	.remount_fs	= ovl_remount,
>  };
>  
>  enum {
> 

I think this exporting of .remount_fs may allow people in userspace
have the ability to remount a filesystem with a new set of mounting
options. Your new adding function do nothing with the passing in
parameters.

I'm not sure if it could be competent for remount case.

Add Cc linux-unionfs.

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