[PATCH 1/8] quota: Allow to pass mount path to quotactl

[Christoph Hellwig]

On Fri, Jan 22, 2021 at 04:15:29PM +0100, Sascha Hauer wrote:
> This patch introduces the Q_PATH flag to the quotactl cmd argument.
> When given, the path given in the special argument to quotactl will
> be the mount path where the filesystem is mounted, instead of a path
> to the block device.
> This is necessary for filesystems which do not have a block device as
> backing store. Particularly this is done for upcoming UBIFS support.
> 
> Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>

I hate overloading quotactl even more.  Why not add a new quotactl_path
syscall instead?

> +static struct super_block *quotactl_sb(dev_t dev, int cmd)
>  {
>  	struct super_block *sb;
>  	bool excl = false, thawed = false;
>  
>  	if (quotactl_cmd_onoff(cmd)) {
>  		excl = true;
> @@ -901,12 +887,50 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
>  		goto retry;
>  	}
>  	return sb;
> +}
> +
> +/*
> + * look up a superblock on which quota ops will be performed
> + * - use the name of a block device to find the superblock thereon
> + */
> +static struct super_block *quotactl_block(const char __user *special, int cmd)
> +{
> +#ifdef CONFIG_BLOCK
> +	struct filename *tmp = getname(special);
> +	int error;
> +	dev_t dev;
>  
> +	if (IS_ERR(tmp))
> +		return ERR_CAST(tmp);
> +	error = lookup_bdev(tmp->name, &dev);
> +	putname(tmp);
> +	if (error)
> +		return ERR_PTR(error);
> +
> +	return quotactl_sb(dev, cmd);
>  #else
>  	return ERR_PTR(-ENODEV);
>  #endif

Normal kernel style would be to keep the ifdef entirely outside the
function.

> +static struct super_block *quotactl_path(const char __user *special, int cmd)
> +{
> +	struct super_block *sb;
> +	struct path path;
> +	int error;
> +
> +	error = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
> +			   &path);

This adds an overly long line.

> +	if (error)
> +		return ERR_PTR(error);
> +
> +	sb = quotactl_sb(path.mnt->mnt_sb->s_dev, cmd);

I think quotactl_sb should take the superblock directly.  This will
need a little refactoring of user_get_super, but will lead to much
better logic.