[PATCH] fs: increase reference count for backing store when loop mounting

[Sascha Hauer]

On Thu, Feb 11, 2021 at 06:11:39PM +0100, Ahmad Fatoum wrote:
> The VFS layer already increase the mount reference count for the mount
> point. This means e.g. following sequence is well-behaving:
> 
>   mkdir -p /mnt/disk0.0/media
>   mount -o loop backing.squashfs /mnt/disk0.0/media
>   umount /mnt/disk0.0
> 
> barebox will do the right thing and report
> 
>   umount: Device or resource busy
> 
> However the reference count of the file where backing.squashfs comes
> from is not incremented on mount with the effect that following sequence
> crashes:
> 
>   mkdir -p /media
>   mount -o loop /mnt/disk0.0/backing.squashfs /media
>   umount /mnt/disk0.0 # should've returned EBUSY
>   umount /media
> 
> Fix this by touching the backing store's mount reference count when
> loop mounting and unmounting.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
> ---
>  fs/fs.c | 38 +++++++++++++++++++++++++++++++++-----
>  1 file changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/fs.c b/fs/fs.c
> index 00b8645fb0e9..10cc08fc93ab 100644
> --- a/fs/fs.c
> +++ b/fs/fs.c
> @@ -89,7 +89,13 @@ static int init_fs(void)
>  
>  postcore_initcall(init_fs);
>  
> +struct filename;
> +
>  static struct fs_device_d *get_fsdevice_by_path(const char *path);
> +static int filename_lookup(int dfd, struct filename *name, unsigned flags,
> +			   struct path *path);;
> +static struct filename *getname(const char *filename);
> +static void path_put(const struct path *path);
>  
>  LIST_HEAD(fs_device_list);
>  
> @@ -683,6 +689,8 @@ static void fs_remove(struct device_d *dev)
>  	struct fs_device_d *fsdev = dev_to_fs_device(dev);
>  	struct super_block *sb = &fsdev->sb;
>  	struct inode *inode, *tmp;
> +	struct path path;
> +	int ret;
>  
>  	if (fsdev->dev.driver) {
>  		dev->driver->remove(dev);
> @@ -695,9 +703,17 @@ static void fs_remove(struct device_d *dev)
>  	if (fsdev->cdev)
>  		cdev_close(fsdev->cdev);
>  
> -	if (fsdev->loop && fsdev->cdev)
> +	if (fsdev->loop && fsdev->cdev) {
>  		cdev_remove_loop(fsdev->cdev);
>  
> +		ret = filename_lookup(AT_FDCWD, getname(fsdev->backingstore),
> +				      LOOKUP_FOLLOW, &path);
> +		if (!ret) {
> +			mntput(path.mnt);
> +			path_put(&path);
> +		}
> +	}
> +
>  	if (fsdev->vfsmount.mountpoint)
>  		fsdev->vfsmount.mountpoint->d_flags &= ~DCACHE_MOUNTED;
>  
> @@ -767,17 +783,29 @@ static const char *detect_fs(const char *filename, const char *fsoptions)
>  int fsdev_open_cdev(struct fs_device_d *fsdev)
>  {
>  	unsigned long long offset = 0;
> +	struct path path = {};
> +	int ret;
>  
>  	parseopt_b(fsdev->options, "loop", &fsdev->loop);
>  	parseopt_llu_suffix(fsdev->options, "offset", &offset);
> -	if (fsdev->loop)
> -		fsdev->cdev = cdev_create_loop(fsdev->backingstore, O_RDWR,
> -					       offset);
> -	else
> +	if (fsdev->loop) {
> +		ret = filename_lookup(AT_FDCWD, getname(fsdev->backingstore),
> +				      LOOKUP_FOLLOW, &path);
> +		if (ret)
> +			return ret;
> +
> +		fsdev->cdev = cdev_create_loop(fsdev->backingstore, O_RDWR, offset);
> +	} else {
>  		fsdev->cdev = cdev_open(fsdev->backingstore, O_RDWR);
> +	}
>  	if (!fsdev->cdev)
>  		return -EINVAL;

path_put() is missing in this error path.

Sascha


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