Re: [PATCH] btrfs: do not allow mounting non-subvolumes via subvol option

[Goffredo Baroncelli <kreijack@libero.it>]

Hi David,

On 07/29/2011 12:14 PM, David Sterba wrote:
> There's a missing test whether the path passed to subvol=path option
> during mount is a real subvolume, allowing any directory located in
> default subovlume to be passed and accepted for mount.
>
> (current btrfs progs prevent this early)
> $ btrfs subvol snapshot . p1-snap
> ERROR: '.' is not a subvolume
>
> (with "is subvolume?" test bypassed)
> $ btrfs subvol snapshot . p1-snap
> Create a snapshot of '.' in './p1-snap'
>
> $ btrfs subvol list -p .
> ID 258 parent 5 top level 5 path subvol
> ID 259 parent 5 top level 5 path subvol1
> ID 260 parent 5 top level 5 path default-subvol1
> ID 262 parent 5 top level 5 path p1/p1-snapshot
> ID 263 parent 259 top level 5 path subvol1/subvol1-snap
>
> The problem I see is that this makes a false impression of snapshotting the
> given subvolume but in fact snapshots the default one: a user expects outcome

Not that matter too much, but the old behavior was to snapshot not the 
"default one" but the one which contains the directory.
This behavior leaded to a lot of misunderstanding about the btrfs 
capability of snapshot subvolume __only__.

Only one question, what happens now if an user pass subvol=<dir> ?

> like ID 263 but in fact gets ID 262 .
>
> This patch makes mount fail with EINVAL with a message in syslog.
>
> Signed-off-by: David Sterba<dsterba@suse.cz>
> ---
>
> I did not find a better errno than EINVAL, probably adding someting like
> ENSUBVOL would be better so that other filesystems with such functionality may
> use it in future.
>
>   fs/btrfs/super.c |   19 +++++++++++++++++++
>   1 files changed, 19 insertions(+), 0 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 15634d4..0c2a1d1 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -753,6 +753,15 @@ static int btrfs_set_super(struct super_block *s, void *data)
>   	return set_anon_super(s, data);
>   }
>
> +/*
> + * subvolumes are identified by ino 256
> + */
> +static inline int is_subvolume_inode(struct inode *inode)
> +{
> +	if (inode&&  inode->i_ino == BTRFS_FIRST_FREE_OBJECTID)
> +		return 1;
> +	return 0;
> +}
>
>   /*
>    * Find a superblock for the given device / mount point.
> @@ -873,6 +882,16 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
>   			error = -ENXIO;
>   			goto error_free_subvol_name;
>   		}
> +
> +		if (!is_subvolume_inode(new_root->d_inode)) {
> +			dput(root);
> +			dput(new_root);
> +			deactivate_locked_super(s);
> +			error = -EINVAL;
> +			printk(KERN_ERR "btrfs: '%s' is not a valid subvolume\n",
> +					subvol_name);
> +			goto error_free_subvol_name;
> +		}
>   		dput(root);
>   		root = new_root;
>   	} else {