Re: [PATCH v2 4/6] Btrfs: fail on mismatched subvol and subvolid mount options

[Qu Wenruo]

-------- Original Message  --------
Subject: [PATCH v2 4/6] Btrfs: fail on mismatched subvol and subvolid 
mount options
From: Omar Sandoval <osandov@xxxxxxxxxxx>
To: Chris Mason <clm@xxxxxx>, Josef Bacik <jbacik@xxxxxx>, David Sterba 
<dsterba@xxxxxxx>, <linux-btrfs@xxxxxxxxxxxxxxx>
Date: 2015å04æ10æ 05:34

> There's nothing to stop a user from passing both subvol= and subvolid=
> to mount, but if they don't refer to the same subvolume, someone is
> going to be surprised at some point. Error out on this case, but allow
> users to pass in both if they do match (which they could, for example,
> get out of /proc/mounts).
Not sure should we do this extra check, as later mount options override 
previous mount option.

I previous tried to do such thing for mount option like inode/noinode, 
but was rejected for that reason.

So not sure such error-out behavior is OK or not.
Maybe only taking the latest subvol/subvolid is a better choice?

Thanks,
Qu
> Signed-off-by: Omar Sandoval <osandov@xxxxxxxxxxx>
> ---
>   fs/btrfs/super.c | 32 ++++++++++++++++++++++++--------
>   1 file changed, 24 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index ab100e5..20b470d 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1163,8 +1163,9 @@ static char *setup_root_args(char *args)
>   	return buf;
>   }
>
> -static struct dentry *mount_subvol(const char *subvol_name, int flags,
> -				   const char *device_name, char *data)
> +static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
> +				   int flags, const char *device_name,
> +				   char *data)
>   {
>   	struct dentry *root;
>   	struct vfsmount *mnt = NULL;
> @@ -1210,12 +1211,26 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
>   	/* mount_subtree() drops our reference on the vfsmount. */
>   	mnt = NULL;
>
> -	if (!IS_ERR(root) && !is_subvolume_inode(root->d_inode)) {
> +	if (!IS_ERR(root)) {
>   		struct super_block *s = root->d_sb;
> -		dput(root);
> -		root = ERR_PTR(-EINVAL);
> -		deactivate_locked_super(s);
> -		pr_err("BTRFS: '%s' is not a valid subvolume\n", subvol_name);
> +		u64 root_objectid = BTRFS_I(root->d_inode)->root->root_key.objectid;
> +
> +		ret = 0;
> +		if (!is_subvolume_inode(root->d_inode)) {
> +			pr_err("BTRFS: '%s' is not a valid subvolume\n",
> +			       subvol_name);
> +			ret = -EINVAL;
> +		}
> +		if (subvol_objectid && root_objectid != subvol_objectid) {
> +			pr_err("BTRFS: subvol '%s' does not match subvolid %llu\n",
> +			       subvol_name, subvol_objectid);
> +			ret = -EINVAL;
> +		}
> +		if (ret) {
> +			dput(root);
> +			root = ERR_PTR(ret);
> +			deactivate_locked_super(s);
> +		}
>   	}
>
>   out:
> @@ -1308,7 +1323,8 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
>
>   	if (subvol_name) {
>   		/* mount_subvol() will free subvol_name. */
> -		return mount_subvol(subvol_name, flags, device_name, data);
> +		return mount_subvol(subvol_name, subvol_objectid, flags,
> +				    device_name, data);
>   	}
>
>   	security_init_mnt_opts(&new_sec_opts);
--
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