Re: [PATCH v2 01/10] fs: add is_idmapped_mnt() helper

[Seth Forshee <sforshee@digitalocean.com>]

On Tue, Nov 30, 2021 at 01:10:23PM +0100, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@ubuntu.com>
> 
> Multiple places open-code the same check to determine whether a given
> mount is idmapped. Introduce a simple helper function that can be used
> instead. This allows us to get rid of the fragile open-coding. We will
> later change the check that is used to determine whether a given mount
> is idmapped. Introducing a helper allows us to do this in a single
> place instead of doing it for multiple places.
> 
> Link: https://lore.kernel.org/r/20211123114227.3124056-2-brauner@kernel.org (v1)
> Cc: Seth Forshee <sforshee@digitalocean.com>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> CC: linux-fsdevel@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Reviewed-by: Seth Forshee <sforshee@digitalocean.com>

> ---
> /* v2 */
> - Amir Goldstein <amir73il@gmail.com>:
>   - s/is_mapped_mnt/is_idmapped_mnt/g
> ---
>  fs/cachefiles/bind.c |  2 +-
>  fs/ecryptfs/main.c   |  2 +-
>  fs/namespace.c       |  2 +-
>  fs/nfsd/export.c     |  2 +-
>  fs/overlayfs/super.c |  2 +-
>  fs/proc_namespace.c  |  2 +-
>  include/linux/fs.h   | 14 ++++++++++++++
>  7 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/cachefiles/bind.c b/fs/cachefiles/bind.c
> index d463d89f5db8..146291be6263 100644
> --- a/fs/cachefiles/bind.c
> +++ b/fs/cachefiles/bind.c
> @@ -117,7 +117,7 @@ static int cachefiles_daemon_add_cache(struct cachefiles_cache *cache)
>  	root = path.dentry;
>  
>  	ret = -EINVAL;
> -	if (mnt_user_ns(path.mnt) != &init_user_ns) {
> +	if (is_idmapped_mnt(path.mnt)) {
>  		pr_warn("File cache on idmapped mounts not supported");
>  		goto error_unsupported;
>  	}
> diff --git a/fs/ecryptfs/main.c b/fs/ecryptfs/main.c
> index d66bbd2df191..2dd23a82e0de 100644
> --- a/fs/ecryptfs/main.c
> +++ b/fs/ecryptfs/main.c
> @@ -537,7 +537,7 @@ static struct dentry *ecryptfs_mount(struct file_system_type *fs_type, int flags
>  		goto out_free;
>  	}
>  
> -	if (mnt_user_ns(path.mnt) != &init_user_ns) {
> +	if (is_idmapped_mnt(path.mnt)) {
>  		rc = -EINVAL;
>  		printk(KERN_ERR "Mounting on idmapped mounts currently disallowed\n");
>  		goto out_free;
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 659a8f39c61a..4994b816a74c 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3936,7 +3936,7 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
>  	 * mapping. It makes things simpler and callers can just create
>  	 * another bind-mount they can idmap if they want to.
>  	 */
> -	if (mnt_user_ns(m) != &init_user_ns)
> +	if (is_idmapped_mnt(m))
>  		return -EPERM;
>  
>  	/* The underlying filesystem doesn't support idmapped mounts yet. */
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index 9421dae22737..668c7527b17e 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -427,7 +427,7 @@ static int check_export(struct path *path, int *flags, unsigned char *uuid)
>  		return -EINVAL;
>  	}
>  
> -	if (mnt_user_ns(path->mnt) != &init_user_ns) {
> +	if (is_idmapped_mnt(path->mnt)) {
>  		dprintk("exp_export: export of idmapped mounts not yet supported.\n");
>  		return -EINVAL;
>  	}
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 265181c110ae..7bb0a47cb615 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -873,7 +873,7 @@ static int ovl_mount_dir_noesc(const char *name, struct path *path)
>  		pr_err("filesystem on '%s' not supported\n", name);
>  		goto out_put;
>  	}
> -	if (mnt_user_ns(path->mnt) != &init_user_ns) {
> +	if (is_idmapped_mnt(path->mnt)) {
>  		pr_err("idmapped layers are currently not supported\n");
>  		goto out_put;
>  	}
> diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
> index 392ef5162655..49650e54d2f8 100644
> --- a/fs/proc_namespace.c
> +++ b/fs/proc_namespace.c
> @@ -80,7 +80,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
>  			seq_puts(m, fs_infop->str);
>  	}
>  
> -	if (mnt_user_ns(mnt) != &init_user_ns)
> +	if (is_idmapped_mnt(mnt))
>  		seq_puts(m, ",idmapped");
>  }
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 1cb616fc1105..426cc7bcbeb8 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2725,6 +2725,20 @@ static inline struct user_namespace *file_mnt_user_ns(struct file *file)
>  {
>  	return mnt_user_ns(file->f_path.mnt);
>  }
> +
> +/**
> + * is_idmapped_mnt - check whether a mount is mapped
> + * @mnt: the mount to check
> + *
> + * If @mnt has an idmapping attached to it @mnt is mapped.
> + *
> + * Return: true if mount is mapped, false if not.
> + */
> +static inline bool is_idmapped_mnt(const struct vfsmount *mnt)
> +{
> +	return mnt_user_ns(mnt) != &init_user_ns;
> +}
> +
>  extern long vfs_truncate(const struct path *, loff_t);
>  int do_truncate(struct user_namespace *, struct dentry *, loff_t start,
>  		unsigned int time_attrs, struct file *filp);
> -- 
> 2.30.2
>