[RFC PATCH v2] hostfs: handle idmapped mounts

[Christian Brauner]

On Tue, Feb 28, 2023 at 07:50:02PM -0600, Glenn Washburn wrote:
> Let hostfs handle idmapped mounts. This allows to have the same hostfs
> mount appear in multiple locations with different id mappings.
> 
> root@(none):/media# id
> uid=0(root) gid=0(root) groups=0(root)
> root@(none):/media# mkdir mnt idmapped
> root@(none):/media# mount -thostfs -o/home/user hostfs mnt
> 
> root@(none):/media# touch mnt/aaa
> root@(none):/media# mount-idmapped --map-mount u:`id -u user`:0:1 --map-mount g:`id -g user`:0:1 /media/mnt /media/idmapped
> root@(none):/media# ls -l mnt/aaa idmapped/aaa
> -rw-r--r-- 1 root root 0 Jan 28 01:23 idmapped/aaa
> -rw-r--r-- 1 user user 0 Jan 28 01:23 mnt/aaa
> 
> root@(none):/media# touch idmapped/bbb
> root@(none):/media# ls -l mnt/bbb idmapped/bbb
> -rw-r--r-- 1 root root 0 Jan 28 01:26 idmapped/bbb
> -rw-r--r-- 1 user user 0 Jan 28 01:26 mnt/bbb
> 
> Signed-off-by: Glenn Washburn <development at efficientek.com>
> ---
> Changes from v1:
>  * Rebase on to tip. The above commands work and have the results expected.
>    The __vfsuid_val(make_vfsuid(...)) seems ugly to get the uid_t, but it
>    seemed like the best one I've come across. Is there a better way?

Sure, I can help you with that. ;)

> 
> Glenn
> ---
>  fs/hostfs/hostfs_kern.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/hostfs/hostfs_kern.c b/fs/hostfs/hostfs_kern.c
> index c18bb50c31b6..9459da99a0db 100644
> --- a/fs/hostfs/hostfs_kern.c
> +++ b/fs/hostfs/hostfs_kern.c
> @@ -786,7 +786,7 @@ static int hostfs_permission(struct mnt_idmap *idmap,
>  		err = access_file(name, r, w, x);
>  	__putname(name);
>  	if (!err)
> -		err = generic_permission(&nop_mnt_idmap, ino, desired);
> +		err = generic_permission(idmap, ino, desired);
>  	return err;
>  }
>  
> @@ -794,13 +794,14 @@ static int hostfs_setattr(struct mnt_idmap *idmap,
>  			  struct dentry *dentry, struct iattr *attr)
>  {
>  	struct inode *inode = d_inode(dentry);
> +	struct user_namespace *fs_userns = i_user_ns(inode);

Fyi, since hostfs can't be mounted in a user namespace
fs_userns == &init_user_ns
so it doesn't really matter what you use.

>  	struct hostfs_iattr attrs;
>  	char *name;
>  	int err;
>  
>  	int fd = HOSTFS_I(inode)->fd;
>  
> -	err = setattr_prepare(&nop_mnt_idmap, dentry, attr);
> +	err = setattr_prepare(idmap, dentry, attr);
>  	if (err)
>  		return err;
>  
> @@ -814,11 +815,11 @@ static int hostfs_setattr(struct mnt_idmap *idmap,
>  	}
>  	if (attr->ia_valid & ATTR_UID) {
>  		attrs.ia_valid |= HOSTFS_ATTR_UID;
> -		attrs.ia_uid = from_kuid(&init_user_ns, attr->ia_uid);
> +		attrs.ia_uid = __vfsuid_val(make_vfsuid(idmap, fs_userns, attr->ia_uid));
>  	}
>  	if (attr->ia_valid & ATTR_GID) {
>  		attrs.ia_valid |= HOSTFS_ATTR_GID;
> -		attrs.ia_gid = from_kgid(&init_user_ns, attr->ia_gid);
> +		attrs.ia_gid = __vfsgid_val(make_vfsgid(idmap, fs_userns, attr->ia_gid));

Heh, if you look include/linux/fs.h:

        /*
         * The two anonymous unions wrap structures with the same member.
         *
         * Filesystems raising FS_ALLOW_IDMAP need to use ia_vfs{g,u}id which
         * are a dedicated type requiring the filesystem to use the dedicated
         * helpers. Other filesystem can continue to use ia_{g,u}id until they
         * have been ported.
         *
         * They always contain the same value. In other words FS_ALLOW_IDMAP
         * pass down the same value on idmapped mounts as they would on regular
         * mounts.
         */
        union {
                kuid_t          ia_uid;
                vfsuid_t        ia_vfsuid;
        };
        union {
                kgid_t          ia_gid;
                vfsgid_t        ia_vfsgid;
        };

this just is:

attrs.ia_uid = from_vfsuid(idmap, fs_userns, attr->ia_vfsuid));
attrs.ia_gid = from_vfsgid(idmap, fs_userns, attr->ia_vfsgid));

(I plan to fully replace ia_{g,u}id at some point.)

Christian