[PATCH review 6/6] userns: Allow the userns root to mount tmpfs.

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> 
> There is no backing store to tmpfs and file creation rules are the
> same as for any other filesystem so it is semantically safe to allow
> unprivileged users to mount it.  ramfs is safe for the same reasons so
> allow either flavor of tmpfs to be mounted by a user namespace root
> user.
> 
> The memory control group successfully limits how much memory tmpfs can
> consume on any system that cares about a user namespace root using
> tmpfs to exhaust memory the memory control group can be deployed.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>

Acked-by: Serge Hallyn <serge.hallyn at canonical.com>

> ---
>  mm/shmem.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5c90d84..197ca5e 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2766,6 +2766,7 @@ static struct file_system_type shmem_fs_type = {
>  	.name		= "tmpfs",
>  	.mount		= shmem_mount,
>  	.kill_sb	= kill_litter_super,
> +	.fs_flags	= FS_USERNS_MOUNT,
>  };
>  
>  int __init shmem_init(void)
> @@ -2823,6 +2824,7 @@ static struct file_system_type shmem_fs_type = {
>  	.name		= "tmpfs",
>  	.mount		= ramfs_mount,
>  	.kill_sb	= kill_litter_super,
> +	.fs_flags	= FS_USERNS_MOUNT,
>  };
>  
>  int __init shmem_init(void)
> -- 
> 1.7.5.4