[PATCHv2 7/7] cgroup: mount cgroupns-root when inside non-init cgroupns

[Andy Lutomirski]

On Fri, Oct 31, 2014 at 12:19 PM, Aditya Kali <adityakali at google.com> wrote:
> This patch enables cgroup mounting inside userns when a process
> as appropriate privileges. The cgroup filesystem mounted is
> rooted at the cgroupns-root. Thus, in a container-setup, only
> the hierarchy under the cgroupns-root is exposed inside the container.
> This allows container management tools to run inside the containers
> without depending on any global state.
> In order to support this, a new kernfs api is added to lookup the
> dentry for the cgroupns-root.
>
> Signed-off-by: Aditya Kali <adityakali at google.com>
> ---
>  fs/kernfs/mount.c      | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/kernfs.h |  2 ++
>  kernel/cgroup.c        | 47 +++++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 95 insertions(+), 2 deletions(-)
>
> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> index f973ae9..e334f45 100644
> --- a/fs/kernfs/mount.c
> +++ b/fs/kernfs/mount.c
> @@ -62,6 +62,54 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
>         return NULL;
>  }
>
> +/**
> + * kernfs_make_root - create new root dentry for the given kernfs_node.
> + * @sb: the kernfs super_block
> + * @kn: kernfs_node for which a dentry is needed
> + *
> + * This can used used by callers which want to mount only a part of the kernfs
> + * as root of the filesystem.
> + */
> +struct dentry *kernfs_obtain_root(struct super_block *sb,
> +                                 struct kernfs_node *kn)
> +{

I can't usefully review this, but kernfs_make_root and
kernfs_obtain_root aren't the same string...

> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 7e5d597..250aaec 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1302,6 +1302,13 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>
>         memset(opts, 0, sizeof(*opts));
>
> +       /* Implicitly add CGRP_ROOT_SANE_BEHAVIOR if inside a non-init cgroup
> +        * namespace.
> +        */
> +       if (current->nsproxy->cgroup_ns != &init_cgroup_ns) {
> +               opts->flags |= CGRP_ROOT_SANE_BEHAVIOR;
> +       }
> +

I don't like this implicit stuff.  Can you just return -EINVAL if sane
behavior isn't requested?

>         while ((token = strsep(&o, ",")) != NULL) {
>                 nr_opts++;
>
> @@ -1391,7 +1398,7 @@ static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
>
>         if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
>                 pr_warn("sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");
> -               if (nr_opts != 1) {
> +               if (nr_opts > 1) {
>                         pr_err("sane_behavior: no other mount options allowed\n");
>                         return -EINVAL;

This looks wrong.  But, if you make the change above, then it'll be right.

> @@ -1685,6 +1701,14 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
>         int ret;
>         int i;
>         bool new_sb;
> +       struct cgroup_namespace *ns =
> +               get_cgroup_ns(current->nsproxy->cgroup_ns);
> +
> +       /* Check if the caller has permission to mount. */
> +       if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
> +               put_cgroup_ns(ns);
> +               return ERR_PTR(-EPERM);
> +       }

Why is this necessary?

> @@ -1862,6 +1904,7 @@ static struct file_system_type cgroup_fs_type = {
>         .name = "cgroup",
>         .mount = cgroup_mount,
>         .kill_sb = cgroup_kill_sb,
> +       .fs_flags = FS_USERNS_MOUNT,

Aargh, another one!  Eric, can you either ack or nack my patch?
Because if my patch goes in, then this line may need to change.  Or
not, but if a stable release with cgroupfs and without my patch
happens, then we'll have an ABI break.

--Andy