[PATCH 5/7] cgroup: mount cgroupns-root when inside non-init cgroupns

[Tejun Heo]

Hello,

On Mon, Dec 07, 2015 at 05:06:20PM -0600, serge.hallyn at ubuntu.com wrote:
>  fs/kernfs/mount.c      |   74 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/kernfs.h |    2 ++
>  kernel/cgroup.c        |   39 ++++++++++++++++++++++++-
>  3 files changed, 114 insertions(+), 1 deletion(-)

Please put kernfs changes in a spearate patch.

> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> index 8eaf417..9219444 100644
> --- a/fs/kernfs/mount.c
> +++ b/fs/kernfs/mount.c
> @@ -62,6 +63,79 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
>  	return NULL;
>  }
>  
> +/*
> + * find the next ancestor in the path down to @child, where @parent was the
> + * parent whose child we want to find.

s/parent/ancestor/ s/child/descendant/ ?

> + *
> + * Say the path is /a/b/c/d.  @child is d, @parent is NULL.  We return the root
> + * node.  If @parent is b, then we return the node for c.
> + * Passing in d as @parent is not ok.
> + */
> +static struct kernfs_node *
> +find_next_ancestor(struct kernfs_node *child, struct kernfs_node *parent)
> +{
> +	if (child == parent) {
> +		pr_crit_once("BUG in find_next_ancestor: called with parent == child");
> +		return NULL;
> +	}
> +
> +	while (child->parent != parent) {
> +		if (!child->parent)
> +			return NULL;
> +		child = child->parent;
> +	}
> +
> +	return child;
> +}
> +
> +/**
> + * kernfs_obtain_root - get a dentry for the given kernfs_node
> + * @sb: the kernfs super_block
> + * @kn: kernfs_node for which a dentry is needed
> + *
> + * This can be 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)

Wouldn't @kn, @sb be a better order?  Also, kernfs super_blocks are
determined by the kernfs_root and its namespace.  I wonder whether
specifying @ns would be better.

> +{
> +	struct dentry *dentry;
> +	struct kernfs_node *knparent = NULL;
> +
> +	BUG_ON(sb->s_op != &kernfs_sops);
> +
> +	dentry = dget(sb->s_root);
> +	if (!kn->parent) // this is the root
			^^^
			Do we do this now?

> +		return dentry;
> +
> +	knparent = find_next_ancestor(kn, NULL);
> +	if (!knparent) {
> +		pr_crit("BUG: find_next_ancestor for root dentry returned NULL\n");

Wouldn't stack dump helpful here?  Why not

	if (WARN_ONCE(!knparent, "find_next..."))
		return ERR_PTR(-EINVAL);

or even just WARN_ON_ONCE().

> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	do {
> +		struct dentry *dtmp;
> +		struct kernfs_node *kntmp;
> +
> +		if (kn == knparent)
> +			return dentry;
> +		kntmp = find_next_ancestor(kn, knparent);
> +		if (!kntmp) {
> +			pr_crit("BUG: find_next_ancestor returned NULL for node\n");

Ditto.  It'd be a kernel bug.  WARN is usually the better way.

> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index a5ab74d..09cd718 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2011,6 +2011,15 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
>  	int ret;
>  	int i;
>  	bool new_sb;
> +	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;

Please move this upwards so that it's below other initialized
variables.

> +
> +	get_cgroup_ns(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);
> +	}
>  
>  	/*
>  	 * The first time anyone tries to mount a cgroup, enable the list
> @@ -2127,6 +2136,11 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
>  		goto out_unlock;
>  	}
>  
> +	if (!opts.none && !capable(CAP_SYS_ADMIN)) {
> +		ret = -EPERM;
> +		goto out_unlock;
> +	}

Hmmm... why is !opts.none necessary?  Please add a comment explaining
why the above is necessary.

>  out_mount:
>  	dentry = kernfs_mount(fs_type, flags, root->kf_root,
>  			      is_v2 ? CGROUP2_SUPER_MAGIC : CGROUP_SUPER_MAGIC,
>  			      &new_sb);
> +
> +	if (!IS_ERR(dentry)) {
> +		/*
> +		 * In non-init cgroup namespace, instead of root cgroup's
> +		 * dentry, we return the dentry corresponding to the
> +		 * cgroupns->root_cgrp.
> +		 */
> +		if (ns != &init_cgroup_ns) {

	if (!IS_ERR(dentry) && ns != &init_cgroup_ns) {

> +			struct dentry *nsdentry;
> +			struct cgroup *cgrp;
> +
> +			cgrp = cset_cgroup_from_root(ns->root_cset, root);
> +			nsdentry = kernfs_obtain_root(dentry->d_sb,
> +				cgrp->kn);

Heh, is kernfs_obtain_root() the right name?  Maybe
kernfs_node_to_inode()?

Thanks.

-- 
tejun