Re: [PATCH] Relax ns_can_attach checks to allow attaching to grandchild cgroups

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Grzegorz Nosek (root-AfQBxy1nhrQ00sYp1HPQUA@public.gmane.org):
> The ns_proxy cgroup allows moving processes to child cgroups only one
> level deep at a time. This commit relaxes this restriction and makes it
> possible to attach tasks directly to grandchild cgroups, e.g.:
> 
> ($pid is in the root cgroup)
> echo $pid > /cgroup/CG1/CG2/tasks
> 
> Previously this operation would fail with -EPERM and would have to be
> performed as two steps:
> echo $pid > /cgroup/CG1/tasks
> echo $pid > /cgroup/CG1/CG2/tasks
> 
> Also, the target cgroup no longer needs to be empty to move a task there.
> 
> Signed-off-by: Grzegorz Nosek <root-AfQBxy1nhrQ00sYp1HPQUA@public.gmane.org>

I'm ok with these semantics.

Acked-by: Serge Hallyn <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

-serge

> ---
>  include/linux/cgroup.h |    4 ++--
>  kernel/cgroup.c        |   11 ++++++-----
>  kernel/ns_cgroup.c     |   14 ++++----------
>  3 files changed, 12 insertions(+), 17 deletions(-)
> 
> Changes since previous version:
>  - removed the restriction of empty target cgroup when moving tasks
>  - updated missed comment (s/child/descendant/)
> 
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 1ba3ded..53188f5 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -301,8 +301,8 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
> 
>  int cgroup_task_count(const struct cgroup *cgrp);
> 
> -/* Return true if the cgroup is a descendant of the current cgroup */
> -int cgroup_is_descendant(const struct cgroup *cgrp);
> +/* Return true if cgrp is a descendant of the task's cgroup */
> +int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task);
> 
>  /* Control Group subsystem type. See Documentation/cgroups.txt for details */
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index ee91952..d881222 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -3104,18 +3104,19 @@ int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
>  }
> 
>  /**
> - * cgroup_is_descendant - see if @cgrp is a descendant of current task's cgrp
> + * cgroup_is_descendant - see if @cgrp is a descendant of @task's cgrp
>   * @cgrp: the cgroup in question
> + * @task: the task in question
>   *
> - * See if @cgrp is a descendant of the current task's cgroup in
> - * the appropriate hierarchy.
> + * See if @cgrp is a descendant of @task's cgroup in the appropriate
> + * hierarchy.
>   *
>   * If we are sending in dummytop, then presumably we are creating
>   * the top cgroup in the subsystem.
>   *
>   * Called only by the ns (nsproxy) cgroup.
>   */
> -int cgroup_is_descendant(const struct cgroup *cgrp)
> +int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task)
>  {
>  	int ret;
>  	struct cgroup *target;
> @@ -3125,7 +3126,7 @@ int cgroup_is_descendant(const struct cgroup *cgrp)
>  		return 1;
> 
>  	get_first_subsys(cgrp, NULL, &subsys_id);
> -	target = task_cgroup(current, subsys_id);
> +	target = task_cgroup(task, subsys_id);
>  	while (cgrp != target && cgrp!= cgrp->top_cgroup)
>  		cgrp = cgrp->parent;
>  	ret = (cgrp == target);
> diff --git a/kernel/ns_cgroup.c b/kernel/ns_cgroup.c
> index 43c2111..890691a 100644
> --- a/kernel/ns_cgroup.c
> +++ b/kernel/ns_cgroup.c
> @@ -35,7 +35,7 @@ int ns_cgroup_clone(struct task_struct *task, struct pid *pid)
> 
>  /*
>   * Rules:
> - *   1. you can only enter a cgroup which is a child of your current
> + *   1. you can only enter a cgroup which is a descendant of your current
>   *     cgroup
>   *   2. you can only place another process into a cgroup if
>   *     a. you have CAP_SYS_ADMIN
> @@ -46,21 +46,15 @@ int ns_cgroup_clone(struct task_struct *task, struct pid *pid)
>  static int ns_can_attach(struct cgroup_subsys *ss,
>  		struct cgroup *new_cgroup, struct task_struct *task)
>  {
> -	struct cgroup *orig;
> -
>  	if (current != task) {
>  		if (!capable(CAP_SYS_ADMIN))
>  			return -EPERM;
> 
> -		if (!cgroup_is_descendant(new_cgroup))
> +		if (!cgroup_is_descendant(new_cgroup, current))
>  			return -EPERM;
>  	}
> 
> -	if (atomic_read(&new_cgroup->count) != 0)
> -		return -EPERM;
> -
> -	orig = task_cgroup(task, ns_subsys_id);
> -	if (orig && orig != new_cgroup->parent)
> +	if (!cgroup_is_descendant(new_cgroup, task))
>  		return -EPERM;
> 
>  	return 0;
> @@ -78,7 +72,7 @@ static struct cgroup_subsys_state *ns_create(struct cgroup_subsys *ss,
> 
>  	if (!capable(CAP_SYS_ADMIN))
>  		return ERR_PTR(-EPERM);
> -	if (!cgroup_is_descendant(cgroup))
> +	if (!cgroup_is_descendant(cgroup, current))
>  		return ERR_PTR(-EPERM);
> 
>  	ns_cgroup = kzalloc(sizeof(*ns_cgroup), GFP_KERNEL);
> -- 
> 1.5.4.3
> 
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