Re: [PATCH v5 5/6] clone3: allow spawning processes into cgroups

Michal Koutný
2020-01-29T13:27:19Z

Hello.

On Tue, Jan 21, 2020 at 04:48:43PM +0100, Christian Brauner <christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org> wrote:
> +static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
> + __acquires(&cgroup_mutex) __acquires(&cgroup_threadgroup_rwsem)
> +{
> + int ret;
> + struct cgroup *dst_cgrp = NULL;
> + struct css_set *cset;
> + struct super_block *sb;
> + struct file *f;
> +
> + if (kargs->flags & CLONE_INTO_CGROUP)
> + mutex_lock(&cgroup_mutex);
> +
> + cgroup_threadgroup_change_begin(current);
> +
> + spin_lock_irq(&css_set_lock);
> + cset = task_css_set(current);
> + get_css_set(cset);
> + spin_unlock_irq(&css_set_lock);
> +
> + if (!(kargs->flags & CLONE_INTO_CGROUP)) {
> + kargs->cset = cset;
Where is this css_set put when CLONE_INTO_CGROUP isn't used?
(Aha, it's passed to child's tsk->cgroups but see my other note below.)

> + dst_cgrp = cgroup_get_from_file(f);
> + if (IS_ERR(dst_cgrp)) {
> + ret = PTR_ERR(dst_cgrp);
> + dst_cgrp = NULL;
> + goto err;
> + }
> +
> + /*
> + * Verify that we the target cgroup is writable for us. This is
> + * usually done by the vfs layer but since we're not going through
> + * the vfs layer here we need to do it "manually".
> + */
> + ret = cgroup_may_write(dst_cgrp, sb);
> + if (ret)
> + goto err;
> +
> + ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb,
> + !!(kargs->flags & CLONE_THREAD));
> + if (ret)
> + goto err;
> +
> + kargs->cset = find_css_set(cset, dst_cgrp);
> + if (!kargs->cset) {
> + ret = -ENOMEM;
> + goto err;
> + }
> +
> + if (cgroup_is_dead(dst_cgrp)) {
> + ret = -ENODEV;
> + goto err;
> + }
I'd move this check right after cgroup_get_from_file. The fork-migration
path is synchrinized via cgroup_mutex with cgroup_destroy_locked and
there's no need checking permissions on cgroup that's going away anyway.

> +static void cgroup_css_set_put_fork(struct kernel_clone_args *kargs)
> + __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
> +{
> + cgroup_threadgroup_change_end(current);
> +
> + if (kargs->flags & CLONE_INTO_CGROUP) {
> + struct cgroup *cgrp = kargs->cgrp;
> + struct css_set *cset = kargs->cset;
> +
> + mutex_unlock(&cgroup_mutex);
> +
> + if (cset) {
> + put_css_set(cset);
> + kargs->cset = NULL;
> + }
> +
> + if (cgrp) {
> + cgroup_put(cgrp);
> + kargs->cgrp = NULL;
> + }
> + }
I don't see any function problem with this ordering, however, I'd
prefer symmetry with the "allocation" path (in cgroup_css_set_fork),
i.e. cgroup_put, put_css_set and lastly mutex_unlock.

> +void cgroup_post_fork(struct task_struct *child,
> + struct kernel_clone_args *kargs)
> + __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
> {
> struct cgroup_subsys *ss;
> - struct css_set *cset;
> + struct css_set *cset = kargs->cset;
> int i;
>
> spin_lock_irq(&css_set_lock);
>
> WARN_ON_ONCE(!list_empty(&child->cg_list));
> - cset = task_css_set(current); /* current is @child's parent */
> - get_css_set(cset);
> cset->nr_tasks++;
> css_set_move_task(child, NULL, cset, false);
So, the reference is passed over from kargs->cset to task->cgroups. I
think it's necessary to zero kargs->cset in order to prevent droping the
reference in cgroup_css_set_put_fork.
Perhaps, a general comment about css_set whereabouts during fork and
kargs passing would be useful.

> @@ -6016,6 +6146,17 @@ void cgroup_post_fork(struct task_struct *child)
> } while_each_subsys_mask();
>
> cgroup_threadgroup_change_end(current);
> +
> + if (kargs->flags & CLONE_INTO_CGROUP) {
> + mutex_unlock(&cgroup_mutex);
> +
> + cgroup_put(kargs->cgrp);
> + kargs->cgrp = NULL;
> + }
> +
> + /* Make the new cset the root_cset of the new cgroup namespace. */
> + if (kargs->flags & CLONE_NEWCGROUP)
> + child->nsproxy->cgroup_ns->root_cset = cset;
root_cset reference (from copy_cgroup_ns) seems leaked here and where is
the additional reference to new cset obtained?

Thanks,
Michal