linux-kernel - Re: [PATCH] cgroup: fix race between fork and cgroup.kill

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Message-ID: <nsh73lyuoctjbeudj7o35jrkcygnfehhwj5dbf4u3hdvawx54v@nz6l2v5q2iin>
Date: Fri, 31 Jan 2025 10:33:38 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Christian Brauner <brauner@...nel.org>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH] cgroup: fix race between fork and cgroup.kill

On Thu, Jan 30, 2025 at 04:05:42PM -0800, Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> @@ -6668,6 +6668,7 @@ void cgroup_post_fork(struct task_struct *child,
>  		      struct kernel_clone_args *kargs)
>  	__releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
>  {
> +	unsigned int cgrp_kill_seq = 0;

This initialization is only needed for the extra "init tasks" branch
(the value should equal to whatever init_css_set.dfl_cgroup has, i.e.
0).

>  	unsigned long cgrp_flags = 0;
>  	bool kill = false;
>  	struct cgroup_subsys *ss;
> @@ -6681,10 +6682,13 @@ void cgroup_post_fork(struct task_struct *child,
>  
>  	/* init tasks are special, only link regular threads */
>  	if (likely(child->pid)) {
> -		if (kargs->cgrp)
> +		if (kargs->cgrp) {
>  			cgrp_flags = kargs->cgrp->flags;
> -		else
> +			cgrp_kill_seq = kargs->cgrp->kill_seq;
> +		} else {

This should not be strictly necessary thanks to cgroup_mutex during
cgroup_kill and CLONE_INTO_CGROUP.

>  			cgrp_flags = cset->dfl_cgrp->flags;
> +			cgrp_kill_seq = cset->dfl_cgrp->kill_seq;
> +		}
>  
>  		WARN_ON_ONCE(!list_empty(&child->cg_list));
>  		cset->nr_tasks++;
> @@ -6719,7 +6723,7 @@ void cgroup_post_fork(struct task_struct *child,
>  		 * child down right after we finished preparing it for
>  		 * userspace.
>  		 */
> -		kill = test_bit(CGRP_KILL, &cgrp_flags);
> +		kill = kargs->kill_seq != cgrp_kill_seq;
>  	}
>  
>  	spin_unlock_irq(&css_set_lock);

The above are only notes for better understanding, I don't think the fix
needs changes in that regard. Thanks

Reviewed-by: Michal Koutný <mkoutny@...e.com>

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