linux-kernel - Re: [PATCH 1/3] cgroup: unify attach permission checking

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Message-ID: <20191218234632.3xpobnq6efprohal@wittgenstein>
Date:   Thu, 19 Dec 2019 00:46:33 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH 1/3] cgroup: unify attach permission checking

On Wed, Dec 18, 2019 at 06:35:14PM +0100, Christian Brauner wrote:
> The core codepaths to check whether a process can be attached to a
> cgroup are the same for threads and thread-group leaders. Only a small
> piece of code verifying that source and destination cgroup are in the
> same domain differentiates the thread permission checking from
> thread-group leader permission checking.
> Since cgroup_migrate_vet_dst() only matters cgroup2 - it is a noop on
> cgroup1 - we can move it out of cgroup_attach_task().
> All checks can now be consolidated into a new helper
> cgroup_attach_permissions() callable from both cgroup_procs_write() and
> cgroup_threads_write().
> 
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Li Zefan <lizefan@...wei.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: cgroups@...r.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
>  kernel/cgroup/cgroup.c | 46 +++++++++++++++++++++++++++++-------------
>  1 file changed, 32 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 735af8f15f95..5ee06c1f7456 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -2719,11 +2719,7 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
>  {
>  	DEFINE_CGROUP_MGCTX(mgctx);
>  	struct task_struct *task;
> -	int ret;
> -
> -	ret = cgroup_migrate_vet_dst(dst_cgrp);
> -	if (ret)
> -		return ret;
> +	int ret = 0;
>  
>  	/* look up all src csets */
>  	spin_lock_irq(&css_set_lock);
> @@ -4690,6 +4686,33 @@ static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
>  	return 0;
>  }
>  
> +static inline bool cgroup_same_domain(const struct cgroup *src_cgrp,
> +				      const struct cgroup *dst_cgrp)
> +{
> +	return src_cgrp->dom_cgrp == dst_cgrp->dom_cgrp;
> +}
> +
> +static int cgroup_attach_permissions(struct cgroup *src_cgrp,
> +				     struct cgroup *dst_cgrp,
> +				     struct super_block *sb, bool thread)
> +{
> +	int ret;
> +
> +	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb);
> +	if (ret)
> +		return ret;
> +
> +	ret = cgroup_migrate_vet_dst(dst_cgrp);
> +	if (ret)
> +		return ret;
> +
> +	if (thread &&
> +	    !cgroup_same_domain(src_cgrp->dom_cgrp, dst_cgrp->dom_cgrp))
> +		ret = -EOPNOTSUPP;
> +
> +	return 0;
> +}
> +
>  static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
>  				  char *buf, size_t nbytes, loff_t off)
>  {
> @@ -4712,8 +4735,8 @@ static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
>  	src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
>  	spin_unlock_irq(&css_set_lock);
>  
> -	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
> -					    of->file->f_path.dentry->d_sb);
> +	ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
> +					of->file->f_path.dentry->d_sb, true);

typo: s/true/false/