linux-kernel - Re: [RFC] [PATCH bpf-next 1/1] bpf: Add a BPF helper for getting the cgroup path of current task

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Date:   Wed, 12 May 2021 15:55:04 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Xufeng Zhang <yunbo.xufeng@...ux.alibaba.com>
Cc:     kpsingh@...nel.org, ast@...nel.org, daniel@...earbox.net,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, revest@...omium.org,
        jackmanb@...omium.org, yhs@...com, songliubraving@...com,
        kafai@...com, john.fastabend@...il.com, joe@...ium.io,
        quentin@...valent.com
Subject: Re: [RFC] [PATCH bpf-next 1/1] bpf: Add a BPF helper for getting the
 cgroup path of current task

On Wed, May 12, 2021 at 05:58:23PM +0800, Xufeng Zhang wrote:
> To implement security rules for application containers by utilizing
> bpf LSM, the container to which the current running task belongs need
> to be known in bpf context. Think about this scenario: kubernetes
> schedules a pod into one host, before the application container can run,
> the security rules for this application need to be loaded into bpf
> maps firstly, so that LSM bpf programs can make decisions based on
> this rule maps.
> 
> However, there is no effective bpf helper to achieve this goal,
> especially for cgroup v1. In the above case, the only available information
> from user side is container-id, and the cgroup path for this container
> is certain based on container-id, so in order to make a bridge between
> user side and bpf programs, bpf programs also need to know the current
> cgroup path of running task.
...
> +#ifdef CONFIG_CGROUPS
> +BPF_CALL_2(bpf_get_current_cpuset_cgroup_path, char *, buf, u32, buf_len)
> +{
> +	struct cgroup_subsys_state *css;
> +	int retval;
> +
> +	css = task_get_css(current, cpuset_cgrp_id);
> +	retval = cgroup_path_ns(css->cgroup, buf, buf_len, &init_cgroup_ns);
> +	css_put(css);
> +	if (retval >= buf_len)
> +		retval = -ENAMETOOLONG;

Manipulating string path to check the hierarchy will be difficult to do
inside bpf prog. It seems to me this helper will be useful only for
simplest cgroup setups where there is no additional cgroup nesting
within containers.
Have you looked at *ancestor_cgroup_id and *cgroup_id helpers?
They're a bit more flexible when dealing with hierarchy and
can be used to achieve the same correlation between kernel and user cgroup ids.