linux-kernel - Re: [PATCH V3 bpf-next 1/2] bpf: add bpf_task_get

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Date: Wed, 20 Mar 2024 07:41:17 -0700
From: Tejun Heo <tj@...nel.org>
To: Jose Fernandez <josef@...flix.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, Tycho Andersen <tycho@...ho.pizza>
Subject: Re: [PATCH V3 bpf-next 1/2] bpf: add bpf_task_get_cgroup kfunc

On Mon, Mar 18, 2024 at 11:03:01PM -0600, Jose Fernandez wrote:
> This patch enhances the BPF helpers by adding a kfunc to retrieve the
> cgroup v2 of a task, addressing a previous limitation where only
> bpf_task_get_cgroup1 was available for cgroup v1. The new kfunc is
> particularly useful for scenarios where obtaining the cgroup ID of a
> task other than the "current" one is necessary, which the existing
> bpf_get_current_cgroup_id helper cannot accommodate. A specific use
> case at Netflix involved the sched_switch tracepoint, where we had to
> get the cgroup IDs of both the prev and next tasks.
> 
> The bpf_task_get_cgroup kfunc acquires and returns a reference to a
> task's default cgroup, ensuring thread-safe access by correctly
> implementing RCU read locking and unlocking. It leverages the existing
> cgroup.h helper, and cgroup_tryget to safely acquire a reference to it.
> 
> Signed-off-by: Jose Fernandez <josef@...flix.com>
> Reviewed-by: Tycho Andersen <tycho@...ho.pizza>
> Acked-by: Yonghong Song <yonghong.song@...ux.dev>
> Acked-by: Stanislav Fomichev <sdf@...gle.com>

Acked-by: Tejun Heo <tj@...nel.org>

but some questions below

> +__bpf_kfunc struct cgroup *bpf_task_get_cgroup(struct task_struct *task)
> +{
> +	struct cgroup *cgrp;
> +
> +	rcu_read_lock();
> +	cgrp = task_dfl_cgroup(task);
> +	if (!cgroup_tryget(cgrp))
> +		cgrp = NULL;
> +	rcu_read_unlock();
> +	return cgrp;
> +}

So, as this is a lot easier in cgroup2, the above can probably be written
directly in BPF (untested and not sure the necessary annotations are in
place, so please take it with a big grain of salt):

	bpf_rcu_read_lock();
	cgrp = task->cgroups->dfl_cgrp;
	cgrp = bpf_cgroup_from_id(cgrp->kn.id);
	bpf_rcu_read_unlock();

If all you need is ID, it's even simpler:

	bpf_rcu_read_lock();
	cgrp_id = task->cgroups->dfl_cgrp->kn.id;
	bpf_rcu_read_unlock();

In the first example, it's not great that we go from task pointer to cgroup
pointer to ID and then back to acquired cgroup pointer. I wonder whether
what we really want is to support something like the following:

	bpf_rcu_read_lock();
	cgrp = bpf_cgroup_tryget(task->cgroups->dfl_cgrp);
	bpf_rcu_read_unlock();

What do you think?

Thanks.

-- 
tejun