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

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Date: Tue, 19 Mar 2024 23:03:14 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Jose Fernandez <josef@...flix.com>, Tejun Heo <tj@...nel.org>
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 10:04 PM Jose Fernandez <josef@...flix.com> 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>
> ---
> V2 -> V3: No changes
> V1 -> V2: Return a pointer to the cgroup instead of the cgroup ID
>
>  kernel/bpf/helpers.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index a89587859571..bbd19d5eedb6 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2266,6 +2266,31 @@ bpf_task_get_cgroup1(struct task_struct *task, int hierarchy_id)
>                 return NULL;
>         return cgrp;
>  }
> +
> +/**
> + * bpf_task_get_cgroup - Acquire a reference to the default cgroup of a task.
> + * @task: The target task
> + *
> + * This function returns the task's default cgroup, primarily
> + * designed for use with cgroup v2. In cgroup v1, the concept of default
> + * cgroup varies by subsystem, and while this function will work with
> + * cgroup v1, it's recommended to use bpf_task_get_cgroup1 instead.
> + * A cgroup returned by this kfunc which is not subsequently stored in a
> + * map, must be released by calling bpf_cgroup_release().
> + *
> + * Return: On success, the cgroup is returned. On failure, NULL is returned.
> + */
> +__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;
> +}

Tejun,

Does this patch look good to you?