netdev - Re: [PATCH v3 2/6] cgroup: add support for eBPF programs

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Date:   Fri, 26 Aug 2016 17:03:27 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Mack <daniel@...que.org>
Cc:     htejun@...com, daniel@...earbox.net, ast@...com,
        davem@...emloft.net, kafai@...com, fw@...len.de,
        pablo@...filter.org, harald@...hat.com, netdev@...r.kernel.org,
        sargun@...gun.me
Subject: Re: [PATCH v3 2/6] cgroup: add support for eBPF programs

On Fri, Aug 26, 2016 at 09:58:48PM +0200, Daniel Mack wrote:
> This patch adds two sets of eBPF program pointers to struct cgroup.
> One for such that are directly pinned to a cgroup, and one for such
> that are effective for it.
> 
> To illustrate the logic behind that, assume the following example
> cgroup hierarchy.
> 
>   A - B - C
>         \ D - E
> 
> If only B has a program attached, it will be effective for B, C, D
> and E. If D then attaches a program itself, that will be effective for
> both D and E, and the program in B will only affect B and C. Only one
> program of a given type is effective for a cgroup.
> 
> Attaching and detaching programs will be done through the bpf(2)
> syscall. For now, ingress and egress inet socket filtering are the
> only supported use-cases.
> 
> Signed-off-by: Daniel Mack <daniel@...que.org>
...
> +	css_for_each_descendant_pre(pos, &cgrp->self) {
> +		struct cgroup *desc = container_of(pos, struct cgroup, self);
> +
> +		/* skip the subtree if the descendant has its own program */
> +		if (desc->bpf.prog[type] && desc != cgrp)

is desc != cgrp really needed?
I thought css_for_each_descendant_pre() shouldn't walk itself
or I'm missing how it works.

> +			pos = css_rightmost_descendant(pos);
> +		else
> +			rcu_assign_pointer(desc->bpf.effective[type],
> +					   effective);
> +	}
> +}
> +