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

From: Daniel Borkmann <[email-masked]>
Date: Tue, 30 Aug 2016 00:42:10 +0200

On 08/26/2016 09:58 PM, 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 <[email-masked]>
[...]
> +void __cgroup_bpf_update(struct cgroup *cgrp,
> +			 struct cgroup *parent,
> +			 struct bpf_prog *prog,
> +			 enum bpf_attach_type type)
> +{
> +	struct bpf_prog *old_prog, *effective;
> +	struct cgroup_subsys_state *pos;
> +
> +	old_prog = xchg(cgrp->bpf.prog + type, prog);
> +
> +	if (prog)
> +		static_branch_inc(&cgroup_bpf_enabled_key);
> +
> +	if (old_prog) {
> +		bpf_prog_put(old_prog);
> +		static_branch_dec(&cgroup_bpf_enabled_key);
> +	}
> +
> +	effective = (!prog && parent) ?
> +		rcu_dereference_protected(parent->bpf.effective[type],
> +					  lockdep_is_held(&cgroup_mutex)) :
> +		prog;
> +
> +	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)
> +			pos = css_rightmost_descendant(pos);
> +		else
> +			rcu_assign_pointer(desc->bpf.effective[type],
> +					   effective);
> +	}

Shouldn't the old_prog reference only be released right here at the end
instead of above (otherwise this could race)?

> +}