netdev - Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG

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Message-ID: <20160817142047.GB4021@htj.duckdns.org>
Date:	Wed, 17 Aug 2016 10:20:48 -0400
From:	Tejun Heo <htejun@...com>
To:	Daniel Mack <daniel@...que.org>
CC:	<daniel@...earbox.net>, <ast@...com>, <davem@...emloft.net>,
	<kafai@...com>, <fw@...len.de>, <pablo@...filter.org>,
	<harald@...hat.com>, <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
 commands

Hello, Daniel.

On Wed, Aug 17, 2016 at 04:00:46PM +0200, Daniel Mack wrote:
> The current implementation walks the tree from the passed cgroup up
> to the root. If there is any program of the given type installed in
> any of the ancestors, the installation is rejected. This is because
> programs subject to restrictions should have no way of escaping if
> a higher-level cgroup has installed a program already. This restriction
> can be revisited at some later point in time.

This seems a bit too restrictive to me.  Given that an entity which
can install a bpf program can remove any programs anyway, wouldn't it
make more sense to just execute the program on the closest ancestor to
the cgroup?  That'd allow selectively overriding programs for specific
subhierarchies while applying generic policies to the rest.

> +static int bpf_prog_attach(const union bpf_attr *attr)
> +{
...
> +		/* Reject installation of a program if any ancestor has one. */
> +		for (pos = cgrp->self.parent; pos; pos = pos->parent) {
> +			struct cgroup *parent;
> +
> +			css_get(pos);

This refcnting pattern doesn't make sense.  If @pos is already
accessible (which it is in this case), getting an extra ref before
accessing it doesn't do anything.  This would necessary iff the
pointer is to be used outside the scope which is protected by the ref
on @cgrp.

> +			parent = container_of(pos, struct cgroup, self);
> +
> +			if ((is_ingress  && parent->bpf_ingress) ||
> +			    (!is_ingress && parent->bpf_egress))
> +				err = -EEXIST;
> +
> +			css_put(pos);
> +		}
> +
> +		if (err < 0) {
> +			bpf_prog_put(prog);
> +			return err;
> +		}
> +
> +		progp = is_ingress ? &cgrp->bpf_ingress : &cgrp->bpf_egress;
> +
> +		rcu_read_lock();
> +		old_prog = rcu_dereference(*progp);
> +		rcu_assign_pointer(*progp, prog);

Wouldn't it make sense to update the descendants to point to the
programs directly so that there's no need to traverse the tree on each
packet?  It's more state to maintain but I think it would make total
sense given that it would reduce per-packet cost.

Otherwise, looks good to me.

Thanks.

-- 
tejun