netdev - Re: [PATCH v2 net-next 1/8] bpf: Add support for recursively running cgroup sock filters

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Date:   Thu, 31 Aug 2017 07:22:01 -0700
From:   Tejun Heo <tj@...nel.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        netdev@...r.kernel.org, daniel@...earbox.net, ast@...nel.org,
        davem@...emloft.net
Subject: Re: [PATCH v2 net-next 1/8] bpf: Add support for recursively running
 cgroup sock filters

Hello, David, Alexei.

Sorry about late reply.

On Sun, Aug 27, 2017 at 08:49:23AM -0600, David Ahern wrote:
> On 8/25/17 8:49 PM, Alexei Starovoitov wrote:
> > 
> >> +	if (prog && curr_recursive && !new_recursive)
> >> +		/* if a parent has recursive prog attached, only
> >> +		 * allow recursive programs in descendent cgroup
> >> +		 */
> >> +		return -EINVAL;
> >> +
> >>  	old_prog = cgrp->bpf.prog[type];
> > 
> > ... I'm struggling to completely understand how it interacts
> > with BPF_F_ALLOW_OVERRIDE.
> 
> The 2 flags are completely independent. The existing override logic is
> unchanged. If a program can not be overridden, then the new recursive
> flag is irrelevant.

I'm not sure all four combo of the two flags makes sense.  Can't we
have something simpler like the following?

1. None: No further bpf programs allowed in the subtree.

2. Overridable: If a sub-cgroup installs the same bpf program, this
   one yields to that one.

3. Recursive: If a sub-cgroup installs the same bpf program, that
   cgroup program gets run in addition to this one.

Note that we can have combinations of overridables and recursives -
both allow further programs in the sub-hierarchy and the only
distinction is whether that specific program behaves when that
happens.

Thanks.

-- 
tejun