Re: [PATCH v2 net] bpf: introduce BPF_F_ALLOW_OVERRIDE flag

[Tejun Heo <tj@kernel.org>]

On Fri, Feb 10, 2017 at 08:28:24PM -0800, Alexei Starovoitov wrote:
> If BPF_F_ALLOW_OVERRIDE flag is used in BPF_PROG_ATTACH command
> to the given cgroup the descendent cgroup will be able to override
> effective bpf program that was inherited from this cgroup.
> By default it's not passed, therefore override is disallowed.
> 
> Examples:
> 1.
> prog X attached to /A with default
> prog Y fails to attach to /A/B and /A/B/C
> Everything under /A runs prog X
> 
> 2.
> prog X attached to /A with allow_override.
> prog Y fails to attach to /A/B with default (non-override)
> prog M attached to /A/B with allow_override.
> Everything under /A/B runs prog M only.
> 
> 3.
> prog X attached to /A with allow_override.
> prog Y fails to attach to /A with default.
> The user has to detach first to switch the mode.
> 
> In the future this behavior may be extended with a chain of
> non-overridable programs.
> 
> Also fix the bug where detach from cgroup where nothing is attached
> was not throwing error. Return ENOENT in such case.
> 
> Add several testcases and adjust libbpf.
> 
> Fixes: 3007098494be ("cgroup: add support for eBPF programs")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

The cgroup part looks good to me.  Please feel free to add

Acked-by: Tejun Heo <tj@kernel.org>

One question tho.  Is there a specific reason to disallow attaching
!overridable program under an overridable one?  Isn't disallowing
attaching programs if the closest ancestor is !overridable enough?

Thanks.

-- 
tejun