netdev - Re: [PATCH bpf-next 1/5] bpf: add support for forced LINK

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Date:   Thu, 30 Jul 2020 12:03:59 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next 1/5] bpf: add support for forced LINK_DETACH command

On Thu, Jul 30, 2020 at 10:43 AM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Jul 29, 2020, at 4:05 PM, Andrii Nakryiko <andriin@...com> wrote:
> >
> > Add LINK_DETACH command to force-detach bpf_link without destroying it. It has
> > the same behavior as auto-detaching of bpf_link due to cgroup dying for
> > bpf_cgroup_link or net_device being destroyed for bpf_xdp_link. In such case,
> > bpf_link is still a valid kernel object, but is defuncts and doesn't hold BPF
> > program attached to corresponding BPF hook. This functionality allows users
> > with enough access rights to manually force-detach attached bpf_link without
> > killing respective owner process.
> >
> > This patch implements LINK_DETACH for cgroup, xdp, and netns links, mostly
> > re-using existing link release handling code.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
>
> The code looks good to me. My only question is, do we need both
> bpf_link_ops->detach and bpf_link_ops->release?

I think so. release() is mandatory for final clean up, after the last
FD was closed, so every type of bpf_link has to implement this.
detach() is optional, though, and potentially can do different things
than release(). It just so happens right now that three bpf_linkl
types can re-use release as-is (with minimal change to netns release
specifically for detach use case). So I think having two is better and
more flexible.

>
> Thanks,
> Song
>
> [...]