netdev - Re: [PATCH bpf-next 11/13] bpf: libbpf: Add STRUCT

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Message-ID: <20191218201933.2ixhiz3n6zw75eg7@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 18 Dec 2019 20:19:37 +0000
From:   Martin Lau <kafai@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        "Kernel Team" <Kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 11/13] bpf: libbpf: Add STRUCT_OPS support

On Wed, Dec 18, 2019 at 10:14:04AM -0800, Andrii Nakryiko wrote:
[ ... ]
> 
> Where we do have problem is with bpf_link__destroy() unconditionally
> also detaching whatever was attached (tracepoint, kprobe, or whatever
> was done to create bpf_link in the first place). Now,
> bpf_link__destroy() has to be called by user (or skeleton) to at least
> free up malloc()'ed structs. But it appears that it's not always
> desirable that upon bpf_link destruction underlying BPF program gets
> detached. I think this will be the case for xdp and others as well.
> 
> I think the good and generic way to go about this is to have this as a
> general concept of destroying the link without detaching BPF programs.
> E.g., what if we have new API call `void bpf_link__unlink()`, which
> will mark that link as not requiring to detach underlying BPF program.
> When bpf_link__destroy() is called later, it will just free resources
> allocated to maintain bpf_link itself, but won't detach any BPF
> programs/resources.
> 
> With this, user will have to explicitly specify that he doesn't want
> to detach even when skeleton/link is destroyed. If we get consensus on
> this, I can add support for this to all the existing bpf_links and you
> can build on that?
Keeping the current struct_ops unreg mechanism (i.e.
bpf_struct_ops__unregister(), to be renamed) and
having a way to opt-out sounds good to me.  Thanks.