netdev - Re: [PATCH bpf-next 5/8] bpf: Add link-based BPF program attachment to network namespace

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Date:   Wed, 27 May 2020 10:48:05 -0700
From:   sdf@...gle.com
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [PATCH bpf-next 5/8] bpf: Add link-based BPF program attachment
 to network namespace

On 05/27, Jakub Sitnicki wrote:
> Add support for bpf() syscall subcommands that operate on
> bpf_link (LINK_CREATE, LINK_UPDATE, OBJ_GET_INFO) for attach points tied  
> to
> network namespaces (that is flow dissector at the moment).

> Link-based and prog-based attachment can be used interchangeably, but only
> one can be in use at a time. Attempts to attach a link when a prog is
> already attached directly, and the other way around, will be met with
> -EBUSY.

> Attachment of multiple links of same attach type to one netns is not
> supported, with the intention to lift it when a use-case presents
> itself. Because of that attempts to create a netns link, when one already
> exists result in -E2BIG error, signifying that there is no space left for
> another attachment.

> Link-based attachments to netns don't keep a netns alive by holding a ref
> to it. Instead links get auto-detached from netns when the latter is being
> destroyed by a pernet pre_exit callback.

> When auto-detached, link lives in defunct state as long there are open FDs
> for it. -ENOLINK is returned if a user tries to update a defunct link.

> Because bpf_link to netns doesn't hold a ref to struct net, special care  
> is
> taken when releasing the link. The netns might be getting torn down when
> the release function tries to access it to detach the link.

> To ensure the struct net object is alive when release function accesses it
> we rely on the fact that cleanup_net(), struct net destructor, calls
> synchronize_rcu() after invoking pre_exit callbacks. If auto-detach from
> pre_exit happens first, link release will not attempt to access struct  
> net.

> Same applies the other way around, network namespace doesn't keep an
> attached link alive because by not holding a ref to it. Instead bpf_links
> to netns are RCU-freed, so that pernet pre_exit callback can safely access
> and auto-detach the link when racing with link release/free.

[..]
> +	rcu_read_lock();
>   	for (type = 0; type < MAX_NETNS_BPF_ATTACH_TYPE; type++) {
> -		if (rcu_access_pointer(net->bpf.progs[type]))
> +		if (rcu_access_pointer(net->bpf.links[type]))
> +			bpf_netns_link_auto_detach(net, type);
> +		else if (rcu_access_pointer(net->bpf.progs[type]))
>   			__netns_bpf_prog_detach(net, type);
>   	}
> +	rcu_read_unlock();
Aren't you doing RCU_INIT_POINTER in __netns_bpf_prog_detach?
Is it allowed under rcu_read_load?