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Date:   Wed, 27 May 2020 21:54:23 +0200
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     sdf@...gle.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 Wed, May 27, 2020 at 07:48 PM CEST, sdf@...gle.com wrote:
> 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?

Yes, that's true. __netns_bpf_prog_detach does

	RCU_INIT_POINTER(net->bpf.progs[type], NULL);

RCU read lock is here for the rcu_dereference() that happens in
bpf_netns_link_auto_detach (netns doesn't hold a ref to bpf_link):

/* Called with RCU read lock. */
static void __net_exit
bpf_netns_link_auto_detach(struct net *net, enum netns_bpf_attach_type type)
{
	struct bpf_netns_link *net_link;
	struct bpf_link *link;

	link = rcu_dereference(net->bpf.links[type]);
	if (!link)
		return;
	net_link = to_bpf_netns_link(link);
	RCU_INIT_POINTER(net_link->net, NULL);
}

I've pulled it up, out of the loop, perhaps too eagerly and just made it
confusing, considering we're iterating over a 1-item array :-)

Now, I'm also doing RCU_INIT_POINTER on the *contents of bpf_link* in
bpf_netns_link_auto_detach. Is that allowed? I'm not sure, that bit is
hazy to me.

There are no concurrent writers to net_link->net, just readers, i.e.
bpf_netns_link_release(). And I know bpf_link won't be freed until the
grace period elapses.

sparse and CONFIG_PROVE_RCU are not shouting at me, but please do if it
doesn't hold up or make sense.

I certainly can push the rcu_read_lock() down into
bpf_netns_link_auto_detach().

-jkbs

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