linux-kernel - Re: [PATCH bpf-next v6 2/5] netfilter: bpf: Support BPF_F_NETFILTER_IP

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Date:   Thu, 27 Jul 2023 18:16:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     daniel@...earbox.net, kadlec@...filter.org, edumazet@...gle.com,
        ast@...nel.org, fw@...len.de, kuba@...nel.org, pabeni@...hat.com,
        pablo@...filter.org, andrii@...nel.org, davem@...emloft.net,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, netdev@...r.kernel.org, dsahern@...nel.org
Subject: Re: [PATCH bpf-next v6 2/5] netfilter: bpf: Support
 BPF_F_NETFILTER_IP_DEFRAG in netfilter link

On Fri, Jul 21, 2023 at 02:22:46PM -0600, Daniel Xu wrote:
> This commit adds support for enabling IP defrag using pre-existing
> netfilter defrag support. Basically all the flag does is bump a refcnt
> while the link the active. Checks are also added to ensure the prog
> requesting defrag support is run _after_ netfilter defrag hooks.
> 
> We also take care to avoid any issues w.r.t. module unloading -- while
> defrag is active on a link, the module is prevented from unloading.
> 
> Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> ---
>  include/uapi/linux/bpf.h       |   5 ++
>  net/netfilter/nf_bpf_link.c    | 123 +++++++++++++++++++++++++++++----
>  tools/include/uapi/linux/bpf.h |   5 ++
>  3 files changed, 118 insertions(+), 15 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 739c15906a65..12a5480314a2 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1187,6 +1187,11 @@ enum bpf_perf_event_type {
>   */
>  #define BPF_F_KPROBE_MULTI_RETURN	(1U << 0)
>  
> +/* link_create.netfilter.flags used in LINK_CREATE command for
> + * BPF_PROG_TYPE_NETFILTER to enable IP packet defragmentation.
> + */
> +#define BPF_F_NETFILTER_IP_DEFRAG (1U << 0)
> +
>  /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
>   * the following extensions:
>   *
> diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
> index c36da56d756f..8fe594bbc7e2 100644
> --- a/net/netfilter/nf_bpf_link.c
> +++ b/net/netfilter/nf_bpf_link.c
> @@ -1,6 +1,8 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/bpf.h>
>  #include <linux/filter.h>
> +#include <linux/kmod.h>
> +#include <linux/module.h>
>  #include <linux/netfilter.h>
>  
>  #include <net/netfilter/nf_bpf_link.h>
> @@ -23,8 +25,88 @@ struct bpf_nf_link {
>  	struct nf_hook_ops hook_ops;
>  	struct net *net;
>  	u32 dead;
> +	const struct nf_defrag_hook *defrag_hook;
>  };
>  
> +static const struct nf_defrag_hook *
> +get_proto_defrag_hook(struct bpf_nf_link *link,
> +		      const struct nf_defrag_hook __rcu *global_hook,
> +		      const char *mod)
> +{
> +	const struct nf_defrag_hook *hook;
> +	int err;
> +
> +	/* RCU protects us from races against module unloading */
> +	rcu_read_lock();
> +	hook = rcu_dereference(global_hook);
> +	if (!hook) {
> +		rcu_read_unlock();
> +		err = request_module(mod);
> +		if (err)
> +			return ERR_PTR(err < 0 ? err : -EINVAL);
> +
> +		rcu_read_lock();
> +		hook = rcu_dereference(global_hook);
> +	}
> +
> +	if (hook && try_module_get(hook->owner)) {
> +		/* Once we have a refcnt on the module, we no longer need RCU */
> +		hook = rcu_pointer_handoff(hook);
> +	} else {
> +		WARN_ONCE(!hook, "%s has bad registration", mod);
> +		hook = ERR_PTR(-ENOENT);
> +	}
> +	rcu_read_unlock();
> +
> +	if (!IS_ERR(hook)) {
> +		err = hook->enable(link->net);
> +		if (err) {
> +			module_put(hook->owner);
> +			hook = ERR_PTR(err);
> +		}
> +	}
> +
> +	return hook;

The rcu + module_get logic looks correct to me, but you've dropped all Florian's acks.
What's going on?

We need explicit acks to merge this through bpf-next.