Received-SPF: pass (google.com: domain of linux-kernel+bounces-3189-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Subject: Re: [PATCH net-next 16/24] net: netkit, veth, tun, virt*: Use
 nested-BH locking for XDP redirect.
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
 linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>, Boqun Feng
 <boqun.feng@gmail.com>, Eric Dumazet <edumazet@google.com>,
 Frederic Weisbecker <frederic@kernel.org>, Ingo Molnar <mingo@redhat.com>,
 Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
 Peter Zijlstra <peterz@infradead.org>, Thomas Gleixner <tglx@linutronix.de>,
 Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>,
 "K. Y. Srinivasan" <kys@microsoft.com>, "Michael S. Tsirkin"
 <mst@redhat.com>, Alexei Starovoitov <ast@kernel.org>,
 Andrii Nakryiko <andrii@kernel.org>, Dexuan Cui <decui@microsoft.com>,
 Haiyang Zhang <haiyangz@microsoft.com>, Hao Luo <haoluo@google.com>,
 Jesper Dangaard Brouer <hawk@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
 John Fastabend <john.fastabend@gmail.com>, Juergen Gross <jgross@suse.com>,
 KP Singh <kpsingh@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>,
 Nikolay Aleksandrov <razor@blackwall.org>, Song Liu <song@kernel.org>,
 Stanislav Fomichev <sdf@google.com>,
 Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wei.liu@kernel.org>,
 Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
 Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
 Yonghong Song <yonghong.song@linux.dev>, bpf@vger.kernel.org,
 virtualization@lists.linux.dev, xen-devel@lists.xenproject.org
References: <20231215171020.687342-1-bigeasy@linutronix.de>
 <20231215171020.687342-17-bigeasy@linutronix.de>
From: Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <74feb818-7109-cb1e-8eec-a037c17a2871@iogearbox.net>
Date: Mon, 18 Dec 2023 09:52:05 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.7.2
Precedence: bulk
MIME-Version: 1.0
In-Reply-To: <20231215171020.687342-17-bigeasy@linutronix.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit

Hi Sebastian,

On 12/15/23 6:07 PM, Sebastian Andrzej Siewior wrote:
> The per-CPU variables used during bpf_prog_run_xdp() invocation and
> later during xdp_do_redirect() rely on disabled BH for their protection.
> Without locking in local_bh_disable() on PREEMPT_RT these data structure
> require explicit locking.
> 
> This is a follow-up on the previous change which introduced
> bpf_run_lock.redirect_lock and uses it now within drivers.
> 
> The simple way is to acquire the lock before bpf_prog_run_xdp() is
> invoked and hold it until the end of function.
> This does not always work because some drivers (cpsw, atlantic) invoke
> xdp_do_flush() in the same context.
> Acquiring the lock in bpf_prog_run_xdp() and dropping in
> xdp_do_redirect() (without touching drivers) does not work because not
> all driver, which use bpf_prog_run_xdp(), do support XDP_REDIRECT (and
> invoke xdp_do_redirect()).
> 
> Ideally the minimal locking scope would be bpf_prog_run_xdp() +
> xdp_do_redirect() and everything else (error recovery, DMA unmapping,
> free/ alloc of memory, …) would happen outside of the locked section.
[...]

>   drivers/net/hyperv/netvsc_bpf.c |  1 +
>   drivers/net/netkit.c            | 13 +++++++----
>   drivers/net/tun.c               | 28 +++++++++++++----------
>   drivers/net/veth.c              | 40 ++++++++++++++++++++-------------
>   drivers/net/virtio_net.c        |  1 +
>   drivers/net/xen-netfront.c      |  1 +
>   6 files changed, 52 insertions(+), 32 deletions(-)
[...]

Please exclude netkit from this set given it does not support XDP, but
instead only accepts tc BPF typed programs.

Thanks,
Daniel

> diff --git a/drivers/net/netkit.c b/drivers/net/netkit.c
> index 39171380ccf29..fbcf78477bda8 100644
> --- a/drivers/net/netkit.c
> +++ b/drivers/net/netkit.c
> @@ -80,8 +80,15 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   	netkit_prep_forward(skb, !net_eq(dev_net(dev), dev_net(peer)));
>   	skb->dev = peer;
>   	entry = rcu_dereference(nk->active);
> -	if (entry)
> -		ret = netkit_run(entry, skb, ret);
> +	if (entry) {
> +		scoped_guard(local_lock_nested_bh, &bpf_run_lock.redirect_lock) {
> +			ret = netkit_run(entry, skb, ret);
> +			if (ret == NETKIT_REDIRECT) {
> +				dev_sw_netstats_tx_add(dev, 1, len);
> +				skb_do_redirect(skb);
> +			}
> +		}
> +	}
>   	switch (ret) {
>   	case NETKIT_NEXT:
>   	case NETKIT_PASS:
> @@ -95,8 +102,6 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   		}
>   		break;
>   	case NETKIT_REDIRECT:
> -		dev_sw_netstats_tx_add(dev, 1, len);
> -		skb_do_redirect(skb);
>   		break;
>   	case NETKIT_DROP:
>   	default: