Commit d9054a1f authored Dec 04, 2020 by Dongdong Wang Committed by Alexei Starovoitov Dec 07, 2020

lwt: Disable BH too in run_lwt_bpf()

The per-cpu bpf_redirect_info is shared among all skb_do_redirect()
and BPF redirect helpers. Callers on RX path are all in BH context,
disabling preemption is not sufficient to prevent BH interruption.

In production, we observed strange packet drops because of the race
condition between LWT xmit and TC ingress, and we verified this issue
is fixed after we disable BH.

Although this bug was technically introduced from the beginning, that
is commit 3a0af8fd ("bpf: BPF for lightweight tunnel infrastructure"),
at that time call_rcu() had to be call_rcu_bh() to match the RCU context.
So this patch may not work well before RCU flavor consolidation has been
completed around v5.0.

Update the comments above the code too, as call_rcu() is now BH friendly.

Signed-off-by: Dongdong Wang <wangdongdong.6@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Link: https://lore.kernel.org/bpf/20201205075946.497763-1-xiyou.wangcong@gmail.com

parent 12c8a8ca

net/core/lwt_bpf.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -39,12 +39,11 @@ static int run_lwt_bpf(struct sk_buff skb, struct bpf_lwt_prog lwt,
		{
		int ret;

		/* Preempt disable is needed to protect per-cpu redirect_info between
		* BPF prog and skb_do_redirect(). The call_rcu in bpf_prog_put() and
		* access to maps strictly require a rcu_read_lock() for protection,
		* mixing with BH RCU lock doesn't work.
		/* Preempt disable and BH disable are needed to protect per-cpu
		* redirect_info between BPF prog and skb_do_redirect().
		*/
		preempt_disable();
		local_bh_disable();
		bpf_compute_data_pointers(skb);
		ret = bpf_prog_run_save_cb(lwt->prog, skb);

		@@ -78,6 +77,7 @@ static int run_lwt_bpf(struct sk_buff skb, struct bpf_lwt_prog lwt,
		break;
		}

		local_bh_enable();
		preempt_enable();

		return ret;

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