Subject: Re: [PATCH net-next 1/2] bpf: don't enable preemption twice in smap_do_verdict

From: John Fastabend <[email-masked]>
Date: Thu, 17 Aug 2017 09:03:25 -0700

On 08/17/2017 08:22 AM, Daniel Borkmann wrote:
> In smap_do_verdict(), the fall-through branch leads to call
> preempt_enable() twice for the SK_REDIRECT, which creates an
> imbalance. Only enable it for all remaining cases again.
> 
> Fixes: 174a79ff9515 ("bpf: sockmap with sk redirect support")
> Reported-by: Alexei Starovoitov <[email-masked]>
> Signed-off-by: Daniel Borkmann <[email-masked]>
> ---
>  kernel/bpf/sockmap.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/sockmap.c b/kernel/bpf/sockmap.c
> index f7e5e6c..39de541 100644
> --- a/kernel/bpf/sockmap.c
> +++ b/kernel/bpf/sockmap.c
> @@ -135,7 +135,8 @@ static void smap_do_verdict(struct smap_psock *psock, struct sk_buff *skb)
>  	/* Fall through and free skb otherwise */
>  	case SK_DROP:
>  	default:
> -		preempt_enable();
> +		if (rc != SK_REDIRECT)
> +			preempt_enable();
>  		kfree_skb(skb);
>  	}
>  }
> 

Yep looks good, nice catch Alexei. Thanks! I'll add a selftests entry
in test_maps to catch this fall through case. Looks like we don't hit
this case during selftests at the moment.

Acked-by: John Fastabend <[email-masked]>