Re: [PATCH] netfilter: force access of RCU protected data in nft_update_chain_stats

[Eric Dumazet <eric.dumazet@gmail.com>]

On 02/24/2019 05:58 PM, Li RongQing wrote:
> basechain->stats is rcu protected data, cannot assure that
> twice accesses have the same result, so dereference it once.
> 
> basechain->stats is allocated by percpu allocater, if it is not NULL,
> its percpu variable does not need to be checked with NULL
> 
> Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> ---
>  net/netfilter/nf_tables_core.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/net/netfilter/nf_tables_core.c b/net/netfilter/nf_tables_core.c
> index 2a00aef7b6d4..9be622c76a62 100644
> --- a/net/netfilter/nf_tables_core.c
> +++ b/net/netfilter/nf_tables_core.c
> @@ -98,20 +98,20 @@ static noinline void nft_update_chain_stats(const struct nft_chain *chain,
>  					    const struct nft_pktinfo *pkt)
>  {
>  	struct nft_base_chain *base_chain;
> -	struct nft_stats *stats;
> +	struct nft_stats *stats, *pstat;
>  
>  	base_chain = nft_base_chain(chain);
> -	if (!rcu_access_pointer(base_chain->stats))
> +
> +	stats = rcu_dereference(base_chain->stats);

This looks bogus to me.

Where is the needed rcu_read_lock() before rcu_dereference() ?

This rcu_access_pointer() test is fine, and avoids a local_bh_disable()/local_bh_enable()
if they are not needed.


> +	if (!stats)
>  		return;
>  
>  	local_bh_disable();
> -	stats = this_cpu_ptr(rcu_dereference(base_chain->stats));
> -	if (stats) {
> -		u64_stats_update_begin(&stats->syncp);
> -		stats->pkts++;
> -		stats->bytes += pkt->skb->len;
> -		u64_stats_update_end(&stats->syncp);
> -	}
> +	pstat = this_cpu_ptr(stats);
> +	u64_stats_update_begin(&pstat->syncp);
> +	pstat->pkts++;
> +	pstat->bytes += pkt->skb->len;
> +	u64_stats_update_end(&pstat->syncp);
>  	local_bh_enable();
>  }
>  
>