linux-kernel - Re: [PATCH] Reduce number of pointer dereferences in IPv4 netfilter LOG module function dump

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Date:	Mon, 15 Nov 2010 01:49:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	Netfilter Core Team <coreteam@...filter.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	netfilter@...r.kernel.org, netfilter-devel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] Reduce number of pointer dereferences in IPv4
 netfilter LOG module function dump_packet()

Le dimanche 14 novembre 2010 à 22:35 +0100, Jesper Juhl a écrit :
> By adding two pointer variables to 
> net/ipv4/netfilter/ipt_LOG.c::dump_packet() we can save 16 bytes of .text 
> and 9 pointer dereferences.
> 
> before this patch we did 20 pointer dereferences and had this object file 
> size:
>    text    data     bss     dec     hex filename
>    6233     600    3080    9913    26b9 net/ipv4/netfilter/ipt_LOG.o
> 
> after this patch we do just 11 pointer dereferences and have this object 
> file size:
>    text    data     bss     dec     hex filename
>    6217     600    3080    9897    26a9 net/ipv4/netfilter/ipt_LOG.o
> 
> 
> Please Cc me on replies.
> 
> 
> Signed-off-by: Jesper Juhl <jj@...osbits.net>
> ---
>  ipt_LOG.c |   16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/net/ipv4/netfilter/ipt_LOG.c b/net/ipv4/netfilter/ipt_LOG.c
> index 72ffc8f..02a92de 100644
> --- a/net/ipv4/netfilter/ipt_LOG.c
> +++ b/net/ipv4/netfilter/ipt_LOG.c
> @@ -39,6 +39,8 @@ static void dump_packet(struct sbuff *m,
>  	struct iphdr _iph;
>  	const struct iphdr *ih;
>  	unsigned int logflags;
> +	struct sock *sk;
> +	struct socket *sk_socket;
>  
>  	if (info->type == NF_LOG_TYPE_LOG)
>  		logflags = info->u.log.logflags;
> @@ -335,13 +337,15 @@ static void dump_packet(struct sbuff *m,
>  	}
>  
>  	/* Max length: 15 "UID=4294967295 " */
> -	if ((logflags & IPT_LOG_UID) && !iphoff && skb->sk) {
> -		read_lock_bh(&skb->sk->sk_callback_lock);
> -		if (skb->sk->sk_socket && skb->sk->sk_socket->file)
> +	sk = skb->sk;
> +	sk_socket = sk->sk_socket;

Really ? sk can be NULL, so you add a NULL dereference.

> +	if ((logflags & IPT_LOG_UID) && !iphoff && sk) {
> +		read_lock_bh(&sk->sk_callback_lock);
> +		if (sk_socket && sk_socket->file)
>  			sb_add(m, "UID=%u GID=%u ",
> -				skb->sk->sk_socket->file->f_cred->fsuid,
> -				skb->sk->sk_socket->file->f_cred->fsgid);
> -		read_unlock_bh(&skb->sk->sk_callback_lock);
> +				sk_socket->file->f_cred->fsuid,
> +				sk_socket->file->f_cred->fsgid);
> +		read_unlock_bh(&sk->sk_callback_lock);
>  	}
>  
>  	/* Max length: 16 "MARK=0xFFFFFFFF " */
> 
> 
> 

Most of these "dereferences" are compiler optimized.

You added a bug in your patch, and make ipt_LOG slower if rule is not
asking for IPT_LOG_UID



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