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Date:   Mon, 10 Aug 2020 08:06:11 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, davem@...emloft.net,
        kuba@...nel.org, pshelar@....org, martin.varghese@...ia.com,
        fw@...len.de, dcaratti@...hat.com, edumazet@...gle.com,
        steffen.klassert@...unet.com, pabeni@...hat.com,
        shmulik@...anetworks.com, kyk.segfault@...il.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: eliminate meaningless memcpy to data in
 pskb_carve_inside_nonlinear()



On 8/10/20 5:28 AM, Miaohe Lin wrote:
> The skb_shared_info part of the data is assigned in the following loop. It
> is meaningless to do a memcpy here.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  net/core/skbuff.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 7e2e502ef519..5b983c9472f5 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5952,9 +5952,6 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
>  
>  	size = SKB_WITH_OVERHEAD(ksize(data));
>  
> -	memcpy((struct skb_shared_info *)(data + size),
> -	       skb_shinfo(skb), offsetof(struct skb_shared_info,
> -					 frags[skb_shinfo(skb)->nr_frags]));
>  	if (skb_orphan_frags(skb, gfp_mask)) {
>  		kfree(data);
>  		return -ENOMEM;
> 

Reminder : net-next is CLOSED.

This is not correct. We still have to copy _something_

Something like :

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 2828f6d5ba898a5e50ccce45589bf1370e474b0f..1c0519426c7ba4b04377fc8054c4223c135879ab 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5953,8 +5953,8 @@ static int pskb_carve_inside_nonlinear(struct sk_buff *skb, const u32 off,
        size = SKB_WITH_OVERHEAD(ksize(data));
 
        memcpy((struct skb_shared_info *)(data + size),
-              skb_shinfo(skb), offsetof(struct skb_shared_info,
-                                        frags[skb_shinfo(skb)->nr_frags]));
+              skb_shinfo(skb), offsetof(struct skb_shared_info, frags[0]));
+
        if (skb_orphan_frags(skb, gfp_mask)) {
                kfree(data);
                return -ENOMEM;

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