linux-kernel - Re: [PATCH] net: handle the return value of pskb_carve_frag

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Date:   Mon, 17 Aug 2020 11:58:05 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     David Miller <davem@...emloft.net>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        "martin.varghese@...ia.com" <martin.varghese@...ia.com>,
        "fw@...len.de" <fw@...len.de>, "pshelar@....org" <pshelar@....org>,
        "dcaratti@...hat.com" <dcaratti@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "steffen.klassert@...unet.com" <steffen.klassert@...unet.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "shmulik@...anetworks.com" <shmulik@...anetworks.com>,
        "kyk.segfault@...il.com" <kyk.segfault@...il.com>,
        "sowmini.varadhan@...cle.com" <sowmini.varadhan@...cle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: handle the return value of pskb_carve_frag_list()
 correctly

David Miller <davem@...emloft.net> wrote:
>> David Miller <davem@...emloft.net> wrote:
>>>> +	/* split line is in frag list */
>>>> +	if (k == 0 && pskb_carve_frag_list(skb, shinfo, off - pos, gfp_mask)) {
>>>> +		/* skb_frag_unref() is not needed here as shinfo->nr_frags = 0. */
>>>> +		if (skb_has_frag_list(skb))
>>>> +			kfree_skb_list(skb_shinfo(skb)->frag_list);
>>>> +		kfree(data);
>>>> +		return -ENOMEM;
>>>
>>>On error, the caller is going to kfree_skb(skb) which will take care of the frag list.
>>>
>> 
>> I'am sorry for my careless. The caller will take care of the frag list and kfree(data) is enough here.
>> Many thanks for review, will send v2 soon.
>
>Actually, reading this again, what about the skb_clone_fraglist() done a few lines up?  Who will release that reference to the fraglist items?
>
>Maybe the kfree_skb_list() is necessary after all?

Yep, it looks really confusing here. On error, the caller calls kfree_skb(skb) but only atomic_sub the skb_shared_info->dataref indeed because skb is cloned
here and it shares the fraglist with origin skbuff. But the skb_clone_fraglist() done a few lines up hold the extra reference to the fraglist for coming new skb->data.
As there is no new skb->data anymore, that reference to the fraglist items won't be release unless we take care of it here.

It seems this patch exactly do the right things already. :)