linux-kernel - Re: [PATCH] netfilter: release skbuf when nlmsg put fail

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Message-ID: <20141013114256.GB6560@breakpoint.cc>
Date:	Mon, 13 Oct 2014 13:42:56 +0200
From:	Florian Westphal <fw@...len.de>
To:	Houcheng Lin <houcheng@...il.com>
Cc:	pablo@...filter.org, kaber@...sh.net, kadlec@...ckhole.kfki.hu,
	davem@...emloft.net, netfilter-devel@...r.kernel.org,
	coreteam@...filter.org, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netfilter: release skbuf when nlmsg put fail

Houcheng Lin <houcheng@...il.com> wrote:
> When system is under heavy loading, the __nfulnl_send() may may failed
> to put nlmsg into skbuf of nfulnl_instance. If not clear the skbuff on failed,
> the __nfulnl_send() will still try to put next nlmsg onto this half-full skbuf
> and cause the user program can never receive packet.
> 
> This patch fix this issue by releasing skbuf immediately after nlmst put
> failed.

Did you observe such problem or is this based on code reading?
I ask because nflog should make sure we always have enough room left in
skb to append a done message, see nfulnl_log_packet():

if (inst->skb &&
    size > skb_tailroom(inst->skb) - sizeof(struct nfgenmsg)) {
	/* flush skb */

Your patch fixes such 'can never send' skb condition by leaking the
skb.  So at the very least you would need to call kfree_skb(), and
perhaps also add WARN_ON() so we catch this and can fix up the size
accounting?
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