From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andreas Ruprecht <rupran@einserver.de>
Subject: Re: [PATCH v2] net: netfilter: Fix undefined reference to nf_nat_redirect_*
 functions
Date: Wed, 26 Nov 2014 11:33:19 +0100
Message-ID: <5475AC6F.9010902@einserver.de>
References: <1416928034-10882-1-git-send-email-rupran@einserver.de> <1416994537-1592-1-git-send-email-rupran@einserver.de> <20141126102402.GA24801@breakpoint.cc>
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Cc: Pablo Neira Ayuso <pablo@netfilter.org>,
	Patrick McHardy <kaber@trash.net>,
	Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>,
	"David S. Miller" <davem@davemloft.net>,
	netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
To: Florian Westphal <fw@strlen.de>
Return-path: <netfilter-devel-owner@vger.kernel.org>
In-Reply-To: <20141126102402.GA24801@breakpoint.cc>
Sender: netfilter-devel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

Sure.

When the file is compiled, i.e. CONFIG_NETFILTER_XT_TARGET_REDIRECT is
selected, all headers will be included and all functions inside the file
will be compiled, regardless of other Kconfig options.

This means redirect_tg6 and redirect_tg4 will be compiled (which doesn't
necessarily mean they will be _called_) but the linker needs to resolve
nf_nat_redirect_ipv4() due to the compilation of the redirect_tg4()
function.

nf_nat_redirect_ip4() is defined in
net/ipv4/netfilter/nf_nat_redirect_ipv4.c but this file is only included
into the build when CONFIG_NF_NAT_REDIRECT_IPV4 is enabled.

Now when a kernel config enables CONFIG_NETFILTER_XT_TARGET_REDIRECT but
_not_ CONFIG_NF_NAT_REDIRECT_IPV4, the declaration of
nf_nat_redirect_ipv4() from the header
<net/netfilter/ipv4/nf_nat_redirect.h> will have no definition (i.e., no
implementation), causing the linker to report an "undefined reference".

Same logic goes for nf_nat_redirect_ipv6().

Hope this helps,

Andreas

On 26.11.2014 11:24, Florian Westphal wrote:
> Andreas Ruprecht <rupran@einserver.de> wrote:
>> Additionally it is necessary to provide stubs for the
>> nf_nat_redirect_ipv{4,6} functions in case the header is included but
>> the corresponding Kconfig feature is not enabled.
> 
> Hmmm, not following.
> 
> Can you elaborate?
> 
> Under which circumstances do we have a call to nf_nat_redirect_ipv4()
> (i.e., linker error) but can safely do a noop operation instead of the
> requested nat redirect...?
>