netdev - Re: [PATCH v2] net: netfilter: Fix undefined reference to nf_nat_redirect

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Date:	Wed, 26 Nov 2014 11:33:19 +0100
From:	Andreas Ruprecht <rupran@...server.de>
To:	Florian Westphal <fw@...len.de>
CC:	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: netfilter: Fix undefined reference to nf_nat_redirect_*
 functions

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@...server.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...?
> 
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