Re: [PATCH nftables] include: fix for musl with iptables v1.8.11

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi,

On Fri, Dec 20, 2024 at 01:33:42PM +0100, Phil Sutter wrote:
> On Fri, Dec 20, 2024 at 01:07:56PM +0100, Alyssa Ross wrote:
> > On Fri, Dec 20, 2024 at 12:50:42PM +0100, Phil Sutter wrote:
> > > Hi Alyssa,
> > >
> > > On Fri, Dec 20, 2024 at 12:10:02AM +0100, Alyssa Ross wrote:
> > > > Since iptables commit 810f8568 (libxtables: xtoptions: Implement
> > > > XTTYPE_ETHERMACMASK), nftables failed to build for musl libc:
> > > >
> > > > 	In file included from /nix/store/bvffdqfhyxvx66bqlqqdmjmkyklkafv6-musl-1.2.5-dev/include/netinet/et…
> > > > 	                 from /nix/store/kz6fymqpgbrj6330s6wv4idcf9pwsqs4-iptables-1.8.10-de…
> > > > 	                 from src/xt.c:30:
> > > > 	/nix/store/bvffdqfhyxvx66bqlqqdmjmkyklkafv6-musl-1.2.5-dev/include/netinet/if_ether.h:115:8: error: redefinition of 'struct ethhdr'
> > > > 	  115 | struct ethhdr {
> > > > 	      |        ^~~~~~
> > > > 	In file included from ./include/linux/netfilter_bridge.h:8,
> > > > 	                 from ./include/linux/netfilter_bridge/ebtables.h:1,
> > > > 	                 from src/xt.c:27:
> > > > 	/nix/store/bvffdqfhyxvx66bqlqqdmjmkyklkafv6-musl-1.2.5-dev/include/linux/if_ether.h:173:8: note: originally defined here
> > > > 	  173 | struct ethhdr {
> > > > 	      |        ^~~~~~
> > > >
> > > > The fix is to use libc's version of if_ether.h before any kernel
> > > > headers, which takes care of conflicts with the kernel's struct ethhdr
> > > > definition by defining __UAPI_DEF_ETHHDR, which will tell the kernel's
> > > > header not to define its own version.
> > >
> > > What I don't like about this is how musl tries to force projects to not
> > > include linux/if_ether.h directly. From the project's view, this is a
> > > workaround not a fix.
> > 
> > My understanding is that it's a general principle of using any libc on
> > Linux that if there's both a libc and kernel header for the same thing,
> > the libc header should be used.  libc headers will of course include
> > other libc headers in preference to kernel headers, so if you also
> > include the kernel headers you're likely to end up with conflicts.
> > Whether conflicts occur in any particular case depends on how a
> > particular libc chooses to expose a particular kernel API.  I could be
> > misremembering, but I believe the same thing can happen with Glibc —
> > some headers under sys/ cause conflicts with their corresponding kernel
> > headers if both are included.  While this case is musl specific, I
> > think the principle applies to all libcs.
> 
> While this may be true for the vast majority of user space programs,
> netfilter tools and libraries are a bit special in how close they
> interface with the kernel. Not all netfilter-related kernel API is
> exposed by glibc, for instance. Including (some) kernel headers is
> therefore unavoidable, and (as your patch shows) order of inclusion
> becomes subtly relevant in ways which won't show when compile-testing
> against glibc only.
> 
> > > > Signed-off-by: Alyssa Ross <hi@alyssa.is>
> > > > ---
> > > > A similar fix would solve the problem properly in iptables, which was
> > > > worked around with 76fce228 ("configure: Determine if musl is used for build").
> > > > The __UAPI_DEF_ETHHDR is supposed to be set by netinet/if_ether.h,
> > > > rather than manually by users.
> > >
> > > Why does 76fce228 not work for you?
> > 
> > It does work, but that's a fix for iptables.  This is a fix for
> > nftables.  I could have submitted a copy of the iptables fix, but I
> > don't think it's the best fix due to its reliance on internal macros
> > that are not part of the public interface.
> 
> Ah, sorry! Patch subject and description managed to confuse me.
> 
> Pablo, what's your opinion? Maybe we should strive for the same solution
> for the problem in all netfilter user space, so either take what we have
> in iptables or adjust iptables to what nftables decides how things
> should be?

I see, you mean to use this (from iptables):

commit 76fce22826f8e860b5eb5b5a2463040c17ff85da
Author: Joshua Lant <joshualant@googlemail.com>
Date:   Wed Aug 28 13:47:31 2024 +0100

    configure: Determine if musl is used for build

and adapt to use it from nftables (and everywhere else).

Alyssa's patch is more simple, but it is mangling a cache kernel
header.

Is this configure.ac workaround needed everywhere in the netfilter.org
trees to make musl happy?

I don't see any better option at this stage.

Thanks.