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.
> > Signed-off-by: Alyssa Ross <hi@xxxxxxxxx>
> > ---
> > 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.
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