Re: [ebtables PATCH 2/2] configure.ac: add option --enable-kernel-64-userland-32

[Florian Westphal <fw@strlen.de>]

Thomas De Schampheleire <patrickdepinguin@gmail.com> wrote:
> 1.  x86_64 kernel 5.4.x + i686 userspace: ebtables works correctly
> 
> 2.  aarch64 kernel 4.1.x + 32-bit ARM userspace: ebtables fails as described
> 
> As mentioned before, in both cases CONFIG_COMPAT=y .
> >
> > Thomas, does unmodified 32bit iptables work on those arch/kernel
> > combinations?
> 
> Yes, iptables 1.8.6 is used successfully without special provisioning
> for bitness. We are using Buildroot 2021.02 to compile.

Ok, so this is 'just' a bug in the ebtables translation layer.

Its likely that there are alignment differences on aarch that the
ebtables i686 fixups are not aware of.

> > ebtables-userspace compat fixups predate the ebtables kernel side
> > support, it was autoenabled on sparc64 in the old makefile:
> >
> > ifeq ($(shell uname -m),sparc64)
> > CFLAGS+=-DEBT_MIN_ALIGN=8 -DKERNEL_64_USERSPACE_32
> > endif
> 
> Yes, in the proposed changes to ebtables userspace, this kind of logic
> is restored, but not based on the machine type but with an autoconf
> flag.
> 
> >
> > I don't even know if the ebtables compat support is compiled in on
> > non-amd64.
> 
> Can you be more specific what you are referring to here?

I meant I wasn't sure if the ebtables compat stuff is compiled in on
non-amd64 platforms.  But I guess they are because iptables works for
you.

> So at this moment it seems to me that the kernel compat support is
> effectively compiled in, and supports x86(_64) but does not support
> the Aarch64/ARM combination (and perhaps others).
> 
> How to proceed now?

The proper solution is to make the existing translation work on aarch64.

It will take me some time to get a crosscompiler+qemu setup going
though.