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

[Thomas De Schampheleire <patrickdepinguin@gmail.com>]

Hello,

El lun, 24 may 2021 a las 17:26, Pablo Neira Ayuso
(<pablo@netfilter.org>) escribió:
>
> On Tue, May 18, 2021 at 08:17:30PM +0200, Thomas De Schampheleire wrote:
> > From: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
> >
> > The ebtables build system seems to assume that 'sparc64' is the
> > only case where KERNEL_64_USERSPACE_32 is relevant, but this is not true.
> > This situation can happen on many architectures, especially in embedded
> > systems. For example, an Aarch64 processor with kernel in 64-bit but
> > userland built for 32-bit Arm. Or a 64-bit MIPS Octeon III processor, with
> > userland running in the 'n32' ABI.
> >
> > While it is possible to set CFLAGS in the environment when calling the
> > configure script, the caller would need to know to not only specify
> > KERNEL_64_USERSPACE_32 but also the EBT_MIN_ALIGN value.
> >
> > Instead, add a configure option. All internal details can then be handled by
> > the configure script.
>
> Are you enabling
>
> CONFIG_NETFILTER_XTABLES_COMPAT
>
> in your kernel build?
>
> KERNEL_64_USERSPACE_32 was deprecated long time ago in favour of
> CONFIG_NETFILTER_XTABLES_COMPAT.

The option you refer to (CONFIG_NETFILTER_XTABLES_COMPAT) was
introduced with commit 47a6959fa331fe892a4fc3b48ca08e92045c6bda
(5.13-rc1). Before that point, it seems CONFIG_COMPAT was the relevant
flag. The checks on CONFIG_COMPAT were already introduced with commit
81e675c227ec60a0bdcbb547dc530ebee23ff931 in 2.6.34.x.

I have seen this problem on Linux 4.1 and 4.9, on an Aarch64 CPU with
64-bit kernel and userspace compiled as 32-bit ARM. In both kernels,
CONFIG_COMPAT was set.

So I am a bit surprised that I bump into this issue after upgrading
ebtables from 2.0.10-4 to 2.0.11 where the padding was removed.
According to your mail and the commits mentioned, it is supposed to
work without ebtables making specific provisions for the 32/64 bit
type difference.

When I apply the patches I submitted to this list, I get correct
behavior. Without them, the kernel complains and ebtables fails.

Best regards,
Thomas