BUG: selinux-testsuite binder tests fail with CONFIG_ANDROID_BINDER_IPC_RUST=y

[Ondrej Mosnáček]

Hello Android binder maintainers,

Fedora Linux has recently enabled CONFIG_ANDROID_BINDER_IPC_RUST [1],
which turns out to cause the binder tests of selinux-testsuite [2] to
fail.

Minimal reproducer (no SELinux involved):
# git clone https://github.com/SELinuxProject/selinux-testsuite
# cd selinux-testsuite/tests/binder
# make
# ./init_binder.sh
# setenforce 0   ### <-- may be needed if SELinux is enabled
# ./manager -n &
# sleep 1
# ./service_provider -n &
# sleep 1
# ./client -n -c -r 3
Client ioctl BINDER_WRITE_READ error: Invalid argument
#

With only CONFIG_ANDROID_BINDER_IPC=y the last command succeeds.

The issue reproduces even on a kernel compiled from commit
eafedbc7c050, so it seems to date back to the origin of the Rust
implementation. I'm not sure if it's a bug in the new kernel code or
some bug/wrong assumption in the testsuite code - can you please look
into it?

Thanks,
Ondrej

[1] https://gitlab.com/cki-project/kernel-ark/-/commit/c12a0ae360f154fcffc0dd72381550391f77793c
[2] https://github.com/SELinuxProject/selinux-testsuite