tests: Add Linuxulator regression test infrastructure

[vova_fbsd.ru]

Summary

Add infrastructure for running regression tests whose test subjects must
execute under the Linux ABI.

Keep the test sources and the reusable Linux-side runtime under
sys/tests/linux. Add tests/sys/linux as the build-system integration
point and register it in the system test hierarchy, installing the tests
under /usr/tests/sys/linux.

Linux test programs are built as static, freestanding ELF binaries. They
do not depend on a Linux base system, libc, an ELF interpreter, or any
files below compat.linux.emul_path. This allows the tests to run on
minimal test systems and in buildworld test sandboxes.

Provide a small reusable runtime for Linux-side test helpers:

• linux_crt0 supplies the ELF entry point, invokes a conventional main(), and terminates the process with the Linux exit_group system call.
• linux_syscalls provides the architecture-specific raw system call interface and a minimal set of syscall wrappers.
• amd64 and arm64 are supported, producing x86-64 and AArch64 Linux ELF executables respectively.

Linux kernel modules required by individual tests are declared through
the ATF require.kmods metadata. Kyua can therefore load linux64.ko before
executing a test, without requiring each test to implement module
management.

Add the first regression test for Linux controlling-terminal semantics.
The native ATF test creates a pseudo-terminal, establishes it as the
controlling terminal of a new session, and executes a static Linux
helper. The helper closes the last descriptors referencing the slave
and then attempts to reopen /dev/tty.

Linux keeps the session's controlling-terminal reference across the last
slave close, so reopening /dev/tty must succeed. This exercises the
devfs behavior fixed by D60701 ("devfs: Keep a Linux controlling
tty across the last slave close").

Also register /usr/tests/sys/linux in BSD.tests.dist so that make check
creates the directory in its temporary installation hierarchy.

Test Plan

# make -C tests/sys/linux check
...
devfs_pty:ctty_last_close  ->  passed  [0.003s]

Results file id is usr_obj_usr_src_arm64.aarch64_tests_sys_linux_checkdir_usr_tests_sys_linux.20261013-152246-721666
Results saved to /root/.kyua/store/results.usr_obj_usr_src_arm64.aarch64_tests_sys_linux_checkdir_usr_tests_sys_linux.20261013-152246-721666.db

1/1 passed (0 broken, 0 failed, 0 skipped)

[glebius]

Thanks a lot for this project! It is definitely very valuable. Adding more reviewers.

sys/tests/linux/Makefile

20–24 ↗

(On Diff #189730)

As more tests are added you definitely don't want to copy over all these CFLAGS. I don't know what is better, to append CFLAGS or to fully override them.

[vova_fbsd.ru]

tests: Consolidate Linux helper build flags and brand binaries

Define the compiler and linker flags used for static Linux test
helpers in one place and use a common rule to build all helpers.

Explicitly brand the resulting binaries as Linux. Clang and lld
otherwise produce static ELF binaries with the System V OSABI and no
ABI note, which only worked when kern.elf64.fallback_brand was set to
Linux.

This makes the tests independent of the host's fallback ELF brand
configuration and simplifies adding more Linux helpers.

[ngie]

Thank you for this, but I'm wondering if the functional testing for Linux syscalls are better suited for LTP. I'm not as active with that project as I used to be, but I'm still able to contribute there. There is some support missing from FreeBSD that would need to be added in order to make those tests work (/dev/loop), and some of the tests assume explicit Linux support which FreeBSD lacks (SCHED_EXT, etc), but there might be some items that can be gleaned from that project that allows us to avoid reinventing the wheel too much.
Treating this as a simple smoke test though for Linuxulator would be ok, but I want to make sure the tests are constrained to smoke tests and we don't go reinventing the wheel too much.

sys/tests/linux/devfs_pty_linux.c

14–16 ↗

(On Diff #189806)

Please use linux_close_range instead.

sys/tests/linux/linux_crt0.c

17 ↗

(On Diff #189806)

Who is expected to define __x86_64__? FreeBSD has typically used __amd64__, not __x86_64__ (this is a Linux-ism).

% rg -c __x86_64__ | awk -F: 'NF == 2 { s += $2 } END { print s }'
2050
% rg -c __amd64__ | awk -F: 'NF == 2 { s += $2 } END { print s }'
2389
%

Most of the __x86_64__ uses are in contrib.

18–33 ↗

(On Diff #189806)

Where did the assembly here come from? FreeBSD source? GNU/Linux source?

sys/tests/linux/linux_crt0.h

1–12 ↗

(On Diff #189806)

Is this header necessary?

sys/tests/linux/linux_syscalls.c

9 ↗

(On Diff #189806)

22–47 ↗

(On Diff #189806)

These are architecture specific syscall numbers. Is there a way that a symbolic constant like __NR_open (the Linux-documented symbol), etc, could be used instead? Per syscall(2), they should be available in sys/syscall.h

54 ↗

(On Diff #189806)

What about syscalls that accept more or less than 4 arguments?

sys/tests/linux/linux_syscalls.h

10 ↗

(On Diff #189806)

This should be defined in the non-test linux_syscalls.h header.

12–15 ↗

(On Diff #189806)

These should use the Linux(-ish) types so the syscalls compile properly on 32-bit and 64-bit.

tests/sys/linux/Makefile

2–4

• Why the indirection?
• How can a user specify that they don't want to try and build and install the tests?
• What about unsupported architectures, e.g., powerpc64?

[vova_fbsd.ru]

In D60702#1393014, @ngie wrote:

Who is expected to define x86_64? FreeBSD has typically used amd64, not x86_64 (this is a Linux-ism).

I assume, it is defined by --target=x86_64-linux-gnu but I agree, FreeBSD-style will be better

[vova_fbsd.ru]

In D60702#1393014, @ngie wrote:

I'm wondering if the functional testing for Linux syscalls are better suited for LTP

I would assime that LTP tests - are good for extended set of tests that require to setup additional software packages (linux base in that case).
As far I understand we have no such beast yet in freebsd-src, so all tests should be buildable/runnable with just base.
Intent was to cover with tests linxlator fixes. I would assume that running whole LTP would be a little challenging, but good to have.

[vova_fbsd.ru]

In D60702#1393014, @ngie wrote:

Where did the assembly here come from? FreeBSD source? GNU/Linux source?

This assembly was written specifically for these freestanding test helpers; it was not copied from FreeBSD or GNU/Linux sources. It provides only the minimum ABI entry point needed to align/initialize the call frame and invoke linux_start(). The helpers do not currently consume argc, argv, envp, or the auxiliary vector.

[kib]

sys/tests/linux/Makefile

17 ↗

(On Diff #189806)

I think you need something like -ffreestanding otherwise compiler is allowed to assume presence of some STD C runtime.

sys/tests/linux/devfs_pty_linux.c

12 ↗

(On Diff #189806)

I think the type should be int.

[vova_fbsd.ru]

tests: Refine Linuxulator test infrastructure

Move the Linuxulator userland tests from sys/tests/linux to
tests/sys/linux. sys/tests is intended for kernel-side test modules,
while ATF tests and their helper programs belong under tests/sys.

Use the generated architecture-specific Linux syscall numbers instead
of embedding raw syscall numbers in the test support code. Extend the
low-level syscall primitive to pass all six Linux ABI arguments on
amd64 and arm64.

Use close_range(2) to close the standard descriptors in the devfs PTY
helper, remove the unnecessary crt0 header, and use the FreeBSD
amd64 spelling for the architecture check.

Only descend into tests/sys/linux on architectures for which the
freestanding Linux helpers can currently be built.

Suggested by: ngie
Differential Revision: https://reviews.freebsd.org/D60702