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==== What’s Missing in FreeBSD Linuxulator? ====
To run systemd in FreeBSD’s Linuxulator, the following gaps need to be addressed:

===== 1. cgroups Support =====
* '''Current State''': FreeBSD has no native cgroups implementation. It uses its own resource control mechanisms (e.g., <code>rctl</code> for resource limits and jails for isolation), which are philosophically and technically different from Linux’s cgroups.
* '''Requirement''': Systemd expects cgroups v1 or v2 for process grouping, resource limiting, and service management. Without this, systemd cannot initialize properly.

===== 2. Namespaces =====
* '''Current State''': FreeBSD supports some namespace-like features via jails and VNET (virtual networking), but these are not equivalent to Linux’s PID, mount, or user namespaces. The Linuxulator does not translate Linux namespace-related system calls (e.g., <code>unshare()</code>, <code>clone()</code> with namespace flags) into FreeBSD equivalents.
* '''Requirement''': Systemd uses namespaces for service isolation and scoping, particularly PID namespaces to manage process trees.

===== 3. System Call Gaps =====
* '''Current State''': The Linuxulator implements a subset of Linux system calls (see <code>sys/compat/linux/linux_sysent.c</code> in the FreeBSD source). Missing calls include newer ones like <code>pidfd_open()</code> (Linux 5.3+), <code>clone3()</code> (Linux 5.3+), and others that systemd may use.
* '''Requirement''': Full compatibility with the system calls systemd depends on, which may require extending the Linuxulator’s syscall table.

===== 4. Fanotify and File System Events =====
* '''Current State''': FreeBSD has <code>kqueue</code> for event notification, but it’s not compatible with Linux’s <code>fanotify</code> API, which systemd uses for file system monitoring.
* '''Requirement''': An emulation layer to map <code>fanotify</code> calls to <code>kqueue</code> or a custom implementation.

===== 5. udev and Device Management =====
* '''Current State''': FreeBSD uses <code>devd</code> for device management, which differs significantly from Linux’s <code>udev</code>. The Linuxulator does not fully emulate <code>udev</code>’s behavior or its integration with the kernel.
* '''Requirement''': Systemd expects <code>udev</code> to manage device events, so a compatibility layer or partial <code>udev</code> emulation is needed.

===== 6. Kernel ABI Stability and Versioning =====
* '''Current State''': The Linuxulator mimics a specific Linux kernel version (e.g., 5.15.0 in FreeBSD 14-STABLE), but systemd may expect features from newer kernels or specific behaviors not yet emulated.
* '''Requirement''': Ensuring the emulated ABI matches a Linux kernel version that systemd supports (e.g., at least 4.4 or later for modern versions).