Blog 021: Filesystem Mounting & /proc Improvements

M4 Phase 4 — mount/umount2, dynamic /proc, /sys stubs

systemd expects to mount filesystems at boot — proc on /proc, sysfs on /sys, tmpfs on /run, cgroup2 on /sys/fs/cgroup. It also reads /proc extensively: /proc/self/stat, /proc/1/cmdline, /proc/meminfo, /proc/mounts. Phase 4 implements all of this.

mount(2) and umount2(2)

The sys_mount handler reads the target path and filesystem type string from userspace, then dispatches on fstype:

• proc — uses the global PROC_FS singleton
• sysfs — uses the global SYS_FS singleton
• tmpfs — creates a fresh TmpFs
• devtmpfs/devpts — silently succeeds (our devfs is always mounted)
• cgroup2/cgroup — creates an empty tmpfs (stub)

If the target directory doesn't exist, mount auto-creates it (like mkdir -p). After mounting, the entry is recorded in a global MountTable so /proc/mounts can report it.

sys_umount2 just removes the entry from the mount table. We don't actually detach the VFS mount — systemd rarely unmounts at runtime and the VFS layer doesn't support it yet.

MountTable

A simple SpinLock<VecDeque<MountEntry>> tracking (fstype, mountpoint) pairs. Initialized at boot with the known mounts (rootfs on /, proc on /proc, devtmpfs on /dev, tmpfs on /tmp). format_mounts() generates Linux-compatible output for /proc/mounts:

rootfs / rootfs rw 0 0
proc /proc proc rw 0 0
devtmpfs /dev devtmpfs rw 0 0
tmpfs /tmp tmpfs rw 0 0

Dynamic /proc

Previously, /proc was a flat tmpfs with a few static files. Now ProcRootDir intercepts lookups:

1. "self" — returns a ProcSelfSymlink that resolves to /proc/<current_pid>
2. Numeric names — parses as PID, returns a ProcPidDir generated on the fly
3. Everything else — delegates to the static tmpfs (mounts, meminfo, etc.)

Per-PID directories (/proc/[pid]/)

ProcPidDir provides five entries:

All entries are synthesized on read from the live process table via Process::find_by_pid(). No data is cached.

System-wide files

/sys stubs

systemd probes /sys at early boot looking for cgroup controllers, device classes, and kernel parameters. SysFs wraps a TmpFs with empty directories:

• /sys/fs/cgroup
• /sys/class
• /sys/devices
• /sys/bus
• /sys/kernel

This is enough for systemd to see sysfs is mounted and continue without errors. The directories are empty — no actual sysfs attributes yet.

Syscall summary

Total implementation: ~900 lines across 10 files. The /proc infrastructure is the most complex piece — the dynamic root directory pattern will extend easily as we add more per-PID entries (fd/, maps, etc.) in later phases.