Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

cmd/init/main.c

@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: Apache-2.0
+// Schunix — minimal init (PID 1)
+
+#include "types.h"
+#include "user.h"
+#include "fcntl.h"
+
+static void
+xputs(int fd, const char *s)
+{
+    int n = 0;
+    while (s[n]) n++;
+    write(fd, (void *)s, n);
+}
+
+int
+main(void)
+{
+    if (getpid() != 1)
+        exit();
+
+    // Open console as fd 0; dup to fd 1 (stdout) and fd 2 (stderr).
+    if (open("console", O_RDWR) < 0) {
+        mknod("console", 1, 1);
+        open("console", O_RDWR);
+    }
+    dup(0);
+    dup(0);
+
+    // --- /dev ------------------------------------------------------------
+    // Minor numbers under major 1 follow Linux's mem-major layout where it
+    // costs nothing (null=2, zero=5, full=7, random=8, urandom=9).
+    // /dev/hole (was minor 4) is gone: it backed an abandoned experiment and
+    // nothing has referenced it since.
+    mkdir("/dev");
+    mknod("/dev/console",  1,  1);
+    mknod("/dev/null",     1,  2);
+    mknod("/dev/keyboard", 1,  3);
+    mknod("/dev/zero",     1,  5);
+    mknod("/dev/full",     1,  7);
+    mknod("/dev/random",   1,  8);
+    mknod("/dev/urandom",  1,  9);
+    mknod("/dev/fb0",      2,  0);
+    mknod("/dev/mouse",    3,  1);
+    mknod("/dev/tty",      1,  1); // alias for console — must pass TCGETS ioctl
+    mknod("/dev/ttyS0",    4, 64);
+    mknod("/dev/stdin",    1,  1);
+    mknod("/dev/stdout",   1,  1);
+    mknod("/dev/stderr",   1,  1);
+
+    // Raw input events, Linux evdev layout (kernel/dev/input.c). /dev/keyboard
+    // and /dev/mouse above are the old character-and-position interfaces; these
+    // are the ones with key releases and button transitions, which is what a
+    // game or a compositor needs and what neither of the others can express.
+    mkdir("/dev/input");
+    mknod("/dev/input/event0", 7, 0);   // keyboard
+    mknod("/dev/input/event1", 7, 1);   // mouse
+
+    // --- /proc -----------------------------------------------------------
+    // Synthesised status files, major 6 (kernel/dev/procfs.c). These are the
+    // global ones; per-process /proc/<pid>/ directories need synthetic
+    // directory support in the filesystem, which does not exist yet.
+    mkdir("/proc");
+    // /proc is a real filesystem now (kernel/fs/procfs.c), not a handful of
+    // mknod'd nodes: it has to be, because one directory per process cannot be
+    // created ahead of time. This is what makes `ps` and `top` work.
+    //
+    // The mknod'd global files below still happen, and are simply covered by
+    // the mount. They stay for now so that a failure here degrades to the old
+    // behaviour instead of losing /proc entirely.
+    mount("proc", "/proc", "proc", 0, 0);
+    mknod("/proc/meminfo",     6, 1);
+    mknod("/proc/uptime",      6, 2);
+    mknod("/proc/cpuinfo",     6, 3);
+    mknod("/proc/version",     6, 4);
+    mknod("/proc/stat",        6, 5);
+    mknod("/proc/loadavg",     6, 6);
+    mknod("/proc/mounts",      6, 7);
+    mknod("/proc/filesystems", 6, 8);
+    // SchFS volume summary: label, generation, free space, and the root
+    // directory. Reachable whether or not the volume is mounted into the
+    // namespace, which is what makes it useful when a mount has gone wrong.
+    mknod("/proc/schfs",       6, 9);
+    // Write 'c' here to crash on purpose. The crash screen is otherwise the
+    // one part of the system that can only be seen by having a bug.
+    mknod("/proc/sysrq",       6, 10);
+    mknod("/proc/pci",         6, 11);
+    mknod("/proc/interrupts",  6, 12);
+
+    mkdir("/sys");
+
+    xputs(1, "\nSchunix\n");
+    xputs(1, "init: ready (pid 1)\n");
+
+    for (;;) {
+        int pid = fork();
+        if (pid < 0) {
+            xputs(2, "init: fork failed\n");
+            sleep(100);
+            continue;
+        }
+
+        if (pid == 0) {
+            // exec.c auto-enables Linux ABI for musl binaries (entry >= 0x100000).
+            // Do NOT call enable_linux_abi() here — it corrupts Schunix syscall
+            // numbers for the exec/exit calls that follow in this child.
+            // Standardized naming: the shell is always /bin/sh, regardless of
+            // which implementation backs it (dash, mksh, etc.) — no ksh/mksh/
+            // other aliases are looked up here.
+            static char *argv[] = { "sh", 0 };
+            exec("/bin/sh", argv);
+            xputs(2, "init: no shell found; place sh in /bin\n");
+            exit();
+        }
+
+        // Reap zombies; the loop restarts the shell when it exits.
+        int wpid;
+        while ((wpid = wait()) >= 0 && wpid != pid)
+            ;
+        // Brief pause before restarting so we don't spin on a missing shell.
+        sleep(100);
+    }
+    return 0;
+}