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.

include/uaccess.h

@@ -0,0 +1,49 @@
+// User-memory access API, modeled on Linux's <linux/uaccess.h>.
+//
+// Rules for kernel code touching user pointers:
+//
+//   1. Never dereference a user-supplied address without validating it
+//      first. A syscall argument can point anywhere, including at kernel
+//      memory or at nothing at all.
+//
+//   2. Never assume a user buffer is physically contiguous. uva2ka() only
+//      translates ONE page; the direct-map alias it returns runs into an
+//      unrelated physical page at the next 4K boundary. Any multi-byte
+//      access through it must stay within a single page.
+//
+// These helpers encapsulate both rules. Because Schunix (like old x86
+// without SMAP) keeps the user page table active in kernel mode, a raw
+// user virtual address is directly dereferenceable once the whole range
+// has been validated — that is what user_ptr() hands back, and why the
+// result is safe across page boundaries where a uva2ka() pointer is not.
+#ifndef SCHUNIX_UACCESS_H
+#define SCHUNIX_UACCESS_H
+
+#include "types.h"
+
+// Nonzero if [uaddr, uaddr+size) is entirely mapped, user-accessible
+// memory in the current process. size == 0 is always ok.
+int access_ok(addr_t uaddr, uint64 size);
+
+// Validate the whole range and return it as a directly usable pointer
+// (the raw user VA). 0 on bad/NULL address.
+void *user_ptr(addr_t uaddr, uint64 size);
+
+// Copy between kernel and user memory, page by page.
+// Return 0 on success, -EFAULT on any unmapped page.
+int copy_from_user(void *dst, addr_t uaddr, uint64 n);
+int copy_to_user(addr_t uaddr, const void *src, uint64 n);
+
+// Fixed-size scalar variants, in the spirit of Linux's get_user/put_user.
+#define get_user(x, uaddr)  copy_from_user(&(x), (addr_t)(uaddr), sizeof(x))
+#define put_user(x, uaddr)  copy_to_user((addr_t)(uaddr), &(x), sizeof(x))
+
+// Longest string fetchstr()/strlen_user() will scan before giving up.
+#define USER_STR_MAX 32768
+
+// Length of the NUL-terminated string at uaddr (excluding the NUL),
+// validating every page it spans. -EFAULT if it runs off mapped memory
+// or exceeds USER_STR_MAX.
+long strlen_user(addr_t uaddr);
+
+#endif // SCHUNIX_UACCESS_H