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.

kernel/core/uaccess.c

@@ -0,0 +1,79 @@
+// User-memory access helpers — see include/uaccess.h for the rules.
+//
+// Everything here validates against the *current* process's page table
+// (myproc()->pgdir), which is also the live CR3 while a syscall runs, so a
+// range that passes access_ok() can be dereferenced directly by kernel
+// code for the remainder of the syscall (single-threaded processes can't
+// unmap it out from under us mid-call).
+
+#include "types.h"
+#include "defs.h"
+#include "param.h"
+#include "memlayout.h"
+#include "mmu.h"
+#include "proc.h"
+#include "errno.h"
+#include "uaccess.h"
+
+int
+access_ok(addr_t uaddr, uint64 size)
+{
+  addr_t a, end;
+
+  if(size == 0)
+    return 1;
+  end = uaddr + size;
+  if(end < uaddr || end > KERNBASE)
+    return 0;
+  for(a = PGROUNDDOWN(uaddr); a < end; a += PGSIZE)
+    if(uva2ka(myproc()->pgdir, (char*)a) == 0)
+      return 0;
+  return 1;
+}
+
+void*
+user_ptr(addr_t uaddr, uint64 size)
+{
+  if(uaddr == 0 || !access_ok(uaddr, size))
+    return 0;
+  return (void*)uaddr;
+}
+
+int
+copy_from_user(void *dst, addr_t uaddr, uint64 n)
+{
+  if(uaddr + n < uaddr || uaddr + n > KERNBASE)
+    return -EFAULT;
+  if(copyin(myproc()->pgdir, uaddr, dst, n) < 0)
+    return -EFAULT;
+  return 0;
+}
+
+int
+copy_to_user(addr_t uaddr, const void *src, uint64 n)
+{
+  if(uaddr + n < uaddr || uaddr + n > KERNBASE)
+    return -EFAULT;
+  if(copyout(myproc()->pgdir, uaddr, (void*)src, n) < 0)
+    return -EFAULT;
+  return 0;
+}
+
+long
+strlen_user(addr_t uaddr)
+{
+  addr_t a = uaddr;
+
+  while(a - uaddr < USER_STR_MAX){
+    // Scan through the per-page kernel alias so an unmapped page is a
+    // clean failure instead of a fault.
+    char *ka = uva2ka(myproc()->pgdir, (char*)a);
+    if(ka == 0)
+      return -EFAULT;
+    addr_t pgend = PGROUNDDOWN(a) + PGSIZE;
+    for(; a < pgend; a++, ka++)
+      if(*ka == 0)
+        return a - uaddr;
+  }
+  return -EFAULT;
+}