diff --git a/arch/sandbox/cpu/os.c b/arch/sandbox/cpu/os.c
index f6d0e84..cb469e0 100644
--- a/arch/sandbox/cpu/os.c
+++ b/arch/sandbox/cpu/os.c
@@ -58,9 +58,29 @@ off_t os_lseek(int fd, off_t offset, int whence)
 	return lseek(fd, offset, whence);
 }
 
-int os_open(const char *pathname, int flags)
+int os_open(const char *pathname, int os_flags)
 {
-	return open(pathname, flags);
+	int flags;
+
+	switch (os_flags & OS_O_MASK) {
+	case OS_O_RDONLY:
+	default:
+		flags = O_RDONLY;
+		break;
+
+	case OS_O_WRONLY:
+		flags = O_WRONLY;
+		break;
+
+	case OS_O_RDWR:
+		flags = O_RDWR;
+		break;
+	}
+
+	if (os_flags & OS_O_CREAT)
+		flags |= O_CREAT;
+
+	return open(pathname, flags, 0777);
 }
 
 int os_close(int fd)
diff --git a/include/os.h b/include/os.h
index f74766d..6b7ee47 100644
--- a/include/os.h
+++ b/include/os.h
@@ -1,4 +1,9 @@
 /*
+ * Operating System Interface
+ *
+ * This provides access to useful OS routines for the sandbox architecture.
+ * They are kept in a separate file so we can include system headers.
+ *
  * Copyright (c) 2011 The Chromium OS Authors.
  * See file CREDITS for list of people who contributed to this
  * project.
@@ -19,12 +24,6 @@
  * MA 02111-1307 USA
  */
 
-/*
- * Operating System Interface
- *
- * This provides access to useful OS routines from the sandbox architecture
- */
-
 #ifndef __OS_H__
 #define __OS_H__
 
@@ -72,6 +71,12 @@ off_t os_lseek(int fd, off_t offset, int whence);
  */
 int os_open(const char *pathname, int flags);
 
+#define OS_O_RDONLY	0
+#define OS_O_WRONLY	1
+#define OS_O_RDWR	2
+#define OS_O_MASK	3	/* Mask for read/write flags */
+#define OS_O_CREAT	0100
+
 /**
  * Access to the OS close() system call
  *