Move from memcpy to memcpy_s.

Change-Id: If7d53c6e54428f01f14528c3f281331d308af56a
diff --git a/src/std.c b/src/std.c
index 00d4704..b7ea2ce 100644
--- a/src/std.c
+++ b/src/std.c
@@ -20,6 +20,7 @@
 
 /* Declare unsafe functions locally so they are not available globally. */
 void *memset(void *s, int c, size_t n);
+void *memcpy(void *dst, const void *src, size_t n);
 
 void memset_s(void *dest, rsize_t destsz, int ch, rsize_t count)
 {
@@ -41,3 +42,39 @@
 fail:
 	panic("memset_s failure");
 }
+
+void memcpy_s(void *dest, rsize_t destsz, const void *src, rsize_t count)
+{
+	uintptr_t d = (uintptr_t)dest;
+	uintptr_t s = (uintptr_t)src;
+
+	if (dest == NULL || src == NULL) {
+		goto fail;
+	}
+
+	if (destsz > RSIZE_MAX || count > RSIZE_MAX) {
+		goto fail;
+	}
+
+	if (count > destsz) {
+		goto fail;
+	}
+
+	/* Destination overlaps the end of source. */
+	if (d > s && d < (s + count)) {
+		goto fail;
+	}
+
+	/* Source overlaps the end of destination. */
+	if (s > d && s < (d + destsz)) {
+		goto fail;
+	}
+
+	/* TODO: consider wrapping? */
+
+	memcpy(dest, src, count);
+	return;
+
+fail:
+	panic("memcpy_s failure");
+}