mm: make get_physaddr return NULL if PD or PT is not present

src/mm/page_tables.c

@@ -22,10 +22,18 @@ void *get_physaddr(void *virtualaddr) {
     unsigned long ptindex = (unsigned long)virtualaddr >> 12 & 0x03FF;
 
     unsigned long *pd = (unsigned long *)0xFFFFF000;
-    // Here you need to check whether the PD entry is present.
+    if(!(pd[pdindex] & PAGE_PRESENT))
+    {
+        debug_log("get_physaddr: no PD (virt address is %X)\n", virtualaddr);
+        return NULL;
+    }
 
     uint32_t *pt = (uint32_t*)(0xFFC00000 + (pdindex << 12));
-    // Here you need to check whether the PT entry is present.
+    if(!(pt[ptindex] & PAGE_PRESENT))
+    {
+        debug_log("get_physaddr: no PT (virt address is %X)\n", virtualaddr);
+        return NULL;
+    }
 
     return (void *)((pt[ptindex] & ~0xFFF) + ((unsigned long)virtualaddr & 0xFFF));
 }