Update stack initialization code to handle all possible overflows and implement a user program to test this

src/userprog/process.c

@@ -28,6 +28,10 @@
    (for the purposes of alignment). */
 #define WORD_SIZE 4
 
+/* Defines non-negative integer division wherein the result is always rounded
+   up. */
+#define DIV_CEIL(x, y) ((x + (y - 1)) / y)
+
 /* Keeps track of the position of pointers to user program arguments
    within a linked list. */
 struct arg_elem
@@ -222,13 +226,19 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
                             + return_addr_size;
 
     /* If pushing the rest of the data required for the stack would cause
-       overflow, allocate an extra page that is contiguous within the
-       virtual address space (below the current address range). */ 
-    if (PHYS_BASE - *esp + remaining_size > PGSIZE)
+       overflow, allocate as many extra pages as needed to the user process
+       contiguously in the virtual address space below the initial page. */
+    int overflow_bytes = (PHYS_BASE - *esp) + remaining_size - PGSIZE;
+    if (overflow_bytes > 0)
       {
-        uint8_t *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
-        if (!install_page (((uint8_t *) PHYS_BASE) - PGSIZE * 2, kpage, true))
-          return false;
+        int pages_needed = DIV_CEIL (overflow_bytes, PGSIZE);
+        for (int i = 1; i < pages_needed + 1; i++)
+          {
+            uint8_t *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
+            if (!install_page (((uint8_t *) PHYS_BASE) - PGSIZE * (i + 1),
+                               kpage, true))
+              return false;
+          }
       }
 
     /* Align stack pointer to word size before pushing argv elements for