src/threads/thread.h

@@ -145,6 +145,8 @@ struct thread
     struct hash open_files;             /* Hash Table of FD -> Struct File. */
 #endif
 
+    void *curr_esp;
+
     /* Owned by thread.c. */
     unsigned magic;                     /* Detects stack overflow. */
   };

src/userprog/exception.c

@@ -3,16 +3,24 @@
 #include <stdio.h>
 #include "userprog/gdt.h"
 #include "userprog/pagedir.h"
+#include "userprog/process.h"
 #include "threads/interrupt.h"
+#include "threads/palloc.h"
 #include "threads/thread.h"
 #include "threads/vaddr.h"
 #include "vm/page.h"
 
+#define MAX_STACK_SIZE (8 * 1024 * 1024) // 8MB
+#define MAX_STACK_OFFSET 32 // 32 bytes offset below stack pointer (ESP)
+
 /* Number of page faults processed. */
 static long long page_fault_cnt;
 
 static void kill (struct intr_frame *);
 static void page_fault (struct intr_frame *);
+
+static bool is_valid_stack_access (const void *fault_addr, const void *esp);
+static bool grow_stack (void *upage);
 bool try_fetch_page (void *upage, bool write);
 
 /* Registers handlers for interrupts that can be caused by user
@@ -149,20 +157,26 @@ page_fault (struct intr_frame *f)
   write = (f->error_code & PF_W) != 0;
   user = (f->error_code & PF_U) != 0;
 
-  if (!user)
+  if (!user || !not_present)
   {
     f->eip = (void *)f->eax;
     f->eax = 0xffffffff;
     return;
   }
 
-
   /* If the fault address is in a user page that is not present, then it might
-     just need to be lazily loaded. So, we check our SPT to see if the page
+   be just that the stack needs to grow or that it needs to be lazily loaded.
+   So we attempt to grow the stack. If this does not work, we check our SPT to see if the page
      is expected to have data loaded in memory. */
   void *upage = pg_round_down (fault_addr);
   if (not_present && is_user_vaddr (upage) && upage != NULL)
     {
+      if (is_valid_stack_access (fault_addr, f->esp))
+        {
+          if (grow_stack (upage))
+            return;
+        }
+
       if (try_fetch_page (upage, write))
         return;
     }
@@ -178,6 +192,54 @@ page_fault (struct intr_frame *f)
   kill (f);
 }
 
+/* Validates whether the fault address is a valid stack access. Access is a
+   valid stack access under the following two conditions:
+     1. The fault address must be within MAX_STACK_OFFSET (32) bytes below
+      the current stack pointer. (Accounts for both PUSH and PUSHA instructions)
+     2. Growing this stack to this address does not cause it to exceed the
+      MAX_STACK_SIZE (8MB) limit.
+
+   Returns true if both conditions are met, false otherwise.
+
+   Pre: fault_addr is a valid user virtual address (so also not NULL). */
+static bool
+is_valid_stack_access (const void *fault_addr, const void *esp)
+{
+  uint32_t new_stack_size = PHYS_BASE - pg_round_down (fault_addr);
+
+  uint32_t *lowest_valid_push_addr = (uint32_t *)esp - MAX_STACK_OFFSET;
+  bool is_within_push_range = (uint32_t *)fault_addr >= lowest_valid_push_addr;
+
+  return is_within_push_range && new_stack_size <= MAX_STACK_SIZE;
+}
+
+/* Attempts to grow the stack by allocating and mapping a new page.
+  This involves:
+    1. Allocating a zeroed page from the user pool
+    2. Installing it into the page table with write permissions
+
+  Returns true if the stack was successfully grown, false if either
+  allocation or installation fails.
+
+  Pre: upage is a valid page-aligned address (so also not NULL). */
+static bool
+grow_stack (void *upage)
+{
+  /* Allocate new page for stack */
+  void *kpage = palloc_get_page (PAL_USER | PAL_ZERO);
+  if (kpage == NULL)
+    return false;
+
+  /* Install the page into user page table */
+  if (!install_page (upage, kpage, true))
+    {
+      palloc_free_page (kpage);
+      return false;
+    }
+
+  return true;
+}
+
 bool
 try_fetch_page (void *upage, bool write)
 {

src/userprog/syscall.c

@@ -1,5 +1,4 @@
 #include "userprog/syscall.h"
-#include "userprog/exception.h"
 #include "devices/shutdown.h"
 #include "devices/input.h"
 #include "filesys/file.h"
@@ -12,6 +11,7 @@
 #include "userprog/process.h"
 #include "userprog/pagedir.h"
 #include <stdio.h>
+#include <stdbool.h>
 #include <syscall-nr.h>
 
 #define MAX_SYSCALL_ARGS 3
@@ -47,8 +47,11 @@ static unsigned syscall_tell (int fd);
 static void syscall_close (int fd);
 
 static struct open_file *fd_get_file (int fd);
-static void validate_user_pointer (const void *start, size_t size, bool write);
-static void validate_user_string (const char *str);
+static void validate_user_pointer (const void *ptr, size_t size,
+                                   bool check_write);
+static void validate_user_string (const char *str, bool check_write);
+static int get_user (const uint8_t *);
+static bool put_user (uint8_t *, uint8_t);
 
 /* A struct defining a syscall_function pointer along with its arity. */
 struct syscall_arguments
@@ -98,7 +101,8 @@ syscall_handler (struct intr_frame *f)
 {
   /* First, read the system call number from the stack. */
   validate_user_pointer (f->esp, sizeof (uintptr_t), false);
-  uintptr_t syscall_number = *(int *) f->esp;
+  uintptr_t syscall_number = *(int *)f->esp;
+  thread_current ()->curr_esp = f->esp;
 
   /* Ensures the number corresponds to a system call that can be handled. */
   if (syscall_number >= LOOKUP_SIZE)
@@ -109,10 +113,9 @@ syscall_handler (struct intr_frame *f)
   /* Next, read and copy the arguments from the stack pointer. */
   validate_user_pointer (f->esp + sizeof (uintptr_t),
                          syscall.arity * sizeof (uintptr_t), false);
-
-  uintptr_t args[MAX_SYSCALL_ARGS] = {0};
+  uintptr_t args[MAX_SYSCALL_ARGS] = { 0 };
   for (int i = 0; i < syscall.arity && i < MAX_SYSCALL_ARGS; i++)
-    args[i] = *(uintptr_t *) (f->esp + sizeof (uintptr_t) * (i + 1));
+    args[i] = *(uintptr_t *)(f->esp + sizeof (uintptr_t) * (i + 1));
 
   /* Call the function that handles this system call with the arguments. When
      there is a return value it is stored in f->eax. */
@@ -141,8 +144,7 @@ syscall_exit (int status)
 static pid_t
 syscall_exec (const char *cmd_line)
 {
-  /* Validate the user string before executing the process. */
-  validate_user_string (cmd_line);
+  validate_user_string (cmd_line, false);
 
   return process_execute (cmd_line); /* Returns the PID of the new process */
 }
@@ -161,8 +163,7 @@ syscall_wait (pid_t pid)
 static bool
 syscall_create (const char *file, unsigned initial_size)
 {
-  /* Validate the user string before creating the file. */
-  validate_user_string (file);
+  validate_user_string (file, false);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
@@ -179,8 +180,7 @@ syscall_create (const char *file, unsigned initial_size)
 static bool
 syscall_remove (const char *file)
 {
-  /* Validate the user string before removing the file. */
-  validate_user_string (file);
+  validate_user_string (file, false);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
@@ -198,8 +198,7 @@ syscall_remove (const char *file)
 static int
 syscall_open (const char *file)
 {
-  /* Validate the user string before opening the file. */
-  validate_user_string (file);
+  validate_user_string (file, false);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
@@ -265,7 +264,6 @@ syscall_read (int fd, void *buffer, unsigned size)
   if (fd < STDIN_FILENO || fd == STDOUT_FILENO)
     return EXIT_FAILURE;
 
-  /* Validate the user buffer for the provided size before reading. */
   validate_user_pointer (buffer, size, true);
 
   if (fd == STDIN_FILENO)
@@ -309,7 +307,6 @@ syscall_write (int fd, const void *buffer, unsigned size)
   if (fd <= 0)
     return 0;
 
-  /* Validate the user buffer for the provided size before writing. */
   validate_user_pointer (buffer, size, false);
 
   if (fd == STDOUT_FILENO)
@@ -452,67 +449,91 @@ fd_get_file (int fd)
   return hash_entry (e, struct open_file, elem);
 }
 
-/* Validates if a block of memory starting at START and of size SIZE bytes is
-   fully contained within user virtual memory. Kills the thread (by exiting with
-   failure) if the memory is invalid. Otherwise, returns (nothing) normally.
-   If the size is 0, the function does no checks and returns the given ptr. */
+/* Validates if a block of memory starting at PTR and of size SIZE bytes is
+   fully contained within valid user virtual memory. thread_exit () if the
+   memory is invalid.
+   If the size is 0, the function does no checks and returns PTR. */
 static void
-validate_user_pointer (const void *start, size_t size, bool write)
+validate_user_pointer (const void *ptr, size_t size, bool check_write)
 {
-  /* If the size is 0, we do not need to check anything. */
   if (size == 0)
     return;
-
-  const void *end = start + size - 1;
-
-  /* Check if the start and end pointers are valid user virtual addresses. */
-  if (start == NULL || !is_user_vaddr (start) || !is_user_vaddr (end))
+  /* ptr < ptr + size - 1, so sufficient to check that (ptr + size -1) is a
+     valid user virtual memory address. */
+  void *last = ptr + size - 1;
+  if (!is_user_vaddr (last))
     syscall_exit (EXIT_FAILURE);
-
-  /* We no longer check if the memory is mapped to physical memory. This is
-     because the data may not necessarily be there at the time of the syscall,
-     but it may be lazily loaded later. In such case, we try to preload the
-     page. If that fails, we exit the thread. */
-  for (void *ptr = pg_round_down (start); ptr <= end; ptr += PGSIZE)
-    if (pagedir_get_page (thread_current ()->pagedir, ptr) == NULL &&
-        !try_fetch_page (ptr, write))
-      syscall_exit (EXIT_FAILURE);
+  ptr = pg_round_down (ptr);
+  while (ptr <= last)
+    {
+      int result;
+      /* Check read access to pointer. */
+      if ((result = get_user (ptr)) == -1)
+        syscall_exit (EXIT_FAILURE);
+      /* Check write access to pointer (if required). */
+      if (check_write && !put_user (ptr, result))
+        syscall_exit (EXIT_FAILURE);
+      ptr += PGSIZE;
+    }
 }
 
-/* Validates if a string is fully contained within user virtual memory. Kills
-   the thread (by exiting with failure) if the memory is invalid. Otherwise,
-   returns (nothing) normally. */
+/* Validates of a C-string starting at ptr is fully contained within valid
+   user virtual memory. thread_exit () if the memory is invalid. */
 static void
-validate_user_string (const char *str)
+validate_user_string (const char *ptr, bool check_write)
 {
-  /* Check if the string pointer is a valid user virtual address. */
-  if (str == NULL || !is_user_vaddr (str))
-    syscall_exit (EXIT_FAILURE);
+  size_t offset = (uintptr_t) ptr % PGSIZE;
 
-  /* Calculate the offset of the string within the (first) page. */
-  size_t offset = (uintptr_t) str % PGSIZE;
-
-  /* We move page by page, checking if the page is mapped to physical memory. */
   for (;;)
-  {
-    void *page = pg_round_down (str);
+    {
+      void *page = pg_round_down (ptr);
 
-    /* If we reach addresses that are not mapped to physical memory before the
-       end of the string, the thread is terminated. */
-    if (!is_user_vaddr(page) ||
-        (pagedir_get_page (thread_current ()->pagedir, page) == NULL &&
-         !try_fetch_page (page, false)))
-      syscall_exit (EXIT_FAILURE);
+      if (!is_user_vaddr (page))
+        syscall_exit (EXIT_FAILURE);
+      if (!is_user_vaddr (ptr))
+        syscall_exit (EXIT_FAILURE);
+      int result;
+      if ((result = get_user ((const uint8_t *)ptr)) == -1)
+        syscall_exit (EXIT_FAILURE);
+      if (check_write && !put_user ((uint8_t *)ptr, result))
+        syscall_exit (EXIT_FAILURE);
 
-    while (offset < PGSIZE)
+      while (offset < PGSIZE)
       {
-        if (*str == '\0')
+        if (*ptr == '\0')
           return; /* We reached the end of the string without issues. */
 
-        str++;
+        ptr++;
         offset++;
       }
 
-    offset = 0; /* Next page will start at the beginning. */
-  }
+      offset = 0;
+
+    }
 }
+
+/* PROVIDED BY SPEC.
+   Reads a byte at user virtual address UADDR.
+   UADDR must be below PHYS_BASE.
+   Returns the byte value if successful, -1 if a segfault occurred. */
+static int
+get_user (const uint8_t *uaddr)
+{
+  int result;
+  asm ("movl $1f, %0; movzbl %1, %0; 1:" : "=&a"(result) : "m"(*uaddr));
+  return result;
+}
+
+/* PROVIDED BY SPEC.
+   Writes BYTE to user address UDST.
+   UDST must be below PHYS_BASE.
+   Returns true if successful, false if a segfault occurred. */
+static bool
+put_user (uint8_t *udst, uint8_t byte)
+{
+  int error_code;
+  asm ("movl $1f, %0; movb %b2, %1; 1:"
+       : "=&a"(error_code), "=m"(*udst)
+       : "q"(byte));
+  return error_code != -1;
+}