Fix Bug: Free all entries in the fd hashtable when the process exits, (Saleh, Ethan)

.gitlab-ci.yml

@@ -23,6 +23,11 @@ test_devices:
   variables:
     DIR: devices
 
+test_filesys:
+  extends: .pintos_tests
+  variables:
+    DIR: filesys
+
 test_threads:
   extends: .pintos_tests
   variables:

src/tests/userprog/Make.tests

@@ -9,14 +9,14 @@ sc-bad-arg sc-bad-num sc-boundary sc-boundary-2 halt exit create-normal		\
 create-empty create-null create-bad-ptr create-long create-exists	\
 create-bound open-normal open-missing open-boundary open-empty		\
 open-null open-bad-ptr open-twice close-normal close-twice close-stdin	\
-close-stdout close-bad-fd read-normal read-bad-ptr read-bad-buf read-boundary	\
-read-zero read-stdout read-bad-fd write-normal write-bad-ptr write-bad-buf	\
+close-stdout close-bad-fd read-normal read-bad-ptr read-boundary	\
+read-zero read-stdout read-bad-fd write-normal write-bad-ptr		\
 write-boundary write-zero write-stdin write-bad-fd exec-once exec-arg	\
 exec-large-arg exec-multiple exec-missing exec-over-arg exec-over-args \
 exec-bad-ptr wait-simple wait-twice wait-killed wait-load-kill \
 wait-bad-pid wait-bad-child multi-recurse multi-child-fd rox-simple	\
 rox-child rox-multichild bad-read bad-write bad-read2 bad-write2        \
-bad-jump bad-jump2 bad-maths overflow-stack)
+bad-jump bad-jump2 bad-maths)
 
 tests/userprog_PROGS = $(tests/userprog_TESTS) $(addprefix \
 tests/userprog/,child-simple child-args child-bad child-close child-rox exec-exit)
@@ -36,7 +36,6 @@ tests/userprog/bad-read2_SRC = tests/userprog/bad-read2.c tests/main.c
 tests/userprog/bad-write2_SRC = tests/userprog/bad-write2.c tests/main.c
 tests/userprog/bad-jump2_SRC = tests/userprog/bad-jump2.c tests/main.c
 tests/userprog/bad-maths_SRC = tests/userprog/bad-maths.c tests/main.c
-tests/userprog/overflow-stack_SRC = tests/userprog/overflow-stack.c tests/main.c
 tests/userprog/sc-boundary_SRC = tests/userprog/sc-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/sc-boundary-2_SRC = tests/userprog/sc-boundary-2.c	\
@@ -67,7 +66,6 @@ tests/userprog/close-stdout_SRC = tests/userprog/close-stdout.c tests/main.c
 tests/userprog/close-bad-fd_SRC = tests/userprog/close-bad-fd.c tests/main.c
 tests/userprog/read-normal_SRC = tests/userprog/read-normal.c tests/main.c
 tests/userprog/read-bad-ptr_SRC = tests/userprog/read-bad-ptr.c tests/main.c
-tests/userprog/read-bad-buf_SRC = tests/userprog/read-bad-buf.c tests/main.c
 tests/userprog/read-boundary_SRC = tests/userprog/read-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/read-zero_SRC = tests/userprog/read-zero.c tests/main.c
@@ -75,7 +73,6 @@ tests/userprog/read-stdout_SRC = tests/userprog/read-stdout.c tests/main.c
 tests/userprog/read-bad-fd_SRC = tests/userprog/read-bad-fd.c tests/main.c
 tests/userprog/write-normal_SRC = tests/userprog/write-normal.c tests/main.c
 tests/userprog/write-bad-ptr_SRC = tests/userprog/write-bad-ptr.c tests/main.c
-tests/userprog/write-bad-buf_SRC = tests/userprog/write-bad-buf.c tests/main.c
 tests/userprog/write-boundary_SRC = tests/userprog/write-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/write-zero_SRC = tests/userprog/write-zero.c tests/main.c
@@ -125,12 +122,10 @@ tests/userprog/close-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/close-twice_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-bad-ptr_PUTFILES += tests/userprog/sample.txt
-tests/userprog/read-bad-buf_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-boundary_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-zero_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-bad-ptr_PUTFILES += tests/userprog/sample.txt
-tests/userprog/write-bad-buf_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-boundary_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-zero_PUTFILES += tests/userprog/sample.txt
 tests/userprog/multi-child-fd_PUTFILES += tests/userprog/sample.txt

src/tests/userprog/Rubric.robustnessCR

@@ -1,9 +1,5 @@
-Full robustness of argument passing and syscall handling code:
-- Test user stack overflow robustness of "exec" system calls and user code.
+Full robustness of argument passing code:
+- Test user stack overflow robustness of "exec" system calls.
 5	exec-over-arg
 5	exec-over-args
-5	overflow-stack
 
-- Test syscall user provided buffer validity checks.
-5	read-bad-buf
-5	write-bad-buf

src/tests/userprog/exec-bad-ptr.ck

@@ -2,7 +2,11 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF']);
+check_expected ([<<'EOF', <<'EOF']);
+(exec-bad-ptr) begin
+(exec-bad-ptr) end
+exec-bad-ptr: exit(0)
+EOF
 (exec-bad-ptr) begin
 exec-bad-ptr: exit(-1)
 EOF

src/tests/userprog/open-bad-ptr.ck

@@ -2,7 +2,11 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF']);
+check_expected ([<<'EOF', <<'EOF']);
+(open-bad-ptr) begin
+(open-bad-ptr) end
+open-bad-ptr: exit(0)
+EOF
 (open-bad-ptr) begin
 open-bad-ptr: exit(-1)
 EOF

src/tests/userprog/overflow-stack.c

@@ -1,17 +0,0 @@
-/* Attempt to overflow the user stack by allocating a 4kB buffer and writing into it.
-   The process must be terminated with -1 exit code until stack growth has been implemented in Task 3
-*/
-
-#include <string.h>
-#include <syscall.h>
-#include "tests/lib.h"
-#include "tests/main.h"
-
-void
-test_main (void)
-{
-  char stack_obj[4096];
-  memset (stack_obj, 'a', sizeof stack_obj);
-  memset (stack_obj+10, '\0', 1);
-  msg ("buffer: %s", stack_obj);
-}

src/tests/userprog/overflow-stack.ck

@@ -1,14 +0,0 @@
-# -*- perl -*-
-use strict;
-use warnings;
-use tests::tests;
-check_expected (IGNORE_USER_FAULTS => 1, [<<'EOF',<<'EOF']);
-(overflow-stack) begin
-overflow-stack: exit(-1)
-EOF
-(overflow-stack) begin
-(overflow-stack) buffer: aaaaaaaaaa
-(overflow-stack) end
-overflow-stack: exit(0)
-EOF
-pass;

src/tests/userprog/read-bad-buf.c

@@ -1,17 +0,0 @@
-/* Passes a buffer to the read system call that starts in valid memory, but runs into kernel space.
-   The process must be terminated with -1 exit code.
-*/
-
-#include <syscall.h>
-#include "tests/lib.h"
-#include "tests/main.h"
-
-void
-test_main (void)
-{
-  int handle;
-  CHECK ((handle = open ("sample.txt")) > 1, "open \"sample.txt\"");
-
-  read (handle, (char *) 0xbfffffe0, 100);
-  fail ("should not have survived read()");
-}

src/tests/userprog/read-bad-buf.ck

@@ -1,10 +0,0 @@
-# -*- perl -*-
-use strict;
-use warnings;
-use tests::tests;
-check_expected (IGNORE_KERNEL_FAULTS => 1, [<<'EOF']);
-(read-bad-buf) begin
-(read-bad-buf) open "sample.txt"
-read-bad-buf: exit(-1)
-EOF
-pass;

src/tests/userprog/read-bad-ptr.ck

@@ -2,7 +2,12 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF']);
+check_expected ([<<'EOF', <<'EOF']);
+(read-bad-ptr) begin
+(read-bad-ptr) open "sample.txt"
+(read-bad-ptr) end
+read-bad-ptr: exit(0)
+EOF
 (read-bad-ptr) begin
 (read-bad-ptr) open "sample.txt"
 read-bad-ptr: exit(-1)

src/tests/userprog/write-bad-buf.c

@@ -1,17 +0,0 @@
-/* Passes a buffer to the write system call that starts in valid memory, but runs into kernel space.
-   The process must be terminated with -1 exit code.
-*/
-
-#include <syscall.h>
-#include "tests/lib.h"
-#include "tests/main.h"
-
-void
-test_main (void)
-{
-  int handle;
-  CHECK ((handle = open ("sample.txt")) > 1, "open \"sample.txt\"");
-
-  write (handle, (char *) 0xbffffff0, 32);
-  fail ("should have exited with -1");
-}

src/tests/userprog/write-bad-buf.ck

@@ -1,10 +0,0 @@
-# -*- perl -*-
-use strict;
-use warnings;
-use tests::tests;
-check_expected (IGNORE_KERNEL_FAULTS => 1, [<<'EOF']);
-(write-bad-buf) begin
-(write-bad-buf) open "sample.txt"
-write-bad-buf: exit(-1)
-EOF
-pass;

src/tests/userprog/write-bad-ptr.ck

@@ -2,7 +2,12 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF']);
+check_expected ([<<'EOF', <<'EOF']);
+(write-bad-ptr) begin
+(write-bad-ptr) open "sample.txt"
+(write-bad-ptr) end
+write-bad-ptr: exit(0)
+EOF
 (write-bad-ptr) begin
 (write-bad-ptr) open "sample.txt"
 write-bad-ptr: exit(-1)

src/userprog/process.c

@@ -47,9 +47,9 @@ struct process_start_data
                           tokens while maintaining state. */
     char file_name[FNAME_MAX_LEN + 1]; /* Name of the file of the process to
                                       be started. */
-    bool success; /* Indicates whether the process was successfully loaded. */
-    struct semaphore loaded; /* Semaphore used to signal that the process has
-                                been loaded. */
+    struct semaphore load_sema;
+
+    bool success;
   };
 
 static thread_func start_process NO_RETURN;
@@ -65,13 +65,7 @@ process_execute (const char *cmd)
   char *cmd_copy;
   tid_t tid;
   
-  struct process_start_data *data = malloc (sizeof (struct process_start_data));
-  if (data == NULL)
-    {
-      return TID_ERROR;
-    }
-  sema_init (&data->loaded, 0);
-  data->success = false;
+  struct process_start_data data;
 
   /* Make a copy of command.
      Otherwise there's a race between the caller and load(). */
@@ -85,7 +79,7 @@ process_execute (const char *cmd)
 
   /* Retrieve first argument of command, which is the file name
      of the process. */
-  char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
+  char *file_name = strtok_r (cmd_copy, " ", &data.cmd_saveptr);
 
   /* NOTE: Currently, the file being executed is closed in load () and then
           reopened here. Because load is an exported public function, this
@@ -100,19 +94,24 @@ process_execute (const char *cmd)
   /* Create a new thread to execute the command, by initializing
      it running the function 'start_process' with the appropriate
      arguments. For details of arguments, see 'start_process'. */
-  data->cmd = cmd_copy;
-  strlcpy (data->file_name, file_name, FNAME_MAX_LEN + 1);
+  data.cmd = cmd_copy;
+  strlcpy (data.file_name, file_name, FNAME_MAX_LEN + 1);
+  sema_init (&data.load_sema, 0);
+  data.success = false;
+
+  tid = thread_create (file_name, PRI_DEFAULT, start_process, &data);
 
-  tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
   if (tid == TID_ERROR)
-    palloc_free_page (cmd_copy);
+    {
+      palloc_free_page (cmd_copy);
+    }
   else
     {
-      sema_down (&data->loaded);
-      if (!data->success)
+      sema_down (&data.load_sema);
+      if (!data.success)
         tid = TID_ERROR;
     }
-  free (data);
+
   return tid;
 }
 
@@ -130,7 +129,6 @@ static void
 start_process (void *proc_start_data)
 {
   struct intr_frame if_;
-  bool success;
 
   struct process_start_data *data = proc_start_data;
 
@@ -144,37 +142,33 @@ start_process (void *proc_start_data)
   
   /* Prevent writing to the file being executed. */
   struct file *exec_file = filesys_open (data->file_name);
-  if (exec_file == NULL)
-    {
-      lock_release (&filesys_lock);
-      goto fail;
-    }
   thread_current ()->exec_file = exec_file;
   file_deny_write (exec_file);
   lock_release (&filesys_lock);
 
-  success = load (data->file_name, &if_.eip, &if_.esp);
+  data->success = load (data->file_name, &if_.eip, &if_.esp);
 
-  /* If load failed, quit. */
-  if (!success) 
+  /* If load failed, free process startup data and quit. */
+  if (!data->success)
     {
       palloc_free_page (data->cmd);
-      goto fail;
+      sema_up (&data->load_sema);
+      thread_exit ();
     }
 
   /* Initialize user process stack and free page used to store the
      command that executed the process. */
-  success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
+  bool success = process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
   palloc_free_page (data->cmd);
+  data->success = success;
+  sema_up (&data->load_sema);
   
-  /* If stack initialization failed, free resources and quit. */
+  /* If stack initialization failed, free process resources and quit. */
   if (!success)
     {
-      goto fail;
+      thread_exit ();
     }
 
-  data->success = true;
-  sema_up (&data->loaded);
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
      threads/intr-stubs.S).  Because intr_exit takes all of its
@@ -183,12 +177,6 @@ start_process (void *proc_start_data)
      and jump to it. */
   asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
   NOT_REACHED ();
-
-  /* If starting the process failed, exit. */
-fail:
-  data->success = false;
-  sema_up (&data->loaded);
-  thread_exit ();
 }
 
 /* Helper function that initializes the stack of a newly created 
@@ -319,7 +307,9 @@ process_wait (tid_t child_tid UNUSED)
         break;
     }
   if (child_result == NULL)
-    return -1;
+    {
+      return -1;
+    }
   /* Wait for child to die. */
   sema_down (&child_result->sema);
   /* We need lock release in process_exit, so we need to acquire (and possibly
@@ -333,6 +323,7 @@ process_wait (tid_t child_tid UNUSED)
   int exit_status = child_result->exit_status;
   lock_release (&child_result->lock);
   free (child_result);
+
   return exit_status;
 }
 
@@ -340,6 +331,8 @@ process_wait (tid_t child_tid UNUSED)
 void
 process_exit (void)
 {
+
+
   struct thread *cur = thread_current ();
   uint32_t *pd;
 

src/userprog/syscall.c

@@ -13,9 +13,6 @@
 #include <stdio.h>
 #include <syscall-nr.h>
 
-#define MAX_SYSCALL_ARGS 3
-#define EXIT_FAILURE -1
-
 static unsigned fd_counter = MIN_USER_FD;
 
 struct open_file
@@ -48,19 +45,18 @@ 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);
-static void validate_user_string (const char *str);
+static void *validate_user_pointer (const void *ptr, size_t size);
 
 /* A struct defining a syscall_function pointer along with its arity. */
-struct syscall_arguments
+typedef struct
   {
     syscall_function function;    /* Function pointer. */
     int arity;                    /* Number of arguments of the function. */
-  };
+  } syscall_arguments;
 
 /* A look-up table mapping numbers to system call functions with their number of
    arguments. */
-static const struct syscall_arguments syscall_lookup[] =
+static const syscall_arguments syscall_lookup[] =
 {
   [SYS_HALT] = {(syscall_function) syscall_halt, 0},
   [SYS_EXIT] = {(syscall_function) syscall_exit, 1},
@@ -80,7 +76,8 @@ static const struct syscall_arguments syscall_lookup[] =
 /* The number of syscall functions (i.e, number of elements) within the
    syscall_lookup table. */
 static const int LOOKUP_SIZE
-  = sizeof (syscall_lookup) / sizeof (struct syscall_arguments);
+  = sizeof (syscall_lookup) / sizeof (syscall_arguments);
+
 
 /* Initialises the syscall handling system, as well as a global lock to
    synchronise all file access between processes. */
@@ -91,29 +88,28 @@ syscall_init (void)
   lock_init (&filesys_lock);
 }
 
-/* Function that takes an interrupt frame containing a syscall and its args.
+/* Function that takes a interrupt frame containing a syscall and its args.
    Validates the arguments and pointers before calling the relevant
    high-level system call function, storing its output (if any) in f->eax */
 static void
 syscall_handler (struct intr_frame *f)
 {
   /* First, read the system call number from the stack. */
-  validate_user_pointer (f->esp, sizeof (uintptr_t));
-  uintptr_t syscall_number = *(int *) f->esp;
+  validate_user_pointer (f->esp, 1);
+  unsigned syscall_number = *(int *) f->esp;
 
   /* Ensures the number corresponds to a system call that can be handled. */
   if (syscall_number >= LOOKUP_SIZE)
-    syscall_exit (EXIT_FAILURE);
+    thread_exit ();
 
-  struct syscall_arguments syscall = syscall_lookup[syscall_number];
+  syscall_arguments syscall = syscall_lookup[syscall_number];
 
   /* Next, read and copy the arguments from the stack pointer. */
   validate_user_pointer (f->esp + sizeof (uintptr_t),
                          syscall.arity * sizeof (uintptr_t));
-
-  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));
+  uintptr_t args[3] = {0};
+  for (int i=0; i < syscall.arity; i++)
+      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. */
@@ -138,13 +134,17 @@ syscall_exit (int status)
 }
 
 /* Executes a given command with the relevant args, by calling process_execute.
-   Returns PID for the process that is running the CMD_LINE. */
+   Acquires the filesystem lock as process_execute accesses the file system.
+   Returns PID for the process that is running the CMD_LINE
+ */
 static pid_t
 syscall_exec (const char *cmd_line)
 {
-  validate_user_string (cmd_line);
+  validate_user_pointer (cmd_line, 1);
 
-  return process_execute (cmd_line); /* Returns the PID of the new process */
+  pid_t pid = process_execute(cmd_line);
+
+  return pid;
 }
 
 /* Handles the syscall of wait. Effectively a wrapper for process_wait as the
@@ -152,16 +152,16 @@ syscall_exec (const char *cmd_line)
 static int
 syscall_wait (pid_t pid)
 {
-  return process_wait (pid); /* Returns the exit status of the waited process */
+  return process_wait (pid);
 }
 
 /* Handles the syscall for file creation. First validates the user file
    pointer. Acquires the file system lock to prevent synchronisation issues,
    and then uses FILESYS_CREATE to create the file, returning the same status */
 static bool
-syscall_create (const char *file, unsigned initial_size)
+syscall_create (const char *file UNUSED, unsigned initial_size UNUSED)
 {
-  validate_user_string (file);
+  validate_user_pointer (file, 1);
 
   lock_acquire (&filesys_lock);
   bool status = filesys_create (file, initial_size);
@@ -176,7 +176,7 @@ syscall_create (const char *file, unsigned initial_size)
 static bool
 syscall_remove (const char *file)
 {
-  validate_user_string (file);
+  validate_user_pointer (file, 1);
 
   lock_acquire (&filesys_lock);
   bool status = filesys_remove (file);
@@ -192,23 +192,20 @@ syscall_remove (const char *file)
 static int
 syscall_open (const char *file)
 {
-  validate_user_string (file);
+  validate_user_pointer (file, 1);
 
   lock_acquire (&filesys_lock);
   struct file *ptr = filesys_open (file);
   lock_release (&filesys_lock);
   if (ptr == NULL)
-    return EXIT_FAILURE;
+    return -1;
 
   /* Allocate space for a struct representing a mapping from an FD to a struct
      file. */
   struct open_file *file_info
     = (struct open_file*) malloc (sizeof (struct open_file));
   if (file_info == NULL)
-    {
-      file_close (ptr);
-      return EXIT_FAILURE;
-    }
+    return -1;
 
   /* Populate the above struct, with a unique FD and the current open file */
   file_info->fd = fd_counter++;
@@ -229,7 +226,7 @@ syscall_filesize (int fd)
 {
   struct open_file *file_info = fd_get_file (fd);
   if (file_info == NULL)
-      return EXIT_FAILURE;
+      return -1;
 
   lock_acquire (&filesys_lock);
   int bytes = file_length (file_info->file);
@@ -248,7 +245,7 @@ syscall_read (int fd, void *buffer, unsigned size)
   /* Only console (fd = 0) or other files, not including STDOUT, (fd > 1) are
      allowed. */
   if (fd < 0 || fd == STDOUT_FILENO)
-    return EXIT_FAILURE;
+    return -1;
 
   validate_user_pointer (buffer, size);
 
@@ -266,7 +263,7 @@ syscall_read (int fd, void *buffer, unsigned size)
       /* Reading from a file. */
       struct open_file *file_info = fd_get_file (fd);
       if (file_info == NULL)
-        return EXIT_FAILURE;
+        return -1;
 
       lock_acquire (&filesys_lock);
       int bytes_written = file_read (file_info->file, buffer, size);
@@ -414,57 +411,18 @@ 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. */
-static void
-validate_user_pointer (const void *start, size_t size)
+/* Validates if a block of memory starting at PTR and of size SIZE bytes is
+   fully contained within user virtual memory. Kills the thread (by calling
+   thread_exit) if the memory is invalid. Otherwise, returns the PTR given.
+   If the size is 0, the function does no checks and returns PTR.*/
+static void *
+validate_user_pointer (const void *ptr, size_t size)
 {
-  if (size == 0)
-    return;
+  if (size > 0 && (ptr == NULL ||
+                   !is_user_vaddr (ptr) ||
+                   !is_user_vaddr (ptr + size - 1) ||
+                   pagedir_get_page (thread_current()->pagedir, ptr) == NULL))
+    thread_exit ();
 
-  const void *end = start + size - 1;
-
-  if (start == NULL || !is_user_vaddr (start) || !is_user_vaddr (end))
-    syscall_exit (EXIT_FAILURE);
-
-  /* We now need to check if the entire memory block is mapped to physical
-     memory by the page table. */
-  for (const void *ptr = start; ptr <= end; ptr += PGSIZE)
-    if (pagedir_get_page (thread_current ()->pagedir, ptr) == NULL)
-      syscall_exit (EXIT_FAILURE);
-}
-
-/* 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. */
-static void
-validate_user_string (const char *str)
-{
-  if (str == NULL || !is_user_vaddr (str))
-    syscall_exit (EXIT_FAILURE);
-
-  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);
-
-    if (!is_user_vaddr(page) ||
-        pagedir_get_page (thread_current ()->pagedir, page) == NULL)
-      syscall_exit (EXIT_FAILURE);
-
-    while (offset < PGSIZE)
-      {
-        if (*str == '\0')
-          return; /* We reached the end of the string without issues. */
-
-        str++;
-        offset++;
-      }
-
-    offset = 0; /* Next page will start at the beginning. */
-  }
+  return (void *) ptr;
 }