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

Add validation to check for missing files in exec() args

See merge request lab2425_autumn/pintos_22!36

src/lib/kernel/list.c

@@ -170,6 +170,9 @@ list_insert (struct list_elem *before, struct list_elem *elem)
 {
   ASSERT (is_interior (before) || is_tail (before));
   ASSERT (elem != NULL);
+  // Sanity checks to prevent (some) loop lists
+  ASSERT (before != elem);
+  ASSERT (before->prev != elem);
 
   elem->prev = before->prev;
   elem->next = before;

src/threads/synch.c

@@ -212,6 +212,7 @@ donate_priority (struct thread *donee) {
   ASSERT (intr_get_level () == INTR_OFF);
 
   struct thread *donor = thread_current ();
+  list_remove (&donor->donor_elem);
   list_push_back (&donee->donors_list, &donor->donor_elem);
   
   while (donee != NULL)
@@ -260,6 +261,7 @@ lock_acquire (struct lock *lock)
   ASSERT (!lock_held_by_current_thread (lock));
 
   struct thread *t = thread_current ();
+  ASSERT (t->waiting_lock == NULL);
 
   enum intr_level old_level = intr_disable ();
   if (lock->holder != NULL)

src/threads/thread.c

@@ -1,5 +1,6 @@
 #include "threads/thread.h"
 #include <debug.h>
+#include <hash.h>
 #include <stddef.h>
 #include <random.h>
 #include <stdio.h>
@@ -9,12 +10,14 @@
 #include "threads/flags.h"
 #include "threads/interrupt.h"
 #include "threads/intr-stubs.h"
+#include "threads/malloc.h"
 #include "threads/palloc.h"
 #include "threads/switch.h"
 #include "threads/synch.h"
 #include "threads/vaddr.h"
 #ifdef USERPROG
 #include "userprog/process.h"
+#include "userprog/syscall.h"
 #endif
 
 /* Random value for struct thread's `magic' member.
@@ -68,6 +71,7 @@ static void kernel_thread (thread_func *, void *aux);
 static void idle (void *aux UNUSED);
 static struct thread *running_thread (void);
 static struct thread *next_thread_to_run (void);
+static void init_process_result (struct thread *t);
 static void init_thread (struct thread *, const char *name, int nice,
                          int priority, fp32_t recent_cpu);
 static bool is_thread (struct thread *) UNUSED;
@@ -110,6 +114,7 @@ thread_init (void)
                initial_thread_recent_cpu);
   initial_thread->status = THREAD_RUNNING;
   initial_thread->tid = allocate_tid ();
+  initial_thread->result = NULL; /* Main thread cannot be waited for. */
 }
 
 /* Starts preemptive thread scheduling by enabling interrupts.
@@ -236,6 +241,11 @@ thread_create (const char *name, int priority,
   struct thread *parent_thread = thread_current ();
   init_thread (t, name, parent_thread->nice, priority, parent_thread->recent_cpu);
   tid = t->tid = allocate_tid ();
+  init_process_result (t);
+
+  #ifdef USERPROG
+    hash_init (&t->open_files, fd_hash, fd_less, NULL);
+  #endif
 
   /* Prepare thread for first run by initializing its stack.
      Do this atomically so intermediate values for the 'stack' 
@@ -259,6 +269,10 @@ thread_create (const char *name, int priority,
 
   intr_set_level (old_level);
 
+  /* No need to synchronise child_results since it is only ever accessed by one
+     thread. By the nature of increasing TIDs, this list is ordered. */
+  list_push_back (&parent_thread->child_results, &t->result->elem);
+
   /* Add to run queue. */
   thread_unblock (t);
 
@@ -359,7 +373,9 @@ thread_exit (void)
      and schedule another process.  That process will destroy us
      when it calls thread_schedule_tail(). */
   intr_disable ();
-  list_remove (&thread_current()->allelem);
+  struct thread *t = thread_current ();
+  list_remove (&t->allelem);
+  list_remove (&t->donor_elem);
   thread_current ()->status = THREAD_DYING;
   schedule ();
   NOT_REACHED ();
@@ -632,6 +648,18 @@ is_thread (struct thread *t)
   return t != NULL && t->magic == THREAD_MAGIC;
 }
 
+/* Allocate and initialise a process result for given thread. */
+static void
+init_process_result (struct thread *t)
+{
+  struct process_result *result = malloc (sizeof (struct process_result));
+  result->tid = t->tid;
+  result->exit_status = t->exit_status;
+  lock_init (&result->lock);
+  sema_init (&result->sema, 0);
+  t->result = result;
+}
+
 /* Does basic initialization of T as a blocked thread named
    NAME. */
 static void
@@ -653,6 +681,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->base_priority
       = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
   list_init (&t->donors_list);
+  list_push_back (&t->donors_list, &t->donor_elem);
   t->waiting_lock = NULL;
 
   t->nice = nice;
@@ -660,6 +689,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->priority = t->base_priority;
 
   t->exit_status = -1;
+  list_init (&t->child_results);
 
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);

src/threads/thread.h

@@ -2,8 +2,11 @@
 #define THREADS_THREAD_H
 
 #include <debug.h>
+#include <hash.h>
 #include <list.h>
 #include <stdint.h>
+#include <stdbool.h>
+#include "threads/synch.h"
 #include "threads/fixed-point.h"
 
 /* States in a thread's life cycle. */
@@ -29,6 +32,18 @@ typedef int tid_t;
 #define NICE_DEFAULT 0 /* Default niceness. */
 #define NICE_MAX 20    /* Highest niceness. */
 
+/* A process result, synchronised between parent and child. */
+struct process_result
+{
+  tid_t tid;             /* The tid of the child process. */
+  int exit_status;       /* The exit status of the child process. Initially set
+                            to -1, then to exit_status when child dies. */
+  struct lock lock;      /* Lock the exit_status and sema. */
+  struct semaphore sema; /* Semaphore to signal the parent that the exit_status
+                            has been set. */
+  struct list_elem elem; /* List element for the parent's children list. */
+};
+
 /* A kernel thread or user process.
 
    Each thread structure is stored in its own 4 kB page.  The
@@ -108,6 +123,12 @@ struct thread
     int nice;                           /* Nice value for this thread */
     fp32_t recent_cpu;                  /* Amount of time this process received */
 
+    /* Process wait properties. */
+    struct process_result *result;      /* Result of the process. */
+    struct list child_results;          /* List of children's of this thread
+                                           process results. */
+    struct file *exec_file;             /* Thread's currently running file */
+
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
@@ -116,6 +137,7 @@ struct thread
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
+    struct hash open_files;             /* Hash Table of FD -> Struct File */
 #endif
 
     /* Owned by thread.c. */

src/userprog/process.c

@@ -1,12 +1,14 @@
 #include "userprog/process.h"
 #include <debug.h>
 #include <inttypes.h>
+#include <list.h>
 #include <round.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "userprog/gdt.h"
 #include "userprog/pagedir.h"
+#include "userprog/syscall.h"
 #include "userprog/tss.h"
 #include "filesys/directory.h"
 #include "filesys/file.h"
@@ -45,6 +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. */
+    struct semaphore load_sema;
+
+    bool success;
   };
 
 static thread_func start_process NO_RETURN;
@@ -60,11 +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;
-    }
+  struct process_start_data data;
 
   /* Make a copy of command.
      Otherwise there's a race between the caller and load(). */
@@ -78,17 +79,39 @@ 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
+          might be necessary. */
+  lock_acquire (&filesys_lock);
+  /* Validates that the current file to be executed is a valid file */
+  bool valid_file = filesys_open (file_name) != NULL;
+  lock_release (&filesys_lock);
+  if (!valid_file)
+    return TID_ERROR;
 
   /* 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.load_sema);
+      if (!data.success)
+        tid = TID_ERROR;
+    }
+
   return tid;
 }
 
@@ -106,7 +129,6 @@ static void
 start_process (void *proc_start_data)
 {
   struct intr_frame if_;
-  bool success;
 
   struct process_start_data *data = proc_start_data;
 
@@ -115,25 +137,36 @@ start_process (void *proc_start_data)
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
-  success = load (data->file_name, &if_.eip, &if_.esp);
   
-  /* If load failed, quit. */
-  if (!success) 
+  lock_acquire (&filesys_lock);
+  
+  /* Prevent writing to the file being executed. */
+  struct file *exec_file = filesys_open (data->file_name);
+  thread_current ()->exec_file = exec_file;
+  file_deny_write (exec_file);
+  lock_release (&filesys_lock);
+
+  data->success = load (data->file_name, &if_.eip, &if_.esp);
+
+  /* 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)
     {
-      process_exit ();
-      goto fail;
+      thread_exit ();
     }
 
   /* Start the user process by simulating a return from an
@@ -144,11 +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, free its common resources and exit. */
- fail:
-  free (data);
-  thread_exit ();
 }
 
 /* Helper function that initializes the stack of a newly created 
@@ -261,23 +289,110 @@ push_to_stack (void **esp, void *data, size_t data_size)
 int
 process_wait (tid_t child_tid UNUSED) 
 {
-  /* As a temporary wait, waiting will just put the thread to sleep for one
-     second (TIMER_FREQ = 100 ticks ~ 1 second). */
-  /* TODO: Implement process_wait () correctly. Remove the next line. */
-  timer_sleep (TIMER_FREQ);
+  struct process_result *child_result = NULL;
+  struct list_elem *e;
+  struct thread *cur = thread_current ();
+  for (e = list_begin (&cur->child_results);
+       e != list_end (&cur->child_results); e = list_next (e))
+    {
+      struct process_result *result
+          = list_entry (e, struct process_result, elem);
+      if (result->tid == child_tid)
+        {
+          child_result = result;
+          break;
+        }
+      /* List is ordered, allowing us to break early. */
+      else if (result->tid > child_tid)
+        break;
+    }
+  if (child_result == NULL)
+    {
+      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
+     wait) for it here to ensure we don't free the lock memory before it is
+     released in process_exit. */
+  lock_acquire (&child_result->lock);
+  /* To prevent waiting for child twice, remove it from the list.
+     No need to use lock since this is the only thread with access to
+     the struct process_result now. */
+  list_remove (&child_result->elem);
+  int exit_status = child_result->exit_status;
+  lock_release (&child_result->lock);
+  free (child_result);
 
-  return 0; /* TODO: Change this too */
+  return exit_status;
 }
 
 /* Free the current process's resources. */
 void
 process_exit (void)
 {
+
+
   struct thread *cur = thread_current ();
   uint32_t *pd;
 
   printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
 
+  /* Clean up all open files */
+  hash_destroy (&cur->open_files, fd_cleanup);
+
+  /* Close the executable file. */
+  if (cur->exec_file != NULL)
+    {
+      lock_acquire (&filesys_lock);
+      file_close (cur->exec_file);
+      lock_release (&filesys_lock);
+    }
+
+  /* Update process result. */
+  if (cur->result != NULL)
+    {
+      lock_acquire (&cur->result->lock);
+      cur->result->exit_status = cur->exit_status;
+      /* Parent has died, child has to free the struct process_result * */
+      if (sema_try_down (&cur->result->sema))
+        {
+          lock_release (&cur->result->lock);
+          free (cur->result);
+        }
+      /* Parent is still alive and will be the one to free the
+         struct process_result *, and may be waiting so call sema_up */
+      else
+        {
+          sema_up (&cur->result->sema);
+          lock_release (&cur->result->lock);
+        }
+    }
+
+  /* Free child process results or signal parent's death. */
+  struct list_elem *e;
+  for (e = list_begin (&cur->child_results);
+       e != list_end (&cur->child_results);)
+    {
+      struct process_result *result
+          = list_entry (e, struct process_result, elem);
+      struct list_elem *next = list_next (e);
+      lock_acquire (&result->lock);
+      /* Child has died (and was not waited for). Free the result. */
+      if (sema_try_down (&result->sema))
+        {
+          lock_release (&result->lock);
+          free (result);
+        }
+      /* Child is still alive, signal via sema that parent has died. */
+      else
+        {
+          sema_up (&result->sema);
+          lock_release (&result->lock);
+        }
+      e = next;
+    }
+
   /* Destroy the current process's page directory and switch back
      to the kernel-only page directory. */
   pd = cur->pagedir;
@@ -394,6 +509,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
   off_t file_ofs;
   bool success = false;
   int i;
+  lock_acquire (&filesys_lock);
 
   /* Allocate and activate page directory. */
   t->pagedir = pagedir_create ();
@@ -493,6 +609,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
  done:
   /* We arrive here whether the load is successful or not. */
   file_close (file);
+  lock_release (&filesys_lock);
   return success;
 }
 

src/userprog/syscall.c

@@ -1,13 +1,27 @@
 #include "userprog/syscall.h"
 #include "devices/shutdown.h"
 #include "devices/input.h"
+#include "filesys/file.h"
+#include "filesys/filesys.h"
 #include "threads/vaddr.h"
 #include "threads/interrupt.h"
+#include "threads/malloc.h"
 #include "threads/thread.h"
+#include "threads/synch.h"
 #include "userprog/process.h"
+#include "userprog/pagedir.h"
 #include <stdio.h>
 #include <syscall-nr.h>
 
+static unsigned fd_counter = MIN_USER_FD;
+
+struct open_file
+  {
+    int fd;                     /* File Descriptor / Identifier */
+    struct file *file;          /* Pointer to the associated file */
+    struct hash_elem elem;      /* elem for a hash table */
+  };
+
 static void syscall_handler (struct intr_frame *);
 
 /* A syscall_function is a function that receives up to 3 arguments, the
@@ -30,6 +44,7 @@ static void syscall_seek (int fd, unsigned position);
 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 *ptr, size_t size);
 
 /* A struct defining a syscall_function pointer along with its arity. */
@@ -63,12 +78,19 @@ static const syscall_arguments syscall_lookup[] =
 static const int LOOKUP_SIZE
   = sizeof (syscall_lookup) / sizeof (syscall_arguments);
 
+
+/* Initialises the syscall handling system, as well as a global lock to
+   synchronise all file access between processes. */
 void
 syscall_init (void) 
 {
   intr_register_int (0x30, 3, INTR_ON, syscall_handler, "syscall");
+  lock_init (&filesys_lock);
 }
 
+/* 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)
 {
@@ -94,6 +116,8 @@ syscall_handler (struct intr_frame *f)
   f->eax = syscall.function (args[0], args[1], args[2]);
 }
 
+/* Called upon a "halt" syscall, resulting in a complete shutdown of the
+   process, via shutdown_power_off (); */
 static void
 syscall_halt (void)
 {
@@ -109,53 +133,118 @@ syscall_exit (int status)
   thread_exit ();
 }
 
+/* Executes a given command with the relevant args, by calling process_execute.
+   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 UNUSED)
+syscall_exec (const char *cmd_line)
 {
-  //TODO
-  return 0;
+  validate_user_pointer (cmd_line, 1);
+
+  pid_t pid = process_execute(cmd_line);
+
+  return pid;
 }
 
+/* Handles the syscall of wait. Effectively a wrapper for process_wait as the
+   necessary validation and such all happens in process_wait anyway. */
 static int
 syscall_wait (pid_t pid)
 {
   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 UNUSED, unsigned initial_size UNUSED)
 {
-  //TODO
-  return 0;
+  validate_user_pointer (file, 1);
+
+  lock_acquire (&filesys_lock);
+  bool status = filesys_create (file, initial_size);
+  lock_release (&filesys_lock);
+
+  return status;
 }
 
+/* Handles the syscall for file removal. First validates the user file pointer.
+   Acquires the file system lock to prevent synchronisation issues, and then
+   uses FILESYS_REMOVE to remove the file, returning the same success status */
 static bool
-syscall_remove (const char *file UNUSED)
+syscall_remove (const char *file)
 {
-  //TODO
-  return 0;
+  validate_user_pointer (file, 1);
+
+  lock_acquire (&filesys_lock);
+  bool status = filesys_remove (file);
+  lock_release (&filesys_lock);
+
+  return status;
 }
 
+/* Handles the syscall for opening a file connection. First, validates the file
+   pointer. Then it acquires a lock for the file system, in order to open the
+   connection without synchronisation issues. It then maps a new fd to this file
+   in the hash table before returning the fd. */
 static int
-syscall_open (const char *file UNUSED)
+syscall_open (const char *file)
 {
-  //TODO
-  return 0;
+  validate_user_pointer (file, 1);
+
+  lock_acquire (&filesys_lock);
+  struct file *ptr = filesys_open (file);
+  lock_release (&filesys_lock);
+  if (ptr == NULL)
+    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)
+    return -1;
+
+  /* Populate the above struct, with a unique FD and the current open file */
+  file_info->fd = fd_counter++;
+  file_info->file = ptr;
+
+  /* Add the new FD->file mapping to the hashtable for the current thread */
+  hash_insert (&thread_current ()->open_files, &file_info->elem);
+
+  /* Return the new FD */
+  return file_info->fd;
 }
 
+/* Handles the syscall for getting a file's size. Converts a provided FD into
+   the asssociated file struct. Acquire the lock for the filesystem and use
+   FILE_LENGTH to calculate the length for return. */
 static int
-syscall_filesize (int fd UNUSED)
+syscall_filesize (int fd)
 {
-  //TODO
-  return 0;
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info == NULL)
+      return -1;
+
+  lock_acquire (&filesys_lock);
+  int bytes = file_length (file_info->file);
+  lock_release (&filesys_lock);
+
+  return bytes;
 }
 
+/* Handles the syscall for reading SIZE bytes from a file referenced by FD.
+   If the FD references the console, use input_getc (), otherwise convert the
+   FD to its associated file struct, acquire the filesystem lock, read up to
+   SIZE bytes and then return the number of bytes read.*/
 static int
 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)
+  if (fd < 0 || fd == STDOUT_FILENO)
     return -1;
 
   validate_user_pointer (buffer, size);
@@ -172,10 +261,21 @@ syscall_read (int fd, void *buffer, unsigned size)
   else
     {
       /* Reading from a file. */
-      return 0; // TODO: Implement Write to Files
+      struct open_file *file_info = fd_get_file (fd);
+      if (file_info == NULL)
+        return -1;
+
+      lock_acquire (&filesys_lock);
+      int bytes_written = file_read (file_info->file, buffer, size);
+      lock_release (&filesys_lock);
+      return bytes_written;
     }
 }
 
+/* Handles the syscall for writing SIZE bytes to a file referenced by FD.
+   If the FD references the console, use put_buf (), otherwise convert the
+   FD to its associated file struct, acquire the filesystem lock, write up to
+   SIZE bytes and then return the number of bytes written.*/
 static int
 syscall_write (int fd, const void *buffer, unsigned size)
 {
@@ -195,27 +295,120 @@ syscall_write (int fd, const void *buffer, unsigned size)
   else
     {
       /* Writing to a file. */
-      return 0; // TODO: Implement Write to Files
+      struct open_file *file_info = fd_get_file (fd);
+      if (file_info == NULL)
+        return 0;
+
+      lock_acquire (&filesys_lock);
+      int bytes = file_write (file_info->file, buffer, size);
+      lock_release (&filesys_lock);
+
+      return bytes;
     }
 }
 
+/* Handles the syscall for seeking to POSITION bytes in a file referenced by
+   FD. Converts the FD to its associated file struct, acquires the filesystem
+   lock and then uses file_seek to adjust the cursor to a specific position in
+   the file.*/
 static void
-syscall_seek (int fd UNUSED, unsigned position UNUSED)
+syscall_seek (int fd, unsigned position)
 {
-  //TODO
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info != NULL)
+  {
+    lock_acquire (&filesys_lock);
+    file_seek (file_info->file, position);
+    lock_release (&filesys_lock);
+  }
 }
 
+/* Handles the syscall for returning the next byte in a file referenced by
+   FD. Converts the FD to its associated file struct, acquires the filesystem
+   lock and then uses file_tell to read the next byte.*/
 static unsigned
-syscall_tell (int fd UNUSED)
+syscall_tell (int fd)
 {
-  //TODO
-  return 0;
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info == NULL)
+    return 0;
+
+  lock_acquire (&filesys_lock);
+  unsigned pos = file_tell (file_info->file);
+  lock_release (&filesys_lock);
+
+  return pos;
 }
 
+/* Handles the syscall for closing a connection to a file. Converts the FD to
+   its associated file struct. If it exists, it removes it from the hash table,
+   acquires the filesystem lock, and uses file_close to close the connection.*/
 static void
-syscall_close (int fd UNUSED)
+syscall_close (int fd)
 {
-  //TODO
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info != NULL)
+    {
+      hash_delete (&thread_current ()->open_files, &file_info->elem);
+
+      lock_acquire (&filesys_lock);
+      file_close (file_info->file);
+      lock_release (&filesys_lock);
+      free (file_info);
+    }
+}
+
+/* Hashing function needed for the open_file table. Returns a hash for an entry,
+   based on its FD. */
+unsigned
+fd_hash (const struct hash_elem *element, void *aux UNUSED)
+{
+  return hash_int (hash_entry (element, struct open_file, elem)->fd);
+}
+
+/* Comparator function for the open_file table. Compares two entries based on
+   the FDs. */
+bool
+fd_less (const struct hash_elem *a_, const struct hash_elem *b_,
+        void *aux UNUSED)
+{
+  struct open_file *a = hash_entry (a_, struct open_file, elem);
+  struct open_file *b = hash_entry (b_, struct open_file, elem);
+
+  return a->fd < b->fd;
+}
+
+/* Function to clean up an open file entry. Closes the file and frees the
+   associated memory. */
+void
+fd_cleanup (struct hash_elem *e, void *aux UNUSED)
+{
+  struct open_file *file_info = hash_entry (e, struct open_file, elem);
+
+  lock_acquire (&filesys_lock);
+  file_close (file_info->file);
+  lock_release (&filesys_lock);
+
+  free (file_info);
+}
+
+/* Gets a file from its descriptor (FD number). If there is no file with the fd
+   FD it returns NULL. */
+static struct open_file *
+fd_get_file (int fd)
+{
+  /* We have to set up a fake open_file in order to be able to search the hash
+     table. See hash.h. */
+  struct open_file fake_file_info;
+  fake_file_info.fd = fd;
+
+  struct hash_elem *e
+    = hash_find (&thread_current ()->open_files, &fake_file_info.elem);
+
+  if (e == NULL)
+      return NULL;
+
+  return hash_entry (e, struct open_file, elem);
 }
 
 /* Validates if a block of memory starting at PTR and of size SIZE bytes is
@@ -227,8 +420,9 @@ validate_user_pointer (const void *ptr, size_t size)
 {
   if (size > 0 && (ptr == NULL ||
                    !is_user_vaddr (ptr) ||
-                   !is_user_vaddr (ptr + size - 1)))
+                   !is_user_vaddr (ptr + size - 1) ||
+                   pagedir_get_page (thread_current()->pagedir, ptr) == NULL))
     thread_exit ();
 
-  return ptr;
+  return (void *) ptr;
 }

src/userprog/syscall.h

@@ -1,8 +1,19 @@
 #ifndef USERPROG_SYSCALL_H
 #define USERPROG_SYSCALL_H
 
+#include <hash.h>
+#include "threads/synch.h"
+
+#define MIN_USER_FD 2
+
 typedef int pid_t;
 
+struct lock filesys_lock;
+
 void syscall_init (void);
 
+unsigned fd_hash (const struct hash_elem *element, void *aux);
+bool fd_less (const struct hash_elem *a, const struct hash_elem *b, void *aux);
+void fd_cleanup (struct hash_elem *e, void *aux);
+
 #endif /* userprog/syscall.h */