Update fd_hash to use the fd itself as the hash value for performance, w/ G & E

Remove args-ovf user program binary, replacing with source for testing

See merge request lab2425_autumn/pintos_22!44

src/examples/Makefile

@@ -4,7 +4,7 @@ SRCDIR = ..
 # To add a new test, put its name on the PROGS list
 # and then add a name_SRC line that lists its source files.
 PROGS = cat cmp cp echo halt hex-dump mcat mcp rm \
-	bubsort insult lineup matmult recursor
+	bubsort insult lineup matmult recursor args-ovf
 
 # Should work from task 2 onward.
 cat_SRC = cat.c
@@ -18,6 +18,7 @@ lineup_SRC = lineup.c
 ls_SRC = ls.c
 recursor_SRC = recursor.c
 rm_SRC = rm.c
+args-ovf_SRC = args-ovf.c
 
 # Should work in task 3; also in task 4 if VM is included.
 bubsort_SRC = bubsort.c

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/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-boundary	\
-read-zero read-stdout read-bad-fd write-normal write-bad-ptr		\
+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	\
 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)
+bad-jump bad-jump2 bad-maths overflow-stack)
 
 tests/userprog_PROGS = $(tests/userprog_TESTS) $(addprefix \
 tests/userprog/,child-simple child-args child-bad child-close child-rox exec-exit)
@@ -36,6 +36,7 @@ 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	\
@@ -66,6 +67,7 @@ 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
@@ -73,6 +75,7 @@ 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
@@ -122,10 +125,12 @@ 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

@@ -1,5 +1,9 @@
-Full robustness of argument passing code:
-- Test user stack overflow robustness of "exec" system calls.
+Full robustness of argument passing and syscall handling code:
+- Test user stack overflow robustness of "exec" system calls and user code.
 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

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

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

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

src/tests/userprog/overflow-stack.c

@@ -0,0 +1,17 @@
+/* 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

@@ -0,0 +1,14 @@
+# -*- 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

@@ -0,0 +1,17 @@
+/* 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

@@ -0,0 +1,10 @@
+# -*- 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

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

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

@@ -0,0 +1,17 @@
+/* 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

@@ -0,0 +1,10 @@
+# -*- 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

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

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
@@ -653,13 +681,16 @@ 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;
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
+  t->fd_counter = MINIMUM_USER_FD;
   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 +32,21 @@ typedef int tid_t;
 #define NICE_DEFAULT 0 /* Default niceness. */
 #define NICE_MAX 20    /* Highest niceness. */
 
+/* File Descriptors. */
+#define MINIMUM_USER_FD 2 /* Minimum file descriptor for user programs. */
+
+/* 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 +126,12 @@ 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 +140,9 @@ struct thread
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
+    unsigned int fd_counter;            /* File descriptor counter for thread's
+                                           open files. */
+    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"
@@ -26,6 +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
@@ -38,13 +44,14 @@ struct arg_elem
    that executes process_start for the purpose of starting a user process. */
 struct process_start_data
   {
-    char *cmd; /* Pointer to a copy of the command used to execute the process.
-                  Allocated a page that must be freed by process_start. */
     char *cmd_saveptr; /* Value pointed to by 'saveptr' argument used by
                           successive calls to strtok_r to split 'cmd' into
                           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
+                                finished attempting to load. */
   };
 
 static thread_func start_process NO_RETURN;
@@ -59,12 +66,7 @@ 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(). */
@@ -78,16 +80,33 @@ 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);
+
+  /* Validates that the current file to be executed can be opened/exists. */
+  lock_acquire (&filesys_lock);
+  struct file *file = filesys_open (file_name);
+  lock_release (&filesys_lock);
+  if (file == NULL)
+    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);
+  strlcpy (data.file_name, file_name, FNAME_MAX_LEN + 1);
+  sema_init (&data.loaded, 0);
+  data.success = false;
+
+  tid = thread_create (file_name, PRI_DEFAULT, start_process, &data);
+
+  /* Wait until process file has finished attempting to load via the child
+     thread before reporting success of starting execution. */
+  if (tid != TID_ERROR)
+    {
+      sema_down (&data.loaded);
+      if (!data.success)
+        tid = TID_ERROR;
+    }
 
-  tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
-  if (tid == TID_ERROR)
   palloc_free_page (cmd_copy);
   return tid;
 }
@@ -100,14 +119,15 @@ static void *push_to_stack (void **esp, void *data, size_t data_size);
 /* Make the current thread execute 'cmd', passing in a copy of the
    command string used for processing, the saveptr used by strtok_r
    (in order to further tokenize the same command and retrieve its
-   arguments), as well as the name of the file being executed. This
-   involves loading the specified file and starting it running. */
+   arguments), the name of the file being executed, and a semaphore that
+   calls sema_up to indicate that the 'success' variable passed to it
+   has been updated to indicate whether the process file loading succeeded. 
+   This involves loading the specified file and calling its main () function
+   with the specified command arguments. */
 static void
 start_process (void *proc_start_data)
 {
   struct intr_frame if_;
-  bool success;
-
   struct process_start_data *data = proc_start_data;
 
   /* Initialize interrupt frame and load executable. */
@@ -115,27 +135,47 @@ 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) 
+  /* Acquire the file system lock to prevent race conditions. */
+  lock_acquire (&filesys_lock);
+
+  struct file *exec_file = filesys_open (data->file_name);
+  if (exec_file == NULL)
     {
-      palloc_free_page (data->cmd);
-      goto fail;
+      /* If the executable file cannot be opened, free resources and quit. */
+      lock_release (&filesys_lock);
+      sema_up (&data->loaded);
+      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);
-  palloc_free_page (data->cmd);
+  /* Deny write to the executable file to prevent writing to it and release the
+     file system lock. */
+  file_deny_write (exec_file);
+  lock_release (&filesys_lock);
 
-  /* If stack initialization failed, free resources and quit. */
-  if (!success)
+  thread_current ()->exec_file = exec_file;
+
+  /* Load the ELF executable file, and store the success of the operation in
+     the 'success' variable in data. */
+  data->success = load (data->file_name, &if_.eip, &if_.esp);
+
+  /* If load was sucessful, initialize user process stack and free page used
+ to store the command that executed the process. */
+  if (data->success)
     {
-      process_exit ();
-      goto fail;
+      data->success =
+        process_init_stack (data->cmd_saveptr, &if_.esp, data->file_name);
     }
 
+  /* Signal that the process has finished attempting to load. */
+  bool success = data->success;
+  sema_up (&data->loaded);
+  
+  /* If the load was unsuccessful or if it was but the stack initialization
+     failed, exit the thread. */
+  if (!success)
+      thread_exit ();
+
   /* 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
@@ -144,11 +184,6 @@ 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 
@@ -156,6 +191,10 @@ start_process (void *proc_start_data)
 static bool
 process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
   {
+    ASSERT (cmd_saveptr != NULL);
+    ASSERT (esp != NULL);
+    ASSERT (file_name != NULL);
+
     /* Load command line argument *data* to user process stack.
        This can't cause overflow due to enforcing that the size of
        command line input must fit in a page. Also keep track
@@ -167,8 +206,12 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
     int arg_count = 0;
     while (arg != NULL)
       {
+        /* filename has already been validated to be a safe-to-access string,
+           so we can safely use strlen here. Filename has already been
+           split from the command line arguments. */
         push_to_stack (esp, arg, (strlen (arg) + 1) * sizeof (char));
 
+        /* Try to allocate memory for the argument pointer. */
         struct arg_elem *arg_elem = malloc (sizeof (struct arg_elem));
         if (arg_elem == NULL)
           {
@@ -177,9 +220,11 @@ process_init_stack (char *cmd_saveptr, void **esp, char *file_name)
             return false;
           }
 
+        /* Store the argument pointer in the linked list. */
         arg_elem->arg = *esp;
         list_push_front (&arg_list, &arg_elem->elem);
 
+        /* Increment the argument count and get the next argument. */
         arg_count++;
         arg = strtok_r (NULL, " ", &cmd_saveptr);
       }
@@ -195,14 +240,23 @@ 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)
+      {
+        /* Calculate the number of pages needed to allocate. */
+        int pages_needed = DIV_CEIL (overflow_bytes, PGSIZE);
+
+        /* Allocate the pages and map them to the user process. */
+        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 * 2, kpage, true))
+            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
        performance. */
@@ -259,14 +313,55 @@ push_to_stack (void **esp, void *data, size_t data_size)
  * This function will be implemented in task 2.
  * For now, it does nothing. */
 int
-process_wait (tid_t child_tid UNUSED) 
+process_wait (tid_t child_tid)
 {
-  /* 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 ();
 
-  return 0; /* TODO: Change this too */
+  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)
+        {
+          /* Found the child process. */
+          child_result = result;
+          break;
+        }
+
+      /* List is ordered, allowing us to break early if the child_tid is
+         greater than the current result's tid. */
+      else if (result->tid > child_tid)
+        break;
+    }
+
+  /* If the child process was not found, return -1. */
+  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);
+
+  /* Get the exit status of the child */
+  int exit_status = child_result->exit_status;
+
+  /* Release the lock */
+  lock_release (&child_result->lock);
+
+  free (child_result);
+  return exit_status;
 }
 
 /* Free the current process's resources. */
@@ -278,6 +373,62 @@ process_exit (void)
 
   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, implicitly allowing it to be written to. */
+  if (cur->exec_file != NULL)
+    {
+      /* Acquire the file system lock to prevent race conditions. */
+      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 +545,7 @@ 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 +645,7 @@ 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 +1,28 @@
 #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>
 
+#define MAX_SYSCALL_ARGS 3
+#define EXIT_FAILURE -1
+
+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
@@ -30,18 +45,20 @@ static void syscall_seek (int fd, unsigned position);
 static unsigned syscall_tell (int fd);
 static void syscall_close (int fd);
 
-static void *validate_user_pointer (const void *ptr, size_t size);
+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);
 
 /* A struct defining a syscall_function pointer along with its arity. */
-typedef struct
+struct syscall_arguments
   {
     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 syscall_arguments syscall_lookup[] =
+static const struct syscall_arguments syscall_lookup[] =
 {
   [SYS_HALT] = {(syscall_function) syscall_halt, 0},
   [SYS_EXIT] = {(syscall_function) syscall_exit, 1},
@@ -61,32 +78,39 @@ static const 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 (syscall_arguments);
+  = sizeof (syscall_lookup) / sizeof (struct 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 an 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, 1);
-  unsigned syscall_number = *(int *) f->esp;
+  validate_user_pointer (f->esp, sizeof (uintptr_t));
+  uintptr_t syscall_number = *(int *) f->esp;
 
   /* Ensures the number corresponds to a system call that can be handled. */
   if (syscall_number >= LOOKUP_SIZE)
-    thread_exit ();
+    syscall_exit (EXIT_FAILURE);
 
-  syscall_arguments syscall = syscall_lookup[syscall_number];
+  struct 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[3] = {0};
-  for (int i=0; i < syscall.arity; i++)
+
+  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));
 
   /* Call the function that handles this system call with the arguments. When
@@ -94,6 +118,8 @@ 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)
 {
@@ -109,55 +135,136 @@ syscall_exit (int status)
   thread_exit ();
 }
 
+/* Executes a given command with the relevant args, by calling process_execute.
+   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 the user string before executing the process. */
+  validate_user_string (cmd_line);
+
+  return process_execute (cmd_line); /* Returns the PID of the new process */
 }
 
+/* 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);
+  return process_wait (pid); /* Returns the exit status of the waited process */
 }
 
+/* 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)
+syscall_create (const char *file, unsigned initial_size)
 {
-  //TODO
-  return 0;
+  /* Validate the user string before creating the file. */
+  validate_user_string (file);
+
+  /* Acquire the file system lock to prevent race conditions. */
+  lock_acquire (&filesys_lock);
+  bool status = filesys_create (file, initial_size);
+  lock_release (&filesys_lock);
+
+  /* Return the status of the file creation. */
+  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 the user string before removing the file. */
+  validate_user_string (file);
+
+  /* Acquire the file system lock to prevent race conditions. */
+  lock_acquire (&filesys_lock);
+  bool status = filesys_remove (file);
+  lock_release (&filesys_lock);
+
+  /* Return the status of the file removal. */
+  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 the user string before opening the file. */
+  validate_user_string (file);
+
+  /* Acquire the file system lock to prevent race conditions. */
+  lock_acquire (&filesys_lock);
+  struct file *ptr = filesys_open (file);
+  lock_release (&filesys_lock);
+
+  /* If the file could not be opened, return failure. */
+  if (ptr == NULL)
+    return EXIT_FAILURE;
+
+  /* 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)
+    {
+      /* If we could not allocate memory for the file_info struct, close the
+         file and return failure. */
+      file_close (ptr);
+      return EXIT_FAILURE;
     }
 
+  /* Populate the above struct, with a unique FD and the current open file */
+  file_info->fd = thread_current ()->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;
+  /* Try to get the file from the FD. If it does not exist, return failure. */
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info == NULL)
+      return EXIT_FAILURE;
+
+  /* Acquire the file system lock to prevent any race conditions. */
+  lock_acquire (&filesys_lock);
+  int bytes = file_length (file_info->file);
+  lock_release (&filesys_lock);
+
+  /* Return the number of bytes in the file. */
+  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)
-    return -1;
+  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);
 
   if (fd == STDIN_FILENO)
@@ -167,15 +274,32 @@ syscall_read (int fd, void *buffer, unsigned size)
       for (int i = 0; i < size; i++)
         write_buffer[i] = input_getc ();
 
+      /* In case of console, read is always (eventually) successful. So return
+         the size for the number of bytes read. */
       return size;
     }
   else
     {
       /* Reading from a file. */
-      return 0; // TODO: Implement Write to Files
+      /* Find the file from the FD. If it does not exist, return failure. */
+      struct open_file *file_info = fd_get_file (fd);
+      if (file_info == NULL)
+        return EXIT_FAILURE;
+
+      /* Acquire the file system lock to prevent race-conditions. */
+      lock_acquire (&filesys_lock);
+      int bytes_written = file_read (file_info->file, buffer, size);
+      lock_release (&filesys_lock);
+
+      /* Return the number of bytes read. */
+      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)
 {
@@ -184,51 +308,214 @@ 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);
 
   if (fd == STDOUT_FILENO)
     {
       /* Writing to the console. */
       putbuf (buffer, size);
+
+      /* In case of console, write is always successful. So return the size for
+         the number of bytes written. */
       return size;
     }
   else
     {
       /* Writing to a file. */
-      return 0; // TODO: Implement Write to Files
-    }
-}
-
-static void
-syscall_seek (int fd UNUSED, unsigned position UNUSED)
-{
-  //TODO
-}
-
-static unsigned
-syscall_tell (int fd UNUSED)
-{
-  //TODO
+      /* Find the file from the FD. If it does not exist, return failure. */
+      struct open_file *file_info = fd_get_file (fd);
+      if (file_info == NULL)
         return 0;
+
+      /* Acquire the file system lock to prevent race conditions. */
+      lock_acquire (&filesys_lock);
+      int bytes = file_write (file_info->file, buffer, size);
+      lock_release (&filesys_lock);
+
+      /* Return the number of bytes written. */
+      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_close (int fd UNUSED)
+syscall_seek (int fd, unsigned position)
 {
-  //TODO
+  /* Find the file from the FD. If it does not exist, do nothing. */
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info != NULL)
+  {
+    /* File exists: Acquire the file system lock to prevent race conditions. */
+    lock_acquire (&filesys_lock);
+    file_seek (file_info->file, position);
+    lock_release (&filesys_lock);
+  }
 }
 
-/* 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)
+/* 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)
 {
-  if (size > 0 && (ptr == NULL ||
-                   !is_user_vaddr (ptr) ||
-                   !is_user_vaddr (ptr + size - 1)))
-    thread_exit ();
+  /* Find the file from the FD. If it does not exist, return 0. */
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info == NULL)
+    return 0;
 
-  return ptr;
+  /* Acquire the file system lock to prevent race conditions. */
+  lock_acquire (&filesys_lock);
+  unsigned pos = file_tell (file_info->file);
+  lock_release (&filesys_lock);
+
+  /* Return the current position in the file. */
+  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)
+{
+  /* Find the file from the FD. If it does not exist, do nothing. */
+  struct open_file *file_info = fd_get_file (fd);
+  if (file_info != NULL)
+    {
+      /* File exists */
+      /* First, remove the file from the hash table of open files. */
+      hash_delete (&thread_current ()->open_files, &file_info->elem);
+
+      /* Then, close the file, acquiring the file system lock to prevent race
+         conditions. */
+      lock_acquire (&filesys_lock);
+      file_close (file_info->file);
+      lock_release (&filesys_lock);
+
+      /* Free the memory allocated for the file_info struct. */
+      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)
+{
+  /* We use the FD as the hash value. This is because the FD is incremented
+     sequentially and is therefore unique for each file. It positively affects
+     the performance of the hash table: 1. It is unique so no need to call
+     expensive hash functions. 2. It being sequential means that the hash table
+     is more likely to be weight balanced. */
+  return 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 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)
+{
+  /* 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))
+    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 = pg_round_down (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)
+{
+  /* Check if the string pointer is a valid user virtual address. */
+  if (str == NULL || !is_user_vaddr (str))
+    syscall_exit (EXIT_FAILURE);
+
+  /* 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);
+
+    /* 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)
+      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. */
+  }
 }

src/userprog/syscall.h

@@ -1,8 +1,17 @@
 #ifndef USERPROG_SYSCALL_H
 #define USERPROG_SYSCALL_H
 
+#include <hash.h>
+#include "threads/synch.h"
+
 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 */