Implemented getcwd sys call, along with a new helper function that wr… (#483)

* Implemented getcwd sys call, along with a new helper function that writes a NULL terminated string to memory.

The reason for this commit is that the sys call was not implemented.

* Fixed logging string for sys_getcwd imp

* Removed semicolon in sys_getcwd function

* Fixed logging string for sys_getcwd imp. The problem was that size is unsigned and the incorrect format symbol was used

* Fixed the following:
1) Included the string param in the doc of the write_string function.
2) Added less verbose code to append a NULL to a string in the write_string function
3) Removed the assert to check if the written string to memory matches with one that is read at a given address in the sys_getcwd function
4) Fixed bug in the return values returned by sys_getcwd

* Fixed error logs in the getcwd function.

* Fixed return values of sys_getcwd function.

* Fixed string format symbol in sys_getcwd function.

* Removed dir exists check from the sys_getcwd function.

* Arranged memory block check in the sys_getcwd function.

* Removed new line after write_string function.

(Thank you @johnfxgalea!)

manticore/core/cpu/abstractcpu.py

@@ -552,6 +552,21 @@ class Cpu(Eventful):
             result.append(Operators.CHR(self.read_int(where + i, 8)))
         return result
 
+    def write_string(self, where, string, max_length=None):
+        '''
+        Writes a string to memory, appending a NULL-terminator at the end.
+        :param int where: Address to write the string to
+        :param str string: The string to write to memory
+        :param int max_length:
+            The size in bytes to cap the string at, or None [default] for no
+            limit. This includes the NULL terminator.
+        '''
+        
+        if max_length is not None:
+            string = string[:max_length-1]
+        
+        self.write_bytes(where, string + '\x00')
+        
     def read_string(self, where, max_length=None):
         '''
         Read a NUL-terminated concrete buffer from memory.

manticore/platforms/linux.py

@@ -1029,6 +1029,34 @@ class Linux(Platform):
         except OSError as e:
             return e.errno
 
+    def sys_getcwd(self, buf, size):
+        '''
+        getcwd - Get the current working directory
+        :param int buf: Pointer to dest array
+        :param size: size in bytes of the array pointed to by the buf 
+        :return: buf (Success), or 0
+        '''
+        
+        try:
+            current_dir = os.getcwd()          
+            length = len(current_dir) + 1
+                      
+            if size > 0 and size < length:
+                logger.info("GETCWD: size is greater than 0, but is smaller than the length"  
+                            "of the path + 1. Returning ERANGE")
+                return -errno.ERANGE
+        
+            if not self.current.memory.access_ok(slice(buf, buf+length), 'w'):
+                logger.info("GETCWD: buf within invalid memory. Returning EFAULT")
+                return -errno.EFAULT
+                      
+            self.current.write_string(buf, current_dir)
+            logger.debug("getcwd(0x%08x, %u) -> <%s> (Size %d)", buf, size, current_dir, length)
+            return length
+        
+        except OSError as e:
+            return -e.errno
+
     def sys_lseek(self, fd, offset, whence):
         '''
         lseek - reposition read/write file offset