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Doc/lib/librestricted.tex

 \chapter{Restricted Execution}
 
-In general, executing Python programs have complete access to the
-underlying operating system through the various functions and classes
-contained in Python's modules.  For example, a Python program can open
-any file\footnote{Provided the underlying OS gives you permission!}
-for reading and writing by using the
-\code{open()} built-in function.  This is exactly what you want for
-most applications.
+In general, Python programs have complete access to the underlying
+operating system throug the various functions and classes, For
+example, a Python program can open any file for reading and writing by
+using the \code{open()} built-in function (provided the underlying OS
+gives you permission!).  This is exactly what you want for most
+applications.
 
-There is a class of applications for which this ``openness'' is
-inappropriate.  Imagine a web browser that accepts ``applets'', snippets of
-Python code, from anywhere on the Internet for execution on the local
-system.  Since the originator of the code is unknown, it is obvious that it
-cannot be trusted with the full resources of the local machine.
+There exists a class of applications for which this ``openness'' is
+inappropriate.  Take Grail: a web browser that accepts ``applets'',
+snippets of Python code, from anywhere on the Internet for execution
+on the local system.  This can be used to improve the user interface
+of forms, for instance.  Since the originator of the code is unknown,
+it is obvious that it cannot be trusted with the full resources of the
+local machine.
 
-\emph{Restricted execution} is the basic Python framework that allows
+\emph{Restricted execution} is the basic framework in Python that allows
 for the segregation of trusted and untrusted code.  It is based on the
 notion that trusted Python code (a \emph{supervisor}) can create a
-``padded cell' (or environment) of limited permissions, and run the
+``padded cell' (or environment) with limited permissions, and run the
 untrusted code within this cell.  The untrusted code cannot break out
 of its cell, and can only interact with sensitive system resources
-through interfaces defined, and managed by the trusted code.  The term
-``restricted execution'' is favored over the term ``safe-Python''
+through interfaces defined and managed by the trusted code.  The term
+``restricted execution'' is favored over ``safe-Python''
 since true safety is hard to define, and is determined by the way the
 restricted environment is created.  Note that the restricted
 environments can be nested, with inner cells creating subcells of
-lesser, but never greater, privledge.
+lesser, but never greater, privilege.
 
 An interesting aspect of Python's restricted execution model is that
-the attributes presented to untrusted code usually have the same names
+the interfaces presented to untrusted code usually have the same names
 as those presented to trusted code.  Therefore no special interfaces
 need to be learned to write code designed to run in a restricted
 environment.  And because the exact nature of the padded cell is
 \code{chroot()}-like operation on the \var{filename} parameter, such
 that root is always relative to some safe ``sandbox'' area of the
 filesystem.  In this case, the untrusted code would still see an
-\code{open()} function in its \code{__builtin__} module, with the same
+built-in \code{open()} function in its environment, with the same
 calling interface.  The semantics would be identical too, with
 \code{IOError}s being raised when the supervisor determined that an
 unallowable parameter is being used.
 
+The Python run-time determines whether a particular code block is
+executing in restricted execution mode based on the identity of the
+\code{__builtins__} object in its global variables: if this is (the
+dictionary of) the standard \code{__builtin__} module, the code is
+deemed to be unrestricted, else it is deemed to be restricted.
+
+Python code executing in restricted mode faces a number of limitations
+that are designed to prevent it from escaping from the padded cell.
+For instance, the function object attribute \code{func_globals} and the
+class and instance object attribute \code{__dict__} are unavailable.
+
 Two modules provide the framework for setting up restricted execution
 environments:
 

Doc/lib/librexec.tex

 \code{r_exec()}, \code{r_eval()}, \code{r_execfile()}, and
 \code{r_import()} methods, which are restricted versions of the standard
 Python functions \code{exec()}, \code{eval()}, \code{execfile()}, and
-\code{import()}.  Code executed in this restricted environment will
+the \code{import} statement.
+Code executed in this restricted environment will
 only have access to modules and functions that are deemed safe; you
 can subclass \code{RExec} to add or remove capabilities as desired.
 
 unsafe operations like reading or writing disk files, or using TCP/IP
 sockets.  However, it does not protect against code using extremely
 large amounts of memory or CPU time.  
-% XXX is there any protection against this?
 
-\begin{funcdesc}{RExec}{\optional{hooks\, verbose} }
+\begin{funcdesc}{RExec}{\optional{hooks\optional{\, verbose}}}
 Returns an instance of the \code{RExec} class.  
 
-% XXX is ihooks.py documented?  If yes, there should be a ref here
-
 \var{hooks} is an instance of the \code{RHooks} class or a subclass of it.
+If it is omitted or \code{None}, the default \code{RHooks} class is
+instantiated.
 Whenever the RExec module searches for a module (even a built-in one)
 or reads a module's code, it doesn't actually go out to the file
 system itself.  Rather, it calls methods of an RHooks instance that
 object that's part of the RExec object.  This allows another level of
 flexibility, e.g. using packages.)
 
-By providing an alternate RHooks object, we can control the actual
+By providing an alternate RHooks object, we can control the
 file system accesses made to import a module, without changing the
 actual algorithm that controls the order in which those accesses are
 made.  For instance, we could substitute an RHooks object that passes
 mechanism such as ILU.  Grail's applet loader uses this to support
 importing applets from a URL for a directory.
 
-% XXX does verbose actually do anything at the moment?
-If \var{verbose} is true, additional debugging output will be sent to
+If \var{verbose} is true, additional debugging output may be sent to
 standard output.
 \end{funcdesc}
 
-RExec instances have the following attributes, which are used by the
+The RExec class has the following class attributes, which are used by the
 \code{__init__} method.  Changing them on an existing instance won't
 have any effect; instead, create a subclass of \code{RExec} and assign
 them new values in the class definition.  Instances of the new class
 \renewcommand{\indexsubitem}{(RExec object attribute)}
 \begin{datadesc}{nok_builtin_names}
 Contains the names of built-in functions which will \emph{not} be
- available to programs running in the restricted environment.  The
- value for \code{RExec} is \code{('open',} \code{reload',}
- \code{__import__')}.
+available to programs running in the restricted environment.  The
+value for \code{RExec} is \code{('open',} \code{'reload',}
+\code{'__import__')}.  (This gives the exceptions, because by far the
+majority of built-in functions are harmless.  A subclass that wants to
+override this variable should probably start with the value from the
+base class and concatenate additional forbidden functions --- when new
+dangerous built-in functions are added to Python, they will also be
+added to this module.)
 \end{datadesc}
 
 \begin{datadesc}{ok_builtin_modules}
 Contains the names of built-in modules which can be safely imported.
-The value for \code{RExec} is \code{('array',} \code{'binascii',} \code{'audioop',}
-\code{'imageop',} \code{'marshal',} \code{'math',} \code{'md5',} \code{'parser',} \code{'regex',} \code{'rotor',}
-\code{'select',} \code{'strop',} \code{'struct',} \code{'time')}.
+The value for \code{RExec} is \code{('audioop',} \code{'array',}
+\code{'binascii',} \code{'cmath',} \code{'errno',} \code{'imageop',}
+\code{'marshal',} \code{'math',} \code{'md5',} \code{'operator',}
+\code{'parser',} \code{'regex',} \code{'rotor',} \code{'select',}
+\code{'strop',} \code{'struct',} \code{'time')}.  A similar remark
+about overriding this variable applies --- use the value from the base
+class as a starting point.
 \end{datadesc}
 
 \begin{datadesc}{ok_path}
 Contains the directories which will be searched when an \code{import}
 is performed in the restricted environment.  
-The value for \code{RExec} is the same as \code{sys.path} for
-unrestricted code.
+The value for \code{RExec} is the same as \code{sys.path} (at the time
+the module is loaded) for unrestricted code.
 \end{datadesc}
 
 \begin{datadesc}{ok_posix_names}
 \end{datadesc}
 
 \begin{datadesc}{ok_sys_names}
-Contains the names of the functions and variables in the \code{sys} module which will be
-available to programs running in the restricted environment.  The
-value for \code{RExec} is \code{('ps1',} \code{'ps2',}
-\code{'copyright',} \code{'version',} \code{'platform',} \code{'exit',}
-\code{'maxint')}.
+Contains the names of the functions and variables in the \code{sys}
+module which will be available to programs running in the restricted
+environment.  The value for \code{RExec} is \code{('ps1',}
+\code{'ps2',} \code{'copyright',} \code{'version',} \code{'platform',}
+\code{'exit',} \code{'maxint')}.
 \end{datadesc}
 
 RExec instances support the following methods:
 \renewcommand{\indexsubitem}{(RExec object method)}
 
 \begin{funcdesc}{r_eval}{code}
-\var{code} must either be a string containing a Python expression, or a compiled code object, which will
-be evaluated in the restricted environment.  The value of the expression or code object will be returned.
+\var{code} must either be a string containing a Python expression, or
+a compiled code object, which will be evaluated in the restricted
+environment's \code{__main__} module.  The value of the expression or
+code object will be returned.
 \end{funcdesc}
 
 \begin{funcdesc}{r_exec}{code}
-\var{code} must either be a string containing one or more lines of Python code,  or a compiled code object,
-which will be executed in the restricted environment.  
+\var{code} must either be a string containing one or more lines of
+Python code, or a compiled code object, which will be executed in the
+restricted environment's \code{__main__} module.
 \end{funcdesc}
 
 \begin{funcdesc}{r_execfile}{filename}
 Execute the Python code contained in the file \var{filename} in the
-restricted environment.
+restricted environment's \code{__main__} module.
 \end{funcdesc}
 
 Methods whose names begin with \code{s_} are similar to the functions
 beginning with \code{r_}, but the code will be granted access to
-restricted versions of \code{sys.stdin}, \code{sys.stderr}, and
-\code{sys.stdout}.  
+restricted versions of the standard I/O streans \code{sys.stdin},
+\code{sys.stderr}, and \code{sys.stdout}.  
 
 \begin{funcdesc}{s_eval}{code}
 \var{code} must be a string containing a Python expression, which will
 restricted environment.
 \end{funcdesc}
 
-\code{RExec} objects must also support various methods which will be implicitly called 
-by code executing in the restricted environment.  Overriding these
-methods in a subclass is used to change the policies enforced by a restricted environment.
+\code{RExec} objects must also support various methods which will be
+implicitly called by code executing in the restricted environment.
+Overriding these methods in a subclass is used to change the policies
+enforced by a restricted environment.
 
-\begin{funcdesc}{r_import}{modulename\optional{\, globals, locals, fromlist}}
-Import the module \var{modulename}, raising an \code{ImportError} exception
-if the module is considered unsafe.  
+\begin{funcdesc}{r_import}{modulename\optional{\, globals\, locals\, fromlist}}
+Import the module \var{modulename}, raising an \code{ImportError}
+exception if the module is considered unsafe.
 \end{funcdesc}
 
 \begin{funcdesc}{r_open}{filename\optional{\, mode\optional{\, bufsize}}}
 and a file object (or a class instance compatible with file objects)
 should be returned.  \code{RExec}'s default behaviour is allow opening
 any file for reading, but forbidding any attempt to write a file.  See
-the example below for an implementation of a less restrictive \code{r_open()}.
+the example below for an implementation of a less restrictive
+\code{r_open()}.
 \end{funcdesc}
 
 \begin{funcdesc}{r_reload}{module}
 \end{funcdesc}
 
 \begin{funcdesc}{r_unload}{module}
-Unload the module object \var{module}.   
-% XXX what are the semantics of this?  
+Unload the module object \var{module} (i.e., remove it from the
+restricted environment's \code{sys.modules} dictionary).
 \end{funcdesc}
 
+And their equivalents with access to restricted standard I/O streams:
+
 \begin{funcdesc}{s_import}{modulename\optional{\, globals, locals, fromlist}}
-Import the module \var{modulename}, raising an \code{ImportError} exception
-if the module is considered unsafe.  
+Import the module \var{modulename}, raising an \code{ImportError}
+exception if the module is considered unsafe.
 \end{funcdesc}
 
 \begin{funcdesc}{s_reload}{module}
 \bcode\begin{verbatim}
 class TmpWriterRExec(rexec.RExec):
     def r_open(self, file, mode='r', buf=-1):
-        if mode in ('r', 'rb'): pass 
-	elif mode in ('w', 'wb'):
-	    # check filename : must begin with /tmp/
-	    if file[0:5]!='/tmp/': 
-		raise IOError, "can't open files for writing outside of /tmp"
-	    elif string.find(file, '/../')!=-1:
-		raise IOError, "'..' in filename; open for writing forbidden"
+        if mode in ('r', 'rb'):
+            pass
+        elif mode in ('w', 'wb', 'a', 'ab'):
+            # check filename : must begin with /tmp/
+            if file[:5]!='/tmp/': 
+                raise IOError, "can't write outside /tmp"
+            elif (string.find(file, '/../') >= 0 or
+                 file[:3] == '../' or file[-3:] == '/..'):
+                raise IOError, "'..' in filename forbidden"
+        else: raise IOError, "Illegal open() mode"
         return open(file, mode, buf)
 \end{verbatim}\ecode
 

Doc/librestricted.tex

 \chapter{Restricted Execution}
 
-In general, executing Python programs have complete access to the
-underlying operating system through the various functions and classes
-contained in Python's modules.  For example, a Python program can open
-any file\footnote{Provided the underlying OS gives you permission!}
-for reading and writing by using the
-\code{open()} built-in function.  This is exactly what you want for
-most applications.
+In general, Python programs have complete access to the underlying
+operating system throug the various functions and classes, For
+example, a Python program can open any file for reading and writing by
+using the \code{open()} built-in function (provided the underlying OS
+gives you permission!).  This is exactly what you want for most
+applications.
 
-There is a class of applications for which this ``openness'' is
-inappropriate.  Imagine a web browser that accepts ``applets'', snippets of
-Python code, from anywhere on the Internet for execution on the local
-system.  Since the originator of the code is unknown, it is obvious that it
-cannot be trusted with the full resources of the local machine.
+There exists a class of applications for which this ``openness'' is
+inappropriate.  Take Grail: a web browser that accepts ``applets'',
+snippets of Python code, from anywhere on the Internet for execution
+on the local system.  This can be used to improve the user interface
+of forms, for instance.  Since the originator of the code is unknown,
+it is obvious that it cannot be trusted with the full resources of the
+local machine.
 
-\emph{Restricted execution} is the basic Python framework that allows
+\emph{Restricted execution} is the basic framework in Python that allows
 for the segregation of trusted and untrusted code.  It is based on the
 notion that trusted Python code (a \emph{supervisor}) can create a
-``padded cell' (or environment) of limited permissions, and run the
+``padded cell' (or environment) with limited permissions, and run the
 untrusted code within this cell.  The untrusted code cannot break out
 of its cell, and can only interact with sensitive system resources
-through interfaces defined, and managed by the trusted code.  The term
-``restricted execution'' is favored over the term ``safe-Python''
+through interfaces defined and managed by the trusted code.  The term
+``restricted execution'' is favored over ``safe-Python''
 since true safety is hard to define, and is determined by the way the
 restricted environment is created.  Note that the restricted
 environments can be nested, with inner cells creating subcells of
-lesser, but never greater, privledge.
+lesser, but never greater, privilege.
 
 An interesting aspect of Python's restricted execution model is that
-the attributes presented to untrusted code usually have the same names
+the interfaces presented to untrusted code usually have the same names
 as those presented to trusted code.  Therefore no special interfaces
 need to be learned to write code designed to run in a restricted
 environment.  And because the exact nature of the padded cell is
 \code{chroot()}-like operation on the \var{filename} parameter, such
 that root is always relative to some safe ``sandbox'' area of the
 filesystem.  In this case, the untrusted code would still see an
-\code{open()} function in its \code{__builtin__} module, with the same
+built-in \code{open()} function in its environment, with the same
 calling interface.  The semantics would be identical too, with
 \code{IOError}s being raised when the supervisor determined that an
 unallowable parameter is being used.
 
+The Python run-time determines whether a particular code block is
+executing in restricted execution mode based on the identity of the
+\code{__builtins__} object in its global variables: if this is (the
+dictionary of) the standard \code{__builtin__} module, the code is
+deemed to be unrestricted, else it is deemed to be restricted.
+
+Python code executing in restricted mode faces a number of limitations
+that are designed to prevent it from escaping from the padded cell.
+For instance, the function object attribute \code{func_globals} and the
+class and instance object attribute \code{__dict__} are unavailable.
+
 Two modules provide the framework for setting up restricted execution
 environments:
 
 \code{r_exec()}, \code{r_eval()}, \code{r_execfile()}, and
 \code{r_import()} methods, which are restricted versions of the standard
 Python functions \code{exec()}, \code{eval()}, \code{execfile()}, and
-\code{import()}.  Code executed in this restricted environment will
+the \code{import} statement.
+Code executed in this restricted environment will
 only have access to modules and functions that are deemed safe; you
 can subclass \code{RExec} to add or remove capabilities as desired.
 
 unsafe operations like reading or writing disk files, or using TCP/IP
 sockets.  However, it does not protect against code using extremely
 large amounts of memory or CPU time.  
-% XXX is there any protection against this?
 
-\begin{funcdesc}{RExec}{\optional{hooks\, verbose} }
+\begin{funcdesc}{RExec}{\optional{hooks\optional{\, verbose}}}
 Returns an instance of the \code{RExec} class.  
 
-% XXX is ihooks.py documented?  If yes, there should be a ref here
-
 \var{hooks} is an instance of the \code{RHooks} class or a subclass of it.
+If it is omitted or \code{None}, the default \code{RHooks} class is
+instantiated.
 Whenever the RExec module searches for a module (even a built-in one)
 or reads a module's code, it doesn't actually go out to the file
 system itself.  Rather, it calls methods of an RHooks instance that
 object that's part of the RExec object.  This allows another level of
 flexibility, e.g. using packages.)
 
-By providing an alternate RHooks object, we can control the actual
+By providing an alternate RHooks object, we can control the
 file system accesses made to import a module, without changing the
 actual algorithm that controls the order in which those accesses are
 made.  For instance, we could substitute an RHooks object that passes
 mechanism such as ILU.  Grail's applet loader uses this to support
 importing applets from a URL for a directory.
 
-% XXX does verbose actually do anything at the moment?
-If \var{verbose} is true, additional debugging output will be sent to
+If \var{verbose} is true, additional debugging output may be sent to
 standard output.
 \end{funcdesc}
 
-RExec instances have the following attributes, which are used by the
+The RExec class has the following class attributes, which are used by the
 \code{__init__} method.  Changing them on an existing instance won't
 have any effect; instead, create a subclass of \code{RExec} and assign
 them new values in the class definition.  Instances of the new class
 \renewcommand{\indexsubitem}{(RExec object attribute)}
 \begin{datadesc}{nok_builtin_names}
 Contains the names of built-in functions which will \emph{not} be
- available to programs running in the restricted environment.  The
- value for \code{RExec} is \code{('open',} \code{reload',}
- \code{__import__')}.
+available to programs running in the restricted environment.  The
+value for \code{RExec} is \code{('open',} \code{'reload',}
+\code{'__import__')}.  (This gives the exceptions, because by far the
+majority of built-in functions are harmless.  A subclass that wants to
+override this variable should probably start with the value from the
+base class and concatenate additional forbidden functions --- when new
+dangerous built-in functions are added to Python, they will also be
+added to this module.)
 \end{datadesc}
 
 \begin{datadesc}{ok_builtin_modules}
 Contains the names of built-in modules which can be safely imported.
-The value for \code{RExec} is \code{('array',} \code{'binascii',} \code{'audioop',}
-\code{'imageop',} \code{'marshal',} \code{'math',} \code{'md5',} \code{'parser',} \code{'regex',} \code{'rotor',}
-\code{'select',} \code{'strop',} \code{'struct',} \code{'time')}.
+The value for \code{RExec} is \code{('audioop',} \code{'array',}
+\code{'binascii',} \code{'cmath',} \code{'errno',} \code{'imageop',}
+\code{'marshal',} \code{'math',} \code{'md5',} \code{'operator',}
+\code{'parser',} \code{'regex',} \code{'rotor',} \code{'select',}
+\code{'strop',} \code{'struct',} \code{'time')}.  A similar remark
+about overriding this variable applies --- use the value from the base
+class as a starting point.
 \end{datadesc}
 
 \begin{datadesc}{ok_path}
 Contains the directories which will be searched when an \code{import}
 is performed in the restricted environment.  
-The value for \code{RExec} is the same as \code{sys.path} for
-unrestricted code.
+The value for \code{RExec} is the same as \code{sys.path} (at the time
+the module is loaded) for unrestricted code.
 \end{datadesc}
 
 \begin{datadesc}{ok_posix_names}
 \end{datadesc}
 
 \begin{datadesc}{ok_sys_names}
-Contains the names of the functions and variables in the \code{sys} module which will be
-available to programs running in the restricted environment.  The
-value for \code{RExec} is \code{('ps1',} \code{'ps2',}
-\code{'copyright',} \code{'version',} \code{'platform',} \code{'exit',}
-\code{'maxint')}.
+Contains the names of the functions and variables in the \code{sys}
+module which will be available to programs running in the restricted
+environment.  The value for \code{RExec} is \code{('ps1',}
+\code{'ps2',} \code{'copyright',} \code{'version',} \code{'platform',}
+\code{'exit',} \code{'maxint')}.
 \end{datadesc}
 
 RExec instances support the following methods:
 \renewcommand{\indexsubitem}{(RExec object method)}
 
 \begin{funcdesc}{r_eval}{code}
-\var{code} must either be a string containing a Python expression, or a compiled code object, which will
-be evaluated in the restricted environment.  The value of the expression or code object will be returned.
+\var{code} must either be a string containing a Python expression, or
+a compiled code object, which will be evaluated in the restricted
+environment's \code{__main__} module.  The value of the expression or
+code object will be returned.
 \end{funcdesc}
 
 \begin{funcdesc}{r_exec}{code}
-\var{code} must either be a string containing one or more lines of Python code,  or a compiled code object,
-which will be executed in the restricted environment.  
+\var{code} must either be a string containing one or more lines of
+Python code, or a compiled code object, which will be executed in the
+restricted environment's \code{__main__} module.
 \end{funcdesc}
 
 \begin{funcdesc}{r_execfile}{filename}
 Execute the Python code contained in the file \var{filename} in the
-restricted environment.
+restricted environment's \code{__main__} module.
 \end{funcdesc}
 
 Methods whose names begin with \code{s_} are similar to the functions
 beginning with \code{r_}, but the code will be granted access to
-restricted versions of \code{sys.stdin}, \code{sys.stderr}, and
-\code{sys.stdout}.  
+restricted versions of the standard I/O streans \code{sys.stdin},
+\code{sys.stderr}, and \code{sys.stdout}.  
 
 \begin{funcdesc}{s_eval}{code}
 \var{code} must be a string containing a Python expression, which will
 restricted environment.
 \end{funcdesc}
 
-\code{RExec} objects must also support various methods which will be implicitly called 
-by code executing in the restricted environment.  Overriding these
-methods in a subclass is used to change the policies enforced by a restricted environment.
+\code{RExec} objects must also support various methods which will be
+implicitly called by code executing in the restricted environment.
+Overriding these methods in a subclass is used to change the policies
+enforced by a restricted environment.
 
-\begin{funcdesc}{r_import}{modulename\optional{\, globals, locals, fromlist}}
-Import the module \var{modulename}, raising an \code{ImportError} exception
-if the module is considered unsafe.  
+\begin{funcdesc}{r_import}{modulename\optional{\, globals\, locals\, fromlist}}
+Import the module \var{modulename}, raising an \code{ImportError}
+exception if the module is considered unsafe.
 \end{funcdesc}
 
 \begin{funcdesc}{r_open}{filename\optional{\, mode\optional{\, bufsize}}}
 and a file object (or a class instance compatible with file objects)
 should be returned.  \code{RExec}'s default behaviour is allow opening
 any file for reading, but forbidding any attempt to write a file.  See
-the example below for an implementation of a less restrictive \code{r_open()}.
+the example below for an implementation of a less restrictive
+\code{r_open()}.
 \end{funcdesc}
 
 \begin{funcdesc}{r_reload}{module}
 \end{funcdesc}
 
 \begin{funcdesc}{r_unload}{module}
-Unload the module object \var{module}.   
-% XXX what are the semantics of this?  
+Unload the module object \var{module} (i.e., remove it from the
+restricted environment's \code{sys.modules} dictionary).
 \end{funcdesc}
 
+And their equivalents with access to restricted standard I/O streams:
+
 \begin{funcdesc}{s_import}{modulename\optional{\, globals, locals, fromlist}}
-Import the module \var{modulename}, raising an \code{ImportError} exception
-if the module is considered unsafe.  
+Import the module \var{modulename}, raising an \code{ImportError}
+exception if the module is considered unsafe.
 \end{funcdesc}
 
 \begin{funcdesc}{s_reload}{module}
 \bcode\begin{verbatim}
 class TmpWriterRExec(rexec.RExec):
     def r_open(self, file, mode='r', buf=-1):
-        if mode in ('r', 'rb'): pass 
-	elif mode in ('w', 'wb'):
-	    # check filename : must begin with /tmp/
-	    if file[0:5]!='/tmp/': 
-		raise IOError, "can't open files for writing outside of /tmp"
-	    elif string.find(file, '/../')!=-1:
-		raise IOError, "'..' in filename; open for writing forbidden"
+        if mode in ('r', 'rb'):
+            pass
+        elif mode in ('w', 'wb', 'a', 'ab'):
+            # check filename : must begin with /tmp/
+            if file[:5]!='/tmp/': 
+                raise IOError, "can't write outside /tmp"
+            elif (string.find(file, '/../') >= 0 or
+                 file[:3] == '../' or file[-3:] == '/..'):
+                raise IOError, "'..' in filename forbidden"
+        else: raise IOError, "Illegal open() mode"
         return open(file, mode, buf)
 \end{verbatim}\ecode