Let us say that we want a slightly more relaxed policy than the standard RExec
class. For example, if we're willing to allow files in /tmp to be
written, we can subclass the RExec class:
Notice that the above code will occasionally forbid a perfectly valid filename; for example,
code in the restricted environment won't be able to open a file called /tmp/foo/../bar.
To fix this, the r_open() method would have to simplify the filename
to /tmp/bar, which would require splitting apart the filename and
performing various operations on it. In cases where security is at stake, it may be preferable
to write simple code which is sometimes overly restrictive, instead of more general code that
is also more complex and may harbor a subtle security hole.
def r_open(self, file, mode='r', buf=-1):
if mode in ('r', 'rb'):
elif mode in ('w', 'wb', 'a', 'ab'):
# check filename : must begin with /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)