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
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)
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.