[tahoe-dev] question about sharing...

[David-Sarah Hopwood]

On 01/06/11 18:52, Greg Troxel wrote:
> toby cabot <toby at caboteria.org> writes:
> 
>> If I give someone a URL to a directory can I later revoke that URL
>> somehow but still be able to access the directory myself?
> 
> No.  But you can move the files in it to a new directory and hope the
> original directory gets garbage collected.
> 
> But again, if they copied the data, you can't revoke that.
> 
> 
> It's interesting that this comes up in tahoe much more so than in other
> filesystems.  People don't seem to ask:
> 
>   if I have a filesystem, and I let someone read a file, and then I
>   "chmod 700" it, how can I be sure they didn't keep a copy?  Isn't it a
>   bug that the filesystem doesn't enforce removing all their copies?
> 
> about other filesystems.

Nor do they ask about the revocation behaviour of Unix file descriptors
or Windows HANDLEs (which in fact is very much like capabilities; in
particular, access is checked only on open [*]).

Perhaps they assume that an attacker can't be running a program on the
same machine, or perhaps they don't trust the Unix or Windows access
control models at all. Still, it's odd that pure capability systems are
criticised for a perceived weakness that is routinely ignored in ACL
systems that have some capability-like abstractions.


[*] For example, on Linux:

    python
    >>> f = open("test", "wb")
    >>> f.flush()               # make sure the file exists
    >>> import os
    >>> os.chmod("test", 0400)  # could be done by another process or user
    >>> open("test", "wb")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IOError: [Errno 13] Permission denied: 'test'

    >>> f.write("abc")          # write succeeds on already-open fd
    >>> f.close()
    >>> open("test", "rb").read()
    'abc'

-- 
David-Sarah Hopwood ⚥ http://davidsarah.livejournal.com

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