projects / OpenSPARC-T2-DV / blame
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| 54 | <H2><A NAME="SECTION0019130000000000000000"> | |
| 55 | 17.1.3 An example</A> | |
| 56 | </H2> | |
| 57 | ||
| 58 | <P> | |
| 59 | Let us say that we want a slightly more relaxed policy than the | |
| 60 | standard <tt class="class">RExec</tt> class. For example, if we're willing to allow | |
| 61 | files in <span class="file">/tmp</span> to be written, we can subclass the <tt class="class">RExec</tt> | |
| 62 | class: | |
| 63 | ||
| 64 | <P> | |
| 65 | <div class="verbatim"><pre> | |
| 66 | class TmpWriterRExec(rexec.RExec): | |
| 67 | def r_open(self, file, mode='r', buf=-1): | |
| 68 | if mode in ('r', 'rb'): | |
| 69 | pass | |
| 70 | elif mode in ('w', 'wb', 'a', 'ab'): | |
| 71 | # check filename : must begin with /tmp/ | |
| 72 | if file[:5]!='/tmp/': | |
| 73 | raise IOError, "can't write outside /tmp" | |
| 74 | elif (string.find(file, '/../') >= 0 or | |
| 75 | file[:3] == '../' or file[-3:] == '/..'): | |
| 76 | raise IOError, "'..' in filename forbidden" | |
| 77 | else: raise IOError, "Illegal open() mode" | |
| 78 | return open(file, mode, buf) | |
| 79 | </pre></div> | |
| 80 | Notice that the above code will occasionally forbid a perfectly valid | |
| 81 | filename; for example, code in the restricted environment won't be | |
| 82 | able to open a file called <span class="file">/tmp/foo/../bar</span>. To fix this, the | |
| 83 | <tt class="method">r_open()</tt> method would have to simplify the filename to | |
| 84 | <span class="file">/tmp/bar</span>, which would require splitting apart the filename and | |
| 85 | performing various operations on it. In cases where security is at | |
| 86 | stake, it may be preferable to write simple code which is sometimes | |
| 87 | overly restrictive, instead of more general code that is also more | |
| 88 | complex and may harbor a subtle security hole. | |
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| 124 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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