<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Off-by-one error in overwrite path of MutationDispatcher::ApplyDictionaryEntry"
href="https://bugs.llvm.org/show_bug.cgi?id=49989">49989</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Off-by-one error in overwrite path of MutationDispatcher::ApplyDictionaryEntry
</td>
</tr>
<tr>
<th>Product</th>
<td>compiler-rt
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>fuzzer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>meumertzheim@code-intelligence.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>In the overwrite branch of MutationDispatcher::ApplyDictionaryEntry in
FuzzerMutate.cpp, the index Idx at which W.size() bytes are overwritten with
the word W is chosen uniformly at random in the interval [0, Size - W.size())
(see
<a href="https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L198">https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L198</a>).
This means that Idx + W.size() will always be strictly less than Size, i.e.,
the last byte of the current unit will never be overwritten.
If my assessment is correct, I will gladly submit the trivial patch that fixes
the issue: Rand(Size - W.size()) would have to be replaced with Rand(Size + 1 -
W.size()).</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>