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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Possible incorrect position hint for persistent auto dictionary entry"
href="https://bugs.llvm.org/show_bug.cgi?id=49992">49992</a>
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<th>Summary</th>
<td>Possible incorrect position hint for persistent auto dictionary entry
</td>
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<th>Product</th>
<td>compiler-rt
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>fuzzer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>meumertzheim@code-intelligence.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>In MutationDispatcher::RecordSuccessfulMutationSequence in FuzzerMutate.cpp,
the persistent auto dictionary entries are recorded with a fixed position hint
of 1 (see
<a href="https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L482">https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L482</a>).
As a result, in 50% of the cases, Mutate_AddWordFromPersistentAutoDictionary
will attempt to apply the entry right after the first byte of the input.
Using a fixed position at this point is surprising and may have resulted from a
confusion with a no longer implemented, separate success count hinted at by a
commented out line of code above (see
<a href="https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L477">https://github.com/llvm/llvm-project/blob/1ede08a290ac70b06df6887a39c1a22ccbfcc09f/compiler-rt/lib/fuzzer/FuzzerMutate.cpp#L477</a>).
The fixed position 1 could be replaced with either DE->GetPositionHint()
(position hint copied from the entry) or std::numeric_limits<size_t>::max() (no
position hint). It remains to be determined which of these would be more
appropriate. If needed, I could try to get some data on this via fuzzbench.</pre>
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