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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - false positive for realloc()?"
href="https://bugs.llvm.org/show_bug.cgi?id=44310">44310</a>
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<th>Summary</th>
<td>false positive for realloc()?
</td>
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>9.0
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Static Analyzer
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<th>Assignee</th>
<td>dcoughlin@apple.com
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<th>Reporter</th>
<td>rgerhards@hq.adiscon.com
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<th>CC</th>
<td>dcoughlin@apple.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>I think I found a false positive in static analyzer's understanding of
realloc(). Please have a look here:
<a href="http://ubuntu16.rsyslog.com/19-12-13_07-32-59/2019-12-13-073355-8933-1/report-da528b.html#EndPath">http://ubuntu16.rsyslog.com/19-12-13_07-32-59/2019-12-13-073355-8933-1/report-da528b.html#EndPath</a>
I do a realloc here:
uchar *const pbuf = realloc(mem->data, mem->len + write_size + 1);
Static analyzer assumes that this causes the memory (mem->data) to be released.
This is fine as it also assume a non-null return value. However, slightly later
in the code I reassign the "freed" memory pointer to the new buffer address
returned by realloc():
mem->data = pbuf;
That seems to be ignored by static analyzer as later in the process it claims
that the memory is still freed (report cause 18, after return from function). I
used "const" wherever appropriate and so there should be no doubt about
aliasing in regard to the new pointer.
In the end result, static analyzer diagnoses both a double free and
use-after-free. IMHO this is the effect of not honoring the reassignment of the
buffer pointer.
I have also checked other realloc() uses inside the same code base. They follow
all a similar pattern, but none of them triggers any alert.
I may have overlooked something, but I really don't have any idea anymore of
what could be the real error cause.
As a last resort, I will now go ahead and make the code in question invisible
to static analyzer via preprocessor #if, but I really don't like this idea.</pre>
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