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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - static analyzer false positives due to pointer cast"
href="https://llvm.org/bugs/show_bug.cgi?id=26902">26902</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>static analyzer false positives due to pointer cast
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<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>Static Analyzer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>kremenek@apple.com
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nlewycky@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>This testcase is plain C code:
typedef struct Packet_s {
unsigned char lefthanded;
} Packet;
int test(const unsigned char *payload) {
Packet *packet = (Packet*)payload;
if (packet->lefthanded)
return 1;
return 0;
}
I'm running 'clang_tidy -checks=* tc.c --' to exercise the static analyzer. The
first statement warning is a false positive:
/usr/local/google/home/nlewycky/tc.c:6:20: warning: Casting a non-structure
type to a structure type and accessing a field can lead to memory access errors
or data corruption [clang-analyzer-alpha.core.CastToStruct]
Packet *packet = (Packet*)payload;
^
That's true in the general case, but not in the specific case of a struct with
a single unsigned char. This isn't even an ABI guarantee, the standard, I
think, promises that this is safe.
Since it doesn't understand 'packet', it treats the conditional on the member
in a really bad way:
/usr/local/google/home/nlewycky/tc.c:8:12: warning: This statement is never
executed [clang-analyzer-alpha.deadcode.UnreachableCode]
return 1;
^
/usr/local/google/home/nlewycky/tc.c:8:12: note: This statement is never
executed
return 1;
^
/usr/local/google/home/nlewycky/tc.c:9:10: warning: This statement is never
executed [clang-analyzer-alpha.deadcode.UnreachableCode]
return 0;
^
/usr/local/google/home/nlewycky/tc.c:9:10: note: This statement is never
executed
return 0;
^
I don't really understand the chain of logic that led to both of these reports.
It would make sense to treat 'packet' as containing opaque data after a bad
cast, rather than toxic.</pre>
</div>
</p>
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