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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - clang 3.4 UBSan falsely reports "member access with insufficient space" (screwed up 'this' null check in variadic function?)"
href="http://llvm.org/bugs/show_bug.cgi?id=17605">17605</a>
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<th>Summary</th>
<td>clang 3.4 UBSan falsely reports "member access with insufficient space" (screwed up 'this' null check in variadic function?)
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</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>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>corydoras@ridiculousfish.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>Ubuntu Clang version 3.4-1~exp1 (trunk) (based on LLVM 3.4), x86-64
1. Compile <a href="https://gist.github.com/ridiculousfish/7017899">https://gist.github.com/ridiculousfish/7017899</a> like so:
clang++ -O1 -fsanitize=undefined test.cpp
2. Run it
Result: you get a bogus 'member access insufficient space' runtime error.
This does NOT occur at -O0 but DOES occur at -O1 through -O3, and -Os. The bug
is also somewhat fragile: changes to the objects like removing fields prevents
it from reproducing.
When the bug reproduces, the assembly for the 'this' pointer check is as
follows:
setne %al
testb $7, %bl
sete %r15b
andb %al, %r15b
je .LBB1_3
When the bug does not reproduce, the `setne` insn is preceded by a null
comparison like `testq %r10, %r10`. When it does reproduce, the comparison is
omitted, as illustrated above. So the setne just picks up whatever is in ZF,
which happens to be 1 by virtue of a prior comparison. Thus it thinks that
'this' is 0.</pre>
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