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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Undefined behavior in undefined behavior santizer for shifts"
href="https://bugs.llvm.org/show_bug.cgi?id=44597">44597</a>
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<th>Summary</th>
<td>Undefined behavior in undefined behavior santizer for shifts
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<th>Product</th>
<td>compiler-rt
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<th>Version</th>
<td>unspecified
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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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>ubsan
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>cullmann@absint.de
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<pre>Hi,
we did some experiments with the shift sanitizer.
For the following code:
int main() {
signed char c = -2;
c = c << 4; // Okay integer promotion, sanitizer datastructure says 32-bit
wide operand
c = -2;
c <<= 4; // Integer promotion is done, but sanitizer datastructure says 8-bit
wide operand
// Consequence -> This eventually causes undefined behaviour in
getSIntValue() [Sanitizer Handler Library] since shifting a negative 32-bit
value by (128-8) bits to the left.
return 0;
}
the second call to the shift sanitizer routine will get a sext32 extended value
but has a getIntegerBitWidth of 8 internally in its data structures.
That will lead to "shift overflow" for the signed shift in:
SIntMax Value::getSIntValue() const {
CHECK(getType().isSignedIntegerTy());
if (isInlineInt()) {
// Val was zero-extended to ValueHandle. Sign-extend from original width
// to SIntMax.
const unsigned ExtraBits =
sizeof(SIntMax) * 8 - getType().getIntegerBitWidth();
return SIntMax(Val) << ExtraBits >> ExtraBits;
}
I would propose to change this
return SIntMax(Val) << ExtraBits >> ExtraBits;
to
return SIntMax(UIntMax(Val) << ExtraBits) >> ExtraBits;
to avoid the undefined left shift on a signed value that shifts some bits out.
Or are we confused here?</pre>
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