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Fix BitShift UB when shift amount >= bit width #28272

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Merged
tianleiwu merged 3 commits into from
May 4, 2026
+75 −6
Merged

Fix BitShift UB when shift amount >= bit width #28272

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138a94c
Initial plan
Copilot Apr 29, 2026
c265bb7
Fix BitShift UB when shift amount >= bit width
Copilot Apr 29, 2026
878dded
Skip BitShift shift-by-bit-width tests for DirectML EP
Copilot May 2, 2026
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25 changes: 19 additions & 6 deletions onnxruntime/core/providers/cpu/math/element_wise_ops.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1334,6 +1334,19 @@ BitShift<T>::BitShift(const OpKernelInfo& info) : OpKernel(info) {
ORT_THROW("Invalid direction value of '", direction, "'. Valid values are 'LEFT' or 'RIGHT'.");
}

// Shifting by >= the bit width of an unsigned type is undefined behavior in C++.
// On x86, 64-bit shifts mask the shift amount to 6 bits, so shift by 64 acts like shift by 0.
// Guard against this by returning 0 when the shift amount >= the bit width.
template <typename T>
inline T SafeShiftLeft(T value, T shift) {
return shift >= sizeof(T) * 8 ? T{0} : value << shift;
}

template <typename T>
inline T SafeShiftRight(T value, T shift) {
return shift >= sizeof(T) * 8 ? T{0} : value >> shift;
}

template <typename T>
Status BitShift<T>::Compute(OpKernelContext* context) const {
ProcessBroadcastSpanFuncs funcs{
Expand All @@ -1345,11 +1358,11 @@ Status BitShift<T>::Compute(OpKernelContext* context) const {
ptrdiff_t i = 0;
if (shift_left) {
for (const auto& input : input1.array()) {
output[i++] = input0 << input;
output[i++] = SafeShiftLeft(input0, input);
}
} else {
for (const auto& input : input1.array()) {
output[i++] = input0 >> input;
output[i++] = SafeShiftRight(input0, input);
}
}
},
Expand All @@ -1361,11 +1374,11 @@ Status BitShift<T>::Compute(OpKernelContext* context) const {
ptrdiff_t i = 0;
if (shift_left) {
for (const auto& input : input0.array()) {
output[i++] = input << input1;
output[i++] = SafeShiftLeft(input, input1);
}
} else {
for (const auto& input : input0.array()) {
output[i++] = input >> input1;
output[i++] = SafeShiftRight(input, input1);
}
}
},
Expand All @@ -1380,11 +1393,11 @@ Status BitShift<T>::Compute(OpKernelContext* context) const {
auto cur_out = output.begin(), end_out = output.end();
if (shift_left) {
for (; cur0 != end0; ++cur0, ++cur1, ++cur_out) {
*cur_out = *cur0 << *cur1;
*cur_out = SafeShiftLeft(*cur0, *cur1);
}
} else {
for (; cur0 != end0; ++cur0, ++cur1, ++cur_out) {
*cur_out = *cur0 >> *cur1;
*cur_out = SafeShiftRight(*cur0, *cur1);
}
}

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56 changes: 56 additions & 0 deletions onnxruntime/test/providers/cpu/math/element_wise_ops_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -4420,6 +4420,62 @@ TEST(BitShiftOpTest, BroadcastXRight_Uint8) {
test.Run();
}

// Test that shift amounts >= bit width produce 0 (not undefined behavior).
// DirectML EP has the same hardware-level shift masking behavior, so skip these tests for DML.
TEST(BitShiftOpTest, RightShiftByBitWidth_Uint64) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "RIGHT");
test.AddInput<uint64_t>("X", {4}, {1000, 255, 1, 42});
test.AddInput<uint64_t>("Y", {4}, {64, 64, 64, 64});
test.AddOutput<uint64_t>("Z", {4}, {0, 0, 0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(BitShiftOpTest, LeftShiftByBitWidth_Uint64) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "LEFT");
test.AddInput<uint64_t>("X", {4}, {1000, 255, 1, 42});
test.AddInput<uint64_t>("Y", {4}, {64, 64, 64, 64});
test.AddOutput<uint64_t>("Z", {4}, {0, 0, 0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(BitShiftOpTest, RightShiftByBitWidth_Uint32) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "RIGHT");
test.AddInput<uint32_t>("X", {3}, {16, 4, 1});
test.AddInput<uint32_t>("Y", {3}, {32, 32, 32});
test.AddOutput<uint32_t>("Z", {3}, {0, 0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(BitShiftOpTest, RightShiftByMoreThanBitWidth_Uint64) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "RIGHT");
test.AddInput<uint64_t>("X", {2}, {1000, 42});
test.AddInput<uint64_t>("Y", {2}, {65, 128});
test.AddOutput<uint64_t>("Z", {2}, {0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(BitShiftOpTest, ScalarRightShiftByBitWidth_Uint64) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "RIGHT");
test.AddInput<uint64_t>("X", {1}, {1000});
test.AddInput<uint64_t>("Y", {3}, {64, 65, 128});
test.AddOutput<uint64_t>("Z", {3}, {0, 0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(BitShiftOpTest, ScalarLeftShiftByBitWidth_Uint64) {
OpTester test("BitShift", 11);
test.AddAttribute("direction", "LEFT");
test.AddInput<uint64_t>("X", {3}, {1000, 255, 42});
test.AddInput<uint64_t>("Y", {1}, {64});
test.AddOutput<uint64_t>("Z", {3}, {0, 0, 0});
test.Run(OpTester::ExpectResult::kExpectSuccess, "", {kDmlExecutionProvider});
}

TEST(MathOpTest, BitwiseAnd) {
OpTester test("BitwiseAnd", 18);
std::vector<int64_t> dims{3};
Expand Down
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