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[-Wunsafe-buffer-usage] Fix false warnings when const sized array is safely accessed (array [idx %const]) #140113

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Merged
merged 2 commits into from
May 17, 2025
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[-Wunsafe-buffer-usage] Fix false warnings when const sized array is safely accessed (array [idx %const]) #140113

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c6d8a58
[-Wunsafe-buffer-usage] Fix false warnings when const array is safely…
May 15, 2025
334b062
Fixing the formatting issue and added tests.
May 15, 2025
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17 changes: 16 additions & 1 deletion clang/lib/Analysis/UnsafeBufferUsage.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -600,12 +600,27 @@ static bool isSafeArraySubscript(const ArraySubscriptExpr &Node,
} else if (const auto *BE = dyn_cast<BinaryOperator>(IndexExpr)) {
// For an integer expression `e` and an integer constant `n`, `e & n` and
// `n & e` are bounded by `n`:
if (BE->getOpcode() != BO_And)
if (BE->getOpcode() != BO_And && BE->getOpcode() != BO_Rem)
return false;

const Expr *LHS = BE->getLHS();
const Expr *RHS = BE->getRHS();

if (BE->getOpcode() == BO_Rem) {
// If n is a negative number, then n % const can be greater than const
if (!LHS->getType()->isUnsignedIntegerType()) {
return false;
}

if (!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
llvm::APSInt result = EVResult.Val.getInt();
if (result.isNonNegative() && result.getLimitedValue() <= limit)
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Copilot AI May 15, 2025

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[nitpick] Consider adding a comment to explain the purpose and derivation of the 'limit' variable to enhance clarity on how it determines the safe boundary for remainder operations.

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return true;
}

return false;
}

if ((!LHS->isValueDependent() &&
LHS->EvaluateAsInt(EVResult, Ctx)) || // case: `n & e`
(!RHS->isValueDependent() &&
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16 changes: 15 additions & 1 deletion clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -35,7 +35,21 @@ void constant_idx_safe0(unsigned idx) {
buffer[0] = 0;
}

int array[10]; // expected-warning {{'array' is an unsafe buffer that does not perform bounds checks}}
int array[10]; // expected-warning 3{{'array' is an unsafe buffer that does not perform bounds checks}}

void circular_access_unsigned(unsigned idx) {
array[idx % 10];
array[idx % 11]; // expected-note {{used in buffer access here}}
array[(idx + 3) % 10];
array[(--idx) % 8];
array[idx & 9 % 10];
array[9 & idx % 11];
array [12 % 10];
}

void circular_access_signed(int idx) {
array[idx % 10]; // expected-note {{used in buffer access here}}
}

void masked_idx1(unsigned long long idx, Foo f) {
// Bitwise and operation
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