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[Wunsafe-buffer-usage] False positives for & expression indexing constant size array (arr[anything & 0]) #112284

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Feb 24, 2025
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[Wunsafe-buffer-usage] False positives for & expression indexing constant size array (arr[anything & 0]) #112284

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[Wunsafe-buffer-usage] False positives for & expression indexing cons…
Oct 11, 2024
da96efb
Add more tests and a comment explaining the idea behind the change.
Feb 21, 2025
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19 changes: 19 additions & 0 deletions clang/lib/Analysis/UnsafeBufferUsage.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -462,6 +462,25 @@ AST_MATCHER(ArraySubscriptExpr, isSafeArraySubscript) {
// bug
if (ArrIdx.isNonNegative() && ArrIdx.getLimitedValue() < limit)
return true;
} 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)
return false;

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

if ((!LHS->isValueDependent() &&
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@ziqingluo-90 ziqingluo-90 Feb 21, 2025 •
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if LHS->isValueDependent(), no constant value can be derived from LHS anyway. So I suspect this condition is not needed.

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The Expr::EvaluateAsInt(...) method assumes the expression it operates on in not value dependent. So, we have to check this before hand to make sure the method does not crash. We can only skip this check if we can be guaranteed the expression is never value dependent. I don't think that is true here.

LHS->EvaluateAsInt(EVResult,
Finder->getASTContext())) || // case: `n & e`
(!RHS->isValueDependent() &&
RHS->EvaluateAsInt(EVResult, Finder->getASTContext()))) { // `e & n`
llvm::APSInt result = EVResult.Val.getInt();
if (result.isNonNegative() && result.getLimitedValue() < limit)
return true;
}
return false;
}
return false;
}
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33 changes: 33 additions & 0 deletions clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
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Expand Up @@ -18,7 +18,9 @@ void foo2(unsigned idx) {

struct Foo {
int member_buffer[10];
int x;
};

void foo2(Foo& f, unsigned idx) {
f.member_buffer[idx] = 0; // expected-warning{{unsafe buffer access}}
}
Expand All @@ -33,6 +35,37 @@ 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}}

void masked_idx1(unsigned long long idx, Foo f) {
// Bitwise and operation
array[idx & 5] = 10; // no-warning
array[5 &idx] = 12; // no-warning
array[idx & 11 & 5] = 3; // no warning
array[idx & 11] = 20; // expected-note{{used in buffer access here}}
array[idx &=5]; // expected-note{{used in buffer access here}}
array[f.x & 5]; // no-warning
array[5 & f.x]; // no-warning
array[f.x & (-5)]; // expected-note{{used in buffer access here}}
}

typedef unsigned long long uint64_t;
typedef unsigned int uint32_t;
typedef unsigned char uint8_t;

void type_conversions(uint64_t idx1, uint32_t idx2, uint8_t idx3) {
array[(uint32_t)idx1 & 3];
array[idx2 & 3];
array[idx3 & 3];
}

int array2[5]; // expected-warning {{'array2' is an unsafe buffer that does not perform bounds checks}}

void masked_idx_safe(unsigned long long idx) {
array2[6 & 5]; // no warning
array2[6 & idx & (idx + 1) & 5]; // expected-note{{used in buffer access here}}
}

void constant_idx_unsafe(unsigned idx) {
int buffer[10]; // expected-warning{{'buffer' is an unsafe buffer that does not perform bounds checks}}
// expected-note@-1{{change type of 'buffer' to 'std::array' to label it for hardening}}
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