False Positive: cpp/use-after-free on chained assignment after delete
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Descrição
**Description of the false positive**
I have encountered a false positive with the rule cpp/use-after-free.
CodeQL incorrectly flags a variable as being "used after free" when it is assigned via a chained assignment immediately following a delete[].
The analyzer seems to propagate the "freed" state of the dereferenced pointer to the local variable, failing to recognize that the new operator in the right-hand side of the assignment refreshes the pointer before the local variable reads it.
**Code samples or links to source code**
```
#include
void reallocateBuffer(char** sharedPtr, int size) {
// 1. Memory is freed
delete[] *sharedPtr;
char* localPtr;
// 2. Chained assignment:
// C++ guarantees right-to-left associativity.
// 'new' happens first, updates '*sharedPtr', and THEN 'localPtr' takes that value.
localPtr = *sharedPtr = new char[size];
// 3. CodeQL flags 'localPtr' as Use-After-Free here
if (localPtr) {
localPtr[0] = 'A';
}
}
int main() {
char* data = new char[10];
reallocateBuffer(&data, 50);
delete[] data;
return 0;
}
```
**Expected Behavior**
CodeQL should recognize that localPtr is assigned the result of the new allocation (via *sharedPtr) and is therefore safe to use.
**Actual Behavior**
CodeQL reports cpp/use-after-free on the line localPtr[0] = 'A';, claiming localPtr points to memory that was freed by delete[] *sharedPtr.
**Query / Rule ID**
cpp/use-after-free
Guia de contribuição
Direção de pesquisa
Comece com a consulta cpp/use-after-free mencionada na issue e use o reprodutor C++ fornecido para rastrear o relatório na atribuição encadeada. A tarefa estará concluída quando o analisador não relatar mais localPtr[0] como use-after-free e continuar detectando casos genuínos.
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Avaliação
- Stack de tecnologia
- cpp
- Domínio
- security
- Tipo de issue
- Bug
- Dificuldade
- 4/5
- Tempo estimado
- 3-5 dias
- Status de atividade
- Estagnada
- Clareza
- Razoavelmente clara
- Facilidade para iniciantes
- 48/100