False Positive: cpp/use-after-free on chained assignment after delete
Nobody has claimed this yet.
Assessment
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Newbie friendliness
- 48/100
Research direction
Start with the cpp/use-after-free query named in the issue and use the supplied C++ reproducer to trace the report on the chained assignment. Done means the analyzer no longer reports localPtr[0] as a use-after-free while continuing to detect genuine cases.
Written by the indexing model from the issue text.
Description
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 <new>
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
- Dominant language
- CodeQL
- Stars
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- Forks
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- Avg merge
- 2d 16h
- Merged PRs (30d)
- 143
Contributor guide
First steps
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