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C++: Fix FPs to cpp/return-stack-allocated-memory #18309

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C++: Fix FPs to cpp/return-stack-allocated-memory #18309

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14cef6a
C++: Fix FPs to cpp/return-stack-allocated-memory
calumgrant Dec 17, 2024
fabaceb
C++: Change note
calumgrant Dec 18, 2024
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2 changes: 2 additions & 0 deletions cpp/ql/src/Likely Bugs/Memory Management/ReturnStackAllocatedMemory.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,8 @@ class ReturnStackAllocatedMemoryConfig extends MustFlowConfiguration {
or
node2.(PointerOffsetInstruction).getLeftOperand() = node1
}

override predicate isBarrier(Instruction n) { n.getResultType() instanceof ErroneousType }
}

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4 changes: 4 additions & 0 deletions cpp/ql/src/change-notes/2024-12-18-return-stack-allocated-memory.md
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* The "Returning stack-allocated memory" query (`cpp/return-stack-allocated-memory`) no longer produces results if there is an extraction error in the type of the function.
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This is not accurate. It no longer produces a result when there's an ErroneousType anywhere on the potential must-flow path (but that is too much detail for a change note). So it might be that the return type of the function is known, but that some critical type within the function is not.

Something along the following lines (modifying the example from the QL file):

   int* test() {
      UNKNOWN_TYPE s;
      return &s.x; // BAD: &s.x is an address of a variable on the stack.
   }

9 changes: 7 additions & 2 deletions cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/test.cpp
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@@ -1,4 +1,4 @@
// semmle-extractor-options: -std=c++14
// semmle-extractor-options: -std=c++14 --expect_errors
class MyClass
{
public:
Expand Down Expand Up @@ -248,4 +248,9 @@ char* test_strdupa(const char* s) {
void* test_strndupa(const char* s, size_t size) {
char* s2 = strndupa(s, size);
return s2; // BAD
}
}

UNKNOWN_TYPE test_error_type() {
UNKNOWN_TYPE x;
return x; // GOOD: Don't report error types
}
Comment on lines +253 to +256
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I don't recall what we did in other cases, but it might be better to have all test cases with parse errors in separate files, because that will mean we catch parse errors that are introduced by accident in "regular" tests.

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That's not a bad idea to start doing that.

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I think this example is actually fine, because no pointer to x is being returned.

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