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

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Merged
calumgrant merged 5 commits into from
Dec 20, 2024
Merged

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
e067ac9
C++: Address review comments
calumgrant Dec 19, 2024
7abe700
Update cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnSt…
calumgrant Dec 19, 2024
3193fe8
C++: Update comments
calumgrant Dec 20, 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 }
}

from
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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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@jketema jketema Dec 18, 2024

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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
}
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@jketema jketema Dec 18, 2024

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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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@calumgrant calumgrant Dec 18, 2024

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

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@jketema jketema Dec 18, 2024

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

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@calumgrant calumgrant Dec 19, 2024

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There was a regression (embarrassing result), where this was actually flagged as an alert. But the commit history does not contain the failing test being fixed.

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@jketema jketema Dec 19, 2024

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👍

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