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

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Dec 20, 2024
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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
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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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 returned expression.
5 changes: 3 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
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{
public:
Expand Down Expand Up @@ -248,4 +248,5 @@ char* test_strdupa(const char* s) {
void* test_strndupa(const char* s, size_t size) {
char* s2 = strndupa(s, size);
return s2; // BAD
}
}

16 changes: 16 additions & 0 deletions ...test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/test_errors.cpp
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@@ -0,0 +1,16 @@
// semmle-extractor-options: --expect_errors

UNKNOWN_TYPE test_error_value() {
UNKNOWN_TYPE x;
return x; // GOOD: Error return type
}

void* test_error_pointer() {
UNKNOWN_TYPE x;
return &x; // GOOD: Don't know what &x means
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This is "BAD" (I guess unless & is actually an overloaded operator, because who knows in that case), because we're taking the address of a local variable.

I'm fine with silencing these, but the comment needs to be updated.

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I agree, they are "probably" bad. Do we even need a comment?

Suggested change
return &x; // GOOD: Don't know what &x means
return &x; // BAD [FALSE NEGATIVE]: &x is an error

vs.

Suggested change
return &x; // GOOD: Don't know what &x means
return &x; // BAD [FALSE NEGATIVE]

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Just // BAD [FALSE NEGATIVE] is sufficient in both cases, I think.

}

int* test_error_pointer_member() {
UNKNOWN_TYPE x;
return &x.y; // GOOD: Don't know what x.y means
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Same as above.

}
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