fix double-free in CZString copy-assignment operator - #1706
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| Filename | Overview |
|---|---|
| src/lib_json/json_value.cpp | Reworks CZString copy assignment to use copy-and-swap, but the previously reported inactive-union-member access remains in the invoked swap implementation. |
| src/test_lib_json/main.cpp | Adds focused regression coverage confirming that both source and destination retain the copied string after assignment. |
Reviews (2): Last reviewed commit: "Merge branch 'master' into czstring-copy..." | Re-trigger Greptile
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Coverage Report for CI Build 32808338607Coverage increased (+0.1%) to 90.022%Details
Uncovered ChangesNo uncovered changes found. Coverage RegressionsNo coverage regressions found. Coverage Stats
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…On Wed, Aug 26, 2026 at 8:43 PM greptile-apps[bot] ***@***.***> wrote:
***@***.***[bot]* commented on this pull request.
------------------------------
In src/lib_json/json_value.cpp
<#1706 (comment)>
:
> @@ -317,8 +317,11 @@ void Value::CZString::swap(CZString& other) {
}
Value::CZString& Value::CZString::operator=(const CZString& other) {
- cstr_ = other.cstr_;
- index_ = other.index_;
+ // Copy-and-swap so a duplicate-policy buffer is deep-copied and the old one
+ // released once. The prior shallow copy aliased other.cstr_, double-freeing
+ // an owned string and leaking the overwritten one.
+ CZString temp(other);
+ swap(temp);
You're right. swap() is pre-existing, and CZString already treats the
ArrayIndex/StringStorage union as a word-sized representation; the
existing destructor and copy constructor follow the same convention. Given
that invariant, swap(temp) at src/lib_json/json_value.cpp:320 preserves
the packed length/policy metadata and does not introduce a new issue in
this PR. The ASan/UBSan results and regression coverage also address the
actual ownership bug. I'll withdraw the inactive-union-member finding; no
further change is needed here.
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CZString is the key type behind Json::Value's object and array maps, and a key that owns its buffer (the duplicate policy) frees that buffer in its destructor. The copy-assignment operator only shallow-copied cstr_ and the packed index_/policy word, so after
a = bboth keys pointed at b's allocation while a's old buffer was never released. On scope exit both destructors free the same pointer, which AddressSanitizer reports as a double free, and the overwritten buffer leaks. I ran into it while filling out the CZString assignment coverage from #1654, which exercised the copy and move constructors and the move-assignment but not this operator. The move-assignment already releases then transfers correctly, so I made copy-assignment do the safe thing with copy-and-swap: the temporary deep-copies the source and takes our old buffer to be freed exactly once. Added the missing case to runCZStringTests, which double-frees under ASan before the change and passes after.