[libcxx-commits] [libcxx] [libcxx] Don't deallocate non-pointer data in string assignment. (PR #67200)

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llvmbot wrote:


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<details>
<summary>Changes</summary>

Previously, assignment to a std::basic_string type with a _custom_ allocator could under certain conditions attempt to interpret part of the target string's "short" string-content as if it was a "long" data pointer, and attempt to deallocate a garbage value.

This is a serious bug, but code in which it might happen is rare. It required:

1. the basic_string must be using a custom allocator type which sets the propagate_on_container_copy_assignment trait to true (thus, it does not affect the default allocator, nor most custom allocators).
2. the allocator for the target string must compare not equal to the allocator for the source string (many allocators always compare equal).
3. the source of the copy must currently contain a "long" string, and the assignment-target must currently contain a "short" string.

Finally, the issue would've typically been innocuous when the bytes misinterpreted as a pointer were all zero, as deallocating a nullptr is typically a no-op. This is why existing test cases did not exhibit an issue: they were all zero-length strings, which do not have data in the bytes interpreted as a pointer.

---
Full diff: https://github.com/llvm/llvm-project/pull/67200.diff


2 Files Affected:

- (modified) libcxx/include/string (+2-1) 
- (modified) libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp (+1) 


``````````diff
diff --git a/libcxx/include/string b/libcxx/include/string
index 4b96273698166dd..123e1d64f910ab7 100644
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1969,7 +1969,8 @@ private:
                     allocator_type __a = __str.__alloc();
                     auto __allocation = std::__allocate_at_least(__a, __str.__get_long_cap());
                     __begin_lifetime(__allocation.ptr, __allocation.count);
-                    __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
+                    if (__is_long())
+                        __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
                     __alloc() = std::move(__a);
                     __set_long_pointer(__allocation.ptr);
                     __set_long_cap(__allocation.count);
diff --git a/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp b/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp
index 311778c35dcbaa9..6db086e78ed0f40 100644
--- a/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp
+++ b/libcxx/test/std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp
@@ -90,6 +90,7 @@ TEST_CONSTEXPR_CXX20 bool test() {
     testAlloc(S(A(5)), S("1"), A());
     testAlloc(S(A(5)), S("1", A(7)), A(7));
     testAlloc(S(A(5)), S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)), A(7));
+    testAlloc(S("12345678901234567890", A(5)), S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)), A(7));
   }
 
 #if TEST_STD_VER >= 11

``````````

</details>


https://github.com/llvm/llvm-project/pull/67200