[llvm-bugs] [Bug 25887] New: UBSan: missing check for accessing an inactive member of an union

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Dec 18 10:11:55 PST 2015


            Bug ID: 25887
           Summary: UBSan: missing check for accessing an inactive member
                    of an union
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalobg88 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Just discussed in this range-v3 issue:

both Eric and Casey give good examples. Here Eric's example:

template<typename F, typename S>
struct pair_data
        F non_const_first_;
        F const first;
        S non_const_second_;
        S const second;

template<typename F, typename S>
struct pair : private pair_data<F, S>
    using pair_data<F, S>::first;
    using pair_data<F, S>::second;
    pair() : pair_data<F, S>{} {}
    pair(F f, S s) : pair_data<F, S>{f, s} {}

int main()
    pair<int, float> f{1, 3.14f};
    return f.first;  // UB
    // more UB: destructors of the non-active members (first, second) are
    // instead of the destructors of the active members (first_, second_)

Aggregate initialization of the union initialize the first member "first_".
Accessing first thus access a non-active member of the union and results in
undefined behavior.

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