[llvm-bugs] [Bug 41114] New: weak_ptr(weak_ptr const &) can segfault in the presence of virtual bases

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41114

            Bug ID: 41114
           Summary: weak_ptr(weak_ptr const &) can segfault in the
                    presence of virtual bases
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

cat >test.cc <<EOF
#include <memory>
#include <stdio.h>

struct A { int i = 0; virtual ~A() {} };
struct B : public virtual A { char d[100000]; };

int main() {
    std::weak_ptr<B> p = std::shared_ptr<B>(new B);
    puts("hello world");
    std::weak_ptr<A> q = p;
}
EOF
clang++ -std=c++11 test.cc
./a.out

Segmentation fault


What's going on here is that the converting constructor weak_ptr(const
weak_ptr&) is implemented as

weak_ptr<_Tp>::weak_ptr(shared_ptr<_Yp> const& __r,
                        typename enable_if<is_convertible<_Yp*, _Tp*>::value,
__nat*>::type)
                         _NOEXCEPT
    : __ptr_(__r.__ptr_),     // LINE XXX
      __cntrl_(__r.__cntrl_)
{
    if (__cntrl_)
        __cntrl_->__add_weak();
}

When `_Tp` is a virtual base of `_Yp`, the conversion on line XXX generally
needs to dereference `__r.__ptr_` to compute the appropriate offset. This is
fine if the weak_ptr stores a null pointer value, and it's fine if the weak_ptr
is not-expired; but if the weak_ptr is expired (or empty) and stores a non-null
pointer value, then we dereference into freed memory and segfault.

The solution adopted by both libstdc++ and MSVC is to implement this
constructor as essentially

weak_ptr<_Tp>::weak_ptr(shared_ptr<_Yp> const& __r,
                        typename enable_if<is_convertible<_Yp*, _Tp*>::value,
__nat*>::type)
                         _NOEXCEPT
    : weak_ptr(__r.lock())
{}

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