libstdc++: ODR violation when using std::regex with and without -D_GLIBCXX_DEBUG

[Jonathan Wakely]

On 08/05/18 16:17 +0100, Jonathan Wakely wrote:
>On 8 May 2018 at 15:45, Marc Glisse <marc.glisse@inria.fr> wrote:
>> On Tue, 8 May 2018, Jonathan Wakely wrote:
>>
>>> On 8 May 2018 at 14:00, Jonathan Wakely wrote:
>>>>
>>>> On 8 May 2018 at 13:44, Stephan Bergmann wrote:
>>>>>
>>>>> I was recently bitten by the following issue (Linux, libstdc++ 8.0.1): A
>>>>> process loads two dynamic libraries A and B both using std::regex, and A
>>>>> is
>>>>> compiled without -D_GLIBCXX_DEBUG while B is compiled with
>>>>> -D_GLIBCXX_DEBUG.
>>>>
>>>>
>>>> This is only supported in very restricted cases.
>>>>
>>>>> B creates an instance of std::regex, which internally creates a
>>>>> std::shared_ptr<std::__detail::_NFA<std::__cxx11::regex_traits<char>>>,
>>>>> where _NFA has various members of std::__debug::vector type (but which
>>>>> isn't
>>>>> reflected in the mangled name of that _NFA instantiation itself).
>>>>>
>>>>> Now, when that instance of std::regex is destroyed again in library B,
>>>>> the
>>>>>
>>>>> std::shared_ptr<std::__detail::_NFA<std::__cxx11::regex_traits<char>>>::~shared_ptr
>>>>> destructor (and functions it in turn calls) that happens to get picked
>>>>> is
>>>>> the (inlined, and exported due to default visibility) instance from
>>>>> library
>>>>> A.  And that assumes that that _NFA instantiation has members of
>>>>> non-debug
>>>>> std::vector type, which causes a crash.
>>>>>
>>>>> Should it be considered a bug that such mixture of debug and non-debug
>>>>> std::regex usage causes ODR violations?
>>>>
>>>>
>>>> Yes, but my frank response is "don't do that".
>>>>
>>>> The right fix here might be to ensure that _NFA always uses the
>>>> non-debug vector even in Debug Mode, but I'm fairly certain there are
>>>> other similar problems lurking.
>>>
>>>
>>> N.B. I think this discussion belongs on the libstdc++ list.
>>
>>
>> Would it make sense to use the abi_tag attribute to help with that? (I
>> didn't really think about it, maybe it doesn't)
>
>Yes, I think we could add it conditionally in debug mode, so that
>types with members that are either std::xxx or __gnu_debug::xxx get a
>different mangled name in debug mode.
>
>For the regex _NFA type I don't think we want the debug mode checking,
>because users can't access it directly so any errors are in the
>libstdc++ implementation and we should have eliminated them ourselves,
>not be pushing detection of those logic errors into users' programs.

I've committed this patch to do that.


>For std::match_results (which derives from std::vector) it's possible
>for users to use invalid iterators obtained from a match_results, so
>Debug Mode can help. In that case we could decide whether to add the
>abi_tag, or always derive from _GLIBCXX_STD_C::vector (i.e. the
>non-debug mode one), or even provide an entire
>__gnu_debug::match_results type.
>
>> "don't do that" remains the most sensible answer.
>
>Yes, it's just asking for trouble.
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