Re: [boost] undefined behavior in Boost.Program_Options

Subject: Re: [boost] undefined behavior in Boost.Program_Options
From: Peter Dimov (pdimov_at_[hidden])
Date: 2019-04-24 21:59:35

Chris Rorvick wrote:
> I am seeing my GCC ubsan builds littered with "downcast of address 
> xxxxxxxx which does not point to an object of type 'holder'" errors [3] 
> coming from Boost.Program_options (Boost 1.69).  The issue has been 
> documented in various places [1][2] but I do not see anything on the 
> mailing list in recent months; apologies if I have missed something.
>
> A workaround is proposed on the StackOverflow post [1].  Does anyone have 
> any thoughts on this?
>
> Many thanks!
>
> Chris
>
> [1] 
> https://stackoverflow.com/questions/38987728/why-does-boostany-exhibit-undefined-behaviour-in-boostprogram-options
> [2] https://github.com/boostorg/program_options/issues/76
> [3]
> .../boost/boost/any.hpp:249:17: runtime error: downcast of address
> 0x602000053610 which does not point to an object of type 'holder'
> 0x602000053610: note: object is of type 'boost::any::holder<std::string>'
>  6f 00 80 67  d0 91 ec 88 fe 7f 00 00  68 33 09 00 40 60 00 00  00 00
> 00 00 00 00 00 00  00 00 00 00
>               ^~~~~~~~~~~~~~~~~~~~~~~
>               vptr for 'boost::any::holder<std::string>'
...

This doesn't seem to be the same issue as in the SO link. The cv qualifiers 
are now being stripped by the constructor at

https://github.com/boostorg/any/blob/develop/include/boost/any.hpp#L49

twice (once by `decay` and once by `remove_cv` for added safety) and in your 
message the held object is non-const:

> note: object is of type 'boost::any::holder<std::string>'

My guess is that `holder` needs BOOST_SYMBOL_VISIBLE. That is,

    template<typename ValueType>
    class holder : public placeholder

at 
https://github.com/boostorg/any/blob/develop/include/boost/any.hpp#L168-L169 
needs to be

    template<typename ValueType>
    class BOOST_SYMBOL_VISIBLE holder : public placeholder