[cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'

Anton Korobeynikov via cfe-dev cfe-dev at lists.llvm.org
Mon May 1 03:16:08 PDT 2017


Martin,

You're seeing the problem with __shared_weak_count::__get_deleter(),
however, it depends on both _LIBCPP_BUILD_STATIC and -fno-rtti
settings. The former could only be set when building libc++ itself and
not the user applications.

So, how the things appeared to be broken in your case provided that
you built libc++ with RTTI enabled?

There should be no issues wrt __shared_ptr_pointer::__get_deleter
because it's not exported from libc++.a. Same for <functional> stuff.

On Sun, Apr 30, 2017 at 9:32 PM, Martin J. O'Riordan
<martin.oriordan at movidius.com> wrote:
> Hi Anton,
>
> Sorry, I did 'Reply' instead of 'Reply All'
>
> Actually, I should've added that the '__shared_ptr_pointer::__get_deleter' is also similarly affected, but is not qualified by '_LIBCPP_BUILD_STATIC', and that the same is true for the 'functional implementation although I have no test cases yet which illustrate this as a problem.
>
> Thanks,
>
>         MartinO
>
> -----Original Message-----
> From: Martin J. O'Riordan [mailto:martin.oriordan at movidius.com]
> Sent: 30 April 2017 20:22
> To: 'Anton Korobeynikov' <anton at korobeynikov.info>
> Subject: RE: [cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'
>
> Exactly.  When building the LibC++ library, '_LIBCPP_BUILD_STATIC' is defined.
>
> But this means that the ABI for the 'vtable' is now a function of RTTI enabled vs disabled.
>
>         MartinO
>
> -----Original Message-----
> From: Anton Korobeynikov [mailto:anton at korobeynikov.info]
> Sent: 30 April 2017 18:39
> To: Martin J. O'Riordan <martin.oriordan at movidius.com>
> Cc: Clang Dev <cfe-dev at lists.llvm.org>
> Subject: Re: [cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'
>
> Martin,
>
> How the '_LIBCPP_BUILD_STATIC' define got set in your case? It should only appear when building libc++ itself.
>
> On Thu, Apr 27, 2017 at 6:27 PM, Martin J. O'Riordan via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>> I have a test that has been failing for a long time, and finally got
>> around to investigating it.  The test is really simple:
>>
>>
>>
>> #include <iostream>
>>
>> #include <memory>
>>
>>
>>
>> struct Test {
>>
>>   int n;
>>
>>   Test(int x) : n(x) { std::cerr << "Creating " << n << std::endl; }
>>
>>   ~Test() { std::cerr << "Deleting " << n << std::endl; }
>>
>> };
>>
>>
>>
>> int main () {
>>
>>   std::shared_ptr<Test> x(std::make_shared<Test>(42));
>>
>>   x.reset();
>>
>>   std::cerr << "Completed " << std::endl;
>>
>> }
>>
>>
>>
>> but it crashes immediately after printing the message from the
>> destructor, and before the message in ‘main’.
>>
>>
>>
>> After investigating, I found that the test works perfectly with RTTI
>> enabled, but crashes if it is disabled which puzzled me.  My LibC++
>> library is built with RTTI enabled as instructed on ‘libcxx.llvm.org’
>> which says that the library must be built with RTTI enabled, though it
>> may be used with RTTI disabled.
>>
>>
>>
>> What I found was that the ‘vtable’ for the ‘shared_ptr’ specialisation
>> is different depending on whether RTTI is enabled or disabled.  With
>> RTTI disabled it is:
>>
>>
>>
>> _ZTVNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE:
>>
>>     .word      0
>>
>>     .word      0
>>
>>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED1Ev
>>
>>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED0Ev
>>
>>     .word
>> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE16__on_zero_s
>> haredEv
>>
>>     .word
>> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_s
>> hared_weakEv
>>
>>
>>
>> but with RTTI enabled it is:
>>
>>
>>
>> _ZTVNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE:
>>
>>     .word      0
>>
>>     .word      _ZTINSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE
>>
>>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED1Ev
>>
>>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED0Ev
>>
>>     .word
>> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE16__on_zero_s
>> haredEv
>>
>>     .word      _ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info
>>
>>     .word
>> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_s
>> hared_weakEv
>>
>>
>>
>> In the library, the implementation (in ‘memory.cpp’ compiler with
>> ‘-frtti’) is attempting to call the function ‘__on_zero_shared_weak’,
>> but using offset 24, and the ‘vtable’ emitted in the test case
>> (compiled with ‘-fno-rtti’) has this function at offset 20.
>>
>>
>>
>> This is caused by the following lines in ‘<memory>’:
>>
>>
>>
>>     // Define the function out only if we build static libc++ without RTTI.
>>
>>     // Otherwise we may break clients who need to compile their
>> projects with
>>
>>     // -fno-rtti and yet link against a libc++.dylib compiled
>>
>>     // without -fno-rtti.
>>
>> #if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
>>
>>     virtual const void* __get_deleter(const type_info&) const
>> _NOEXCEPT;
>>
>> #endif
>>
>>
>>
>> and because the function is virtual, the layout of the ‘vtable’ is
>> different between RTTI enabled and disabled (we are building a static
>> library, so ‘_LIBCPP_BUILD_STATIC’ is true).
>>
>>
>>
>> There are a couple of places in ‘<memory>’ and ‘memory.cpp’ where this
>> happens (always with ‘__get_deleter’), but after sanity checking the
>> other headers, I see that ‘<functional>’ and ‘<__functional_03>’ also
>> have similar issues where the layout of the ‘vtable’ is different
>> depending on whether RTTI is enabled or not; though I don’t have any tests which show this.
>>
>>
>>
>> I don’t know what the best fix is for this because it was clearly
>> introduced to address some issue with dynamic libraries, but locally I
>> have decided to always define ‘__get_deleter’, and make its implementation return ‘nullptr’
>> when RTTI is disabled.  This preserves the ‘vtable’ layout independent
>> of RTTI.
>>
>>
>>
>> Thanks,
>>
>>
>>
>>             MartinO
>>
>>
>>
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
>
>
> --
> With best regards, Anton Korobeynikov
> Department of Statistical Modelling, Saint Petersburg State University
>



-- 
With best regards, Anton Korobeynikov
Department of Statistical Modelling, Saint Petersburg State University



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