[Bug libstdc++/99402] [10/11 Regression] std::copy creates _GLIBCXX_DEBUG false positive for attempt to subscript a dereferenceable (start-of-sequence) iterator

[François Dumont]

I eventually prefer to propose this version.

Compared to the previous one I have the _M_can_advance calling the 
former one with correct number of elements to check for advance. And the 
former _M_can_advance is also properly making use of the 
__dp_sign_max_size precision.

Here is the revisited git log:

     libstdc++: [_GLIBCXX_DEBUG] Fix management of __dp_sign_max_size 
[PR 99402]

     __dp_sign precision indicates that we found out what iterator comes 
first or
     last in the range. __dp_sign_max_size is the same plus it gives the 
information
     of the max size of the range that is to say the max_size value such 
that
     distance(lhs, rhs) < max_size.
     Thanks to this additional information we are able to tell when a 
copy of n elements
     to that range will fail even if we do not know exactly how large it is.

     This patch makes sure that we are properly using this information.

     libstdc++-v3/ChangeLog:

             PR libstdc++/99402
             * include/debug/helper_functions.h 
(__can_advance(_InputIterator,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             (__can_advance(const _Safe_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * include/debug/macros.h 
(__glibcxx_check_can_increment_dist): New,
             use latter.
             (__glibcxx_check_can_increment_range): Adapt to use latter.
             (__glibcxx_check_can_decrement_range): Likewise.
             * include/debug/safe_iterator.h
             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
_Distance_precision>&,
             int)): New.
             (__can_advance(const _Safe_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * include/debug/safe_iterator.tcc
             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
_Distance_precision>&,
             int)): New.
             (_Safe_iterator<>::_M_valid_range(const _Safe_iterator<>&,
             std::pair<difference_type, _Distance_precision>&, bool)): 
Adapt for
             __dp_sign_max_size.
             (__copy_move_a): Adapt to use 
__glibcxx_check_can_increment_dist.
             (__copy_move_backward_a): Likewise.
             (__equal_aux): Likewise.
             * include/debug/stl_iterator.h (__can_advance(const 
std::reverse_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             (__can_advance(const std::move_iterator<>&,
             const std::pair<_Diff, _Distance_precision>&, int)): New.
             * testsuite/25_algorithms/copy/debug/99402.cc: New test.

Ok to commit if tests are all PASS ?

François

On 07/03/21 10:30 pm, François Dumont wrote:
> Here is the patch to correctly deal with the new __dp_sign_max_size.
>
> I prefer to introduce new __can_advance overloads for this to 
> correctly deal with the _Distance_precision in it. _M_valid_range was 
> also ignoring __dp_sign_max_size.
>
>     libstdc++: [_GLIBCXX_DEBUG] Fix management of __dp_sign_max_size 
> [PR 99402]
>
>     libstdc++-v3/ChangeLog:
>
>             PR libstdc++/99402
>             * include/debug/helper_functions.h 
> (__can_advance(_InputIterator,
>             const std::pair<_Diff, _Distance_precision>&, int)): New.
>             (__can_advance(const _Safe_iterator<>&,
>             const std::pair<_Diff, _Distance_precision>&, int)): New.
>             * include/debug/macros.h 
> (__glibcxx_check_can_increment_dist): New,
>             use latter.
>             (__glibcxx_check_can_increment_range): Adapt to use latter.
>             (__glibcxx_check_can_decrement_range): Likewise.
>             * include/debug/safe_iterator.h
>             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
> _Distance_precision>&,
>             int)): New.
>             (__can_advance(const _Safe_iterator<>&,
>             const std::pair<_Diff, _Distance_precision>&, int)): New.
>             * include/debug/safe_iterator.tcc
>             (_Safe_iterator<>::_M_can_advance(const std::pair<_Diff, 
> _Distance_precision>&,
>             int)): New.
>             (_Safe_iterator<>::_M_valid_range(const _Safe_iterator<>&,
>             std::pair<difference_type, _Distance_precision>&, bool)): 
> Adapt for
>             __dp_sign_max_size.
>             (__copy_move_a): Adapt to use 
> __glibcxx_check_can_increment_dist.
>             (__copy_move_backward_a): Likewise.
>             (__equal_aux): Likewise.
>             * include/debug/stl_iterator.h (__can_advance(const 
> std::reverse_iterator<>&,
>             const std::pair<_Diff, _Distance_precision>&, int)): New.
>             (__can_advance(const std::move_iterator<>&,
>             const std::pair<_Diff, _Distance_precision>&, int)): New.
>             * testsuite/25_algorithms/copy/debug/99402.cc: New test.
>
> Tested under Linux x86_64.
>
> Ok to commit ?
>
> François
>

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