Re: [PATCH] Fix -Wstringop-overflow warning in 23_containers/vector/types/1.cc

[Jonathan Wakely <jwakely@redhat.com>]

On 27/05/24 22:07 +0200, François Dumont wrote:
>In C++98 this test fails with:
>
>Excess errors:
>/home/fdumont/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/stl_algobase.h:452: 
>warning: 'void* __builtin_memcpy(void*, const void*, long unsigned 
>int)' writing between 2 and 9223372036854775806 bytes into a region of 
>size 0 overflows the destination [-Wstringop-overflow=]
>
>The attached patch avoids this warning.
>
>    libstdc++: Fix -Wstringop-overflow warning coming from std::vector
>
>    Make vector<>::_M_range_insert implementation more transparent to 
>the compiler checks.
>
>    Extend local copies of members to the whole method scope so that 
>all branches benefit
>    from those.
>
>    libstdc++-v3/ChangeLog:
>
>            * include/bits/vector.tcc
>            (std::vector<>::_M_range_insert(iterator, _FwdIt, _FwdIt, 
>forward_iterator_tag)):
>            Use local copies of members to call the different algorithms.
>
>Ok to commit if all tests passes ?
>
>François

>diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
>index 36b27dce7b9..671929dee55 100644
>--- a/libstdc++-v3/include/bits/vector.tcc
>+++ b/libstdc++-v3/include/bits/vector.tcc
>@@ -885,83 +885,80 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>       {
> 	if (__first != __last)
> 	  {
>+	    // Make local copies of these members because the compiler
>+	    // thinks the allocator can alter them if 'this' is globally
>+	    // reachable.
>+	    pointer __start = this->_M_impl._M_start;
>+	    pointer __end = this->_M_impl._M_end_of_storage;
>+	    pointer __finish = this->_M_impl._M_finish;
>+	    pointer __pos = __position.base();
>+	    _Tp_alloc_type& __allocator = _M_get_Tp_allocator();
>+
>+	    if (__pos < __start || __finish < __pos)
>+	      __builtin_unreachable();

I don't think we should use __builtin_unreachable for something which
is not an invariant of the class. The __position argument is supplied
by the user, so we should not make promises about it being valid,
because we can't know that.

We can promise that __start <= __finish, and that __finish <= end,
because we control those. We can't promise the user won't pass in a
bad __position. Although it's undefined for the user to do that, using
__builtin_unreachable() here makes the effects worse, and makes it
harder to debug.

Also, (__pos < __start) might already trigger undefined behaviour for
fancy pointers, if they don't point to the same memory region.

So this change is not OK.


>+
> 	    const size_type __n = std::distance(__first, __last);
>-	    if (size_type(this->_M_impl._M_end_of_storage
>-			  - this->_M_impl._M_finish) >= __n)
>+	    if (size_type(__end - __finish) >= __n)
> 	      {
>-		const size_type __elems_after = end() - __position;
>-		pointer __old_finish(this->_M_impl._M_finish);
>+		const size_type __elems_after = __end - __pos;
>+		pointer __old_finish(__finish);
> 		if (__elems_after > __n)
> 		  {
> 		    _GLIBCXX_ASAN_ANNOTATE_GROW(__n);
>-		    std::__uninitialized_move_a(this->_M_impl._M_finish - __n,
>-						this->_M_impl._M_finish,
>-						this->_M_impl._M_finish,
>-						_M_get_Tp_allocator());
>-		    this->_M_impl._M_finish += __n;
>+		    __finish = std::__uninitialized_move_a
>+		      (__finish - __n, __finish, __finish, __allocator);
> 		    _GLIBCXX_ASAN_ANNOTATE_GREW(__n);
>-		    _GLIBCXX_MOVE_BACKWARD3(__position.base(),
>-					    __old_finish - __n, __old_finish);
>-		    std::copy(__first, __last, __position);
>+		    _GLIBCXX_MOVE_BACKWARD3
>+		      (__pos, __old_finish - __n, __old_finish);
>+		    std::copy(__first, __last, __pos);
> 		  }
> 		else
> 		  {
> 		    _ForwardIterator __mid = __first;
> 		    std::advance(__mid, __elems_after);
> 		    _GLIBCXX_ASAN_ANNOTATE_GROW(__n);
>-		    std::__uninitialized_copy_a(__mid, __last,
>-						this->_M_impl._M_finish,
>-						_M_get_Tp_allocator());
>-		    this->_M_impl._M_finish += __n - __elems_after;
>+		    __finish = std::__uninitialized_copy_a
>+		      (__mid, __last, __finish, __allocator);
> 		    _GLIBCXX_ASAN_ANNOTATE_GREW(__n - __elems_after);
>-		    std::__uninitialized_move_a(__position.base(),
>-						__old_finish,
>-						this->_M_impl._M_finish,
>-						_M_get_Tp_allocator());
>-		    this->_M_impl._M_finish += __elems_after;
>+		    __finish = std::__uninitialized_move_a
>+		      (__pos, __old_finish, __finish, __allocator);
> 		    _GLIBCXX_ASAN_ANNOTATE_GREW(__elems_after);
>-		    std::copy(__first, __mid, __position);
>+		    std::copy(__first, __mid, __pos);
> 		  }
>+
>+		this->_M_impl._M_finish = __finish;
> 	      }
> 	    else
> 	      {
>-		// Make local copies of these members because the compiler
>-		// thinks the allocator can alter them if 'this' is globally
>-		// reachable.
>-		pointer __old_start = this->_M_impl._M_start;
>-		pointer __old_finish = this->_M_impl._M_finish;
>-
>+		const size_type __size = size_type(__finish - __start);
> 		const size_type __len =
> 		  _M_check_len(__n, "vector::_M_range_insert");
>+		if (__len < __n + __size)
>+		  __builtin_unreachable();
>+
> 		pointer __new_start(this->_M_allocate(__len));
> 		pointer __new_finish(__new_start);
> 		__try
> 		  {
> 		    __new_finish
> 		      = std::__uninitialized_move_if_noexcept_a
>-		      (__old_start, __position.base(),
>-		       __new_start, _M_get_Tp_allocator());
>+		      (__start, __pos, __new_start, __allocator);
> 		    __new_finish
>-		      = std::__uninitialized_copy_a(__first, __last,
>-						    __new_finish,
>-						    _M_get_Tp_allocator());
>+		      = std::__uninitialized_copy_a
>+		      (__first, __last, __new_finish, __allocator);
> 		    __new_finish
> 		      = std::__uninitialized_move_if_noexcept_a
>-		      (__position.base(), __old_finish,
>-		       __new_finish, _M_get_Tp_allocator());
>+		      (__pos, __finish, __new_finish, __allocator);
> 		  }
> 		__catch(...)
> 		  {
>-		    std::_Destroy(__new_start, __new_finish,
>-				  _M_get_Tp_allocator());
>+		    std::_Destroy(__new_start, __new_finish, __allocator);
> 		    _M_deallocate(__new_start, __len);
> 		    __throw_exception_again;
> 		  }
>-		std::_Destroy(__old_start, __old_finish,
>-			      _M_get_Tp_allocator());
>+		std::_Destroy(__start, __finish, __allocator);
> 		_GLIBCXX_ASAN_ANNOTATE_REINIT;
>-		_M_deallocate(__old_start,
>-			      this->_M_impl._M_end_of_storage - __old_start);
>+		_M_deallocate(__start, __end - __start);
> 		this->_M_impl._M_start = __new_start;
> 		this->_M_impl._M_finish = __new_finish;
> 		this->_M_impl._M_end_of_storage = __new_start + __len;