https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109703 From d50f2599d7b23bdba05a9102645d082ed9bcb05f Mon Sep 17 00:00:00 2001 From: Kefu Chai Date: Mon, 1 May 2023 21:24:26 +0100 Subject: [PATCH] libstdc++: Set _M_string_length before calling _M_dispose() [PR109703] This always sets _M_string_length in the constructor for ranges of input iterators, such as stream iterators. We copy from the source range to the local buffer, and then repeatedly reallocate a larger one if necessary. When disposing the old buffer, _M_is_local() is used to tell if the buffer is the local one or not (and so must be deallocated). In addition to comparing the buffer address with the local buffer, _M_is_local() has an optimization hint so that the compiler knows that for a string using the local buffer, there is an invariant that _M_string_length <= _S_local_capacity (added for PR109299 via r13-6915-gbf78b43873b0b7). But we failed to set _M_string_length in the constructor taking a pair of iterators, so the invariant might not hold, and __builtin_unreachable() is reached. This causes UBsan errors, and potentially misoptimization. To ensure the invariant holds, _M_string_length is initialized to zero before doing anything else, so that _M_is_local() doesn't see an uninitialized value. This issue only surfaces when constructing a string with a range of input iterator, and the uninitialized _M_string_length happens to be greater than _S_local_capacity, i.e., 15 for the std::string specialization. libstdc++-v3/ChangeLog: PR libstdc++/109703 * include/bits/basic_string.h (basic_string(Iter, Iter, Alloc)): Initialize _M_string_length. Signed-off-by: Kefu Chai Co-authored-by: Jonathan Wakely (cherry picked from commit cbf6c7a1d16490a1e63e9a5ce00e9a5c44c4c2f2) --- a/libstdc++-v3/include/bits/basic_string.h +++ b/libstdc++-v3/include/bits/basic_string.h @@ -760,7 +760,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 _GLIBCXX20_CONSTEXPR basic_string(_InputIterator __beg, _InputIterator __end, const _Alloc& __a = _Alloc()) - : _M_dataplus(_M_local_data(), __a) + : _M_dataplus(_M_local_data(), __a), _M_string_length(0) { #if __cplusplus >= 201103L _M_construct(__beg, __end, std::__iterator_category(__beg)); -- 2.31.1