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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r9-10060] libstdc++: Fix undefined behaviour in std::string Date: Mon, 9 May 2022 16:40:21 +0000 (GMT) [thread overview] Message-ID: <20220509164021.130533857415@sourceware.org> (raw) https://gcc.gnu.org/g:47c48fcfbec4d606cea7ea885323f453d8336604 commit r9-10060-g47c48fcfbec4d606cea7ea885323f453d8336604 Author: Jonathan Wakely <jwakely@redhat.com> Date: Tue May 4 15:49:38 2021 +0100 libstdc++: Fix undefined behaviour in std::string This fixes a ubsan error when constructing a string with a null pointer: bits/basic_string.h:534:21: runtime error: applying non-zero offset 18446744073709551615 to null pointer The _M_construct function only cares whether the second pointer is non-null, so create a non-null value without undefined arithmetic. We can also pass the random_access_iterator_tag directly to the _M_construct function, to avoid going via the tag dispatching _M_construct_aux, because we know we have pointers not integers here. libstdc++-v3/ChangeLog: * include/bits/basic_string.h (basic_string(const CharT*, const A&)): Do not do arithmetic on null pointer. (cherry picked from commit 789c57bc5fe023fc6dc72ade4afcb0916ff788d3) Diff: --- libstdc++-v3/include/bits/basic_string.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h index 6c11135fa8c..5977b5cb7c0 100644 --- a/libstdc++-v3/include/bits/basic_string.h +++ b/libstdc++-v3/include/bits/basic_string.h @@ -528,7 +528,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11 #endif basic_string(const _CharT* __s, const _Alloc& __a = _Alloc()) : _M_dataplus(_M_local_data(), __a) - { _M_construct(__s, __s ? __s + traits_type::length(__s) : __s+npos); } + { + const _CharT* __end = __s ? __s + traits_type::length(__s) + // We just need a non-null pointer here to get an exception: + : reinterpret_cast<const _CharT*>(__alignof__(_CharT)); + _M_construct(__s, __end, random_access_iterator_tag()); + } /** * @brief Construct string as multiple characters.
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