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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r14-5469] libstdc++: Fix UB in weekday::weekday(sys_days) and add test Date: Tue, 14 Nov 2023 22:46:16 +0000 (GMT) [thread overview] Message-ID: <20231114224616.8E35F385AC1C@sourceware.org> (raw) https://gcc.gnu.org/g:f6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22 commit r14-5469-gf6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22 Author: Cassio Neri <cassio.neri@gmail.com> Date: Sun Nov 12 01:33:52 2023 +0000 libstdc++: Fix UB in weekday::weekday(sys_days) and add test The following has undefined behaviour (signed overflow) [1]: weekday max{sys_days{days{numeric_limits<days::rep>::max()}}}; The issue is in this line when __n is very large and __n + 4 overflows: return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6); In addition to fixing this bug, the new implementation makes the compiler emit shorter and branchless code for x86-64 and ARM [2]. [1] https://godbolt.org/z/1s5bv7KfT [2] https://godbolt.org/z/zKsabzrhs libstdc++-v3/ChangeLog: * include/std/chrono (weekday::_S_from_days): Fix UB. * testsuite/std/time/weekday/1.cc: Add test for overflow. Diff: --- libstdc++-v3/include/std/chrono | 11 +++++++++-- libstdc++-v3/testsuite/std/time/weekday/1.cc | 9 +++++++++ 2 files changed, 18 insertions(+), 2 deletions(-) diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 45e7d269e4b..bd21ec189a5 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -930,8 +930,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION static constexpr weekday _S_from_days(const days& __d) { - auto __n = __d.count(); - return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6); + using _Rep = days::rep; + using _URep = make_unsigned_t<_Rep>; + const auto __n = __d.count(); + const auto __m = static_cast<_URep>(__n); + + // 1970-01-01 (__n = 0, __m = 0 ) -> Thursday (4) + // 1969-31-12 (__n = -1, __m = _URep(-1)) -> Wednesday (3) + const auto __offset = __n >= 0 ? _URep(4) : 3 - _URep(-1) % 7 - 7; + return weekday((__m + __offset) % 7); } public: diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc b/libstdc++-v3/testsuite/std/time/weekday/1.cc index 00278c8b01c..e89fca47d4b 100644 --- a/libstdc++-v3/testsuite/std/time/weekday/1.cc +++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc @@ -20,6 +20,7 @@ // Class template day [time.cal.weekday] #include <chrono> +#include <limits> constexpr void constexpr_weekday() @@ -37,6 +38,14 @@ constexpr_weekday() static_assert(weekday{3}[2].weekday() == weekday{3}); static_assert(weekday{3}[last].weekday() == weekday{3}); + // Test for UB (overflow). + { + using rep = days::rep; + using std::numeric_limits; + constexpr weekday max{sys_days{days{numeric_limits<rep>::max()}}}; + constexpr weekday min{sys_days{days{numeric_limits<rep>::min()}}}; + } + static_assert(weekday{sys_days{1900y/January/1}} == Monday); static_assert(weekday{sys_days{1970y/January/1}} == Thursday); static_assert(weekday{sys_days{2020y/August/21}} == Friday);
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