From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id EDC903857C55; Wed, 13 Mar 2024 09:52:48 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org EDC903857C55 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1710323568; bh=tbFqPHUzqoyHqnyZD96vxfsLHWZ1tk0Z0iV3PGTSXqE=; h=From:To:Subject:Date:From; b=bv5ZMVZ7CQ8IqWuupQrDfRyFCiJGDpwzwl6CQwh4VQX285BLwjYdexYKabDDEVQMX ie8toM44lzRls1NqN/jpokCqR3pSpbWF1KYKeLq1F2qxMZNGpvdWb05XD4a9iS1PU8 m2EvnBfHraLAl4i2vz3cpp+98ehk4ONpLASmV9+Q= MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-10208] libstdc++: Fix UB in weekday::weekday(sys_days) and add test X-Act-Checkin: gcc X-Git-Author: Cassio Neri X-Git-Refname: refs/heads/releases/gcc-12 X-Git-Oldrev: 6a858b742c12b82496cd45cfd8b776fbcb474d6b X-Git-Newrev: 2d5eee092aa3cbfb50496ed4b224600de2e7fb08 Message-Id: <20240313095248.EDC903857C55@sourceware.org> Date: Wed, 13 Mar 2024 09:52:48 +0000 (GMT) List-Id: https://gcc.gnu.org/g:2d5eee092aa3cbfb50496ed4b224600de2e7fb08 commit r12-10208-g2d5eee092aa3cbfb50496ed4b224600de2e7fb08 Author: Cassio Neri 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::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. (cherry picked from commit f6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22) 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 83e0e5b8bd0..f5a88331b1b 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -573,8 +573,15 @@ _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 6e7989ec103..96cd5ebc0d8 100644 --- a/libstdc++-v3/testsuite/std/time/weekday/1.cc +++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc @@ -21,6 +21,7 @@ // Class template day [time.cal.weekday] #include +#include constexpr void constexpr_weekday() @@ -38,6 +39,14 @@ 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::max()}}}; + constexpr weekday min{sys_days{days{numeric_limits::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);