From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 7FC313857C72; Wed, 13 Mar 2024 09:53:09 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 7FC313857C72 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1710323589; bh=gBxNjAi4OJzF3BxXFZPpWJ/OWfOFHgTqH+vGwmeowcQ=; h=From:To:Subject:Date:From; b=XhZsEZTk1rIU/YPtT+lPqPP1IHAvcGBXdAlN0JaXg4xCdG4g9vp29xVw+xOmRc28R M9XEjawXjJNodcejqUFpVu0/yN9AMZrhZtHrhnZ3ILmzv46iPaGP6a2uoswN0Nh464 TkIHOapbvCfsrYjW00ySUc1yeQ7Qje1X0uH2dAAI= 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 r12-10212] libstdc++: Remove UB from month and weekday additions and subtractions. X-Act-Checkin: gcc X-Git-Author: Cassio Neri X-Git-Refname: refs/heads/releases/gcc-12 X-Git-Oldrev: a491dd0eac360758dd20b29bef78b27d005547a1 X-Git-Newrev: 2f09b1c7b936f83da597b4a37aa5619c5e36dfc4 Message-Id: <20240313095309.7FC313857C72@sourceware.org> Date: Wed, 13 Mar 2024 09:53:09 +0000 (GMT) List-Id: https://gcc.gnu.org/g:2f09b1c7b936f83da597b4a37aa5619c5e36dfc4 commit r12-10212-g2f09b1c7b936f83da597b4a37aa5619c5e36dfc4 Author: Cassio Neri Date: Sun Dec 10 11:31:31 2023 +0000 libstdc++: Remove UB from month and weekday additions and subtractions. The following invoke signed integer overflow (UB) [1]: month + months{MAX} // where MAX is the maximum value of months::rep month + months{MIN} // where MIN is the maximum value of months::rep month - months{MIN} // where MIN is the minimum value of months::rep weekday + days {MAX} // where MAX is the maximum value of days::rep weekday - days {MIN} // where MIN is the minimum value of days::rep For the additions to MAX, the crux of the problem is that, in libstdc++, months::rep and days::rep are int64_t. Other implementations use int32_t, cast operands to int64_t and perform arithmetic operations without risk of overflowing. For month + months{MIN}, the implementation follows the Standard's "returns clause" and evaluates: modulo(static_cast(unsigned{__x}) + (__y.count() - 1), 12); Overflow occurs when MIN - 1 is evaluated. Casting to a larger type could help but, unfortunately again, this is not possible for libstdc++. For the subtraction of MIN, the problem is that -MIN is not representable. It's fair to say that the intention is for these additions/subtractions to be performed in modulus (12 or 7) arithmetic so that no overflow is expected. To fix these UB, this patch implements: template unsigned __add_modulo(unsigned __x, _T __y); template unsigned __sub_modulo(unsigned __x, _T __y); which respectively, returns the remainder of Euclidean division of, __x + __y and __x - __y by __d without overflowing. These functions replace constexpr unsigned __modulo(long long __n, unsigned __d); which also calculates the reminder of __n, where __n is the result of the addition or subtraction. Hence, these operations might invoke UB before __modulo is called and thus, __modulo can't do anything to remediate the issue. In addition to solve the UB issues, __add_modulo and __sub_modulo allow better codegen (shorter and branchless) on x86-64 and ARM [2]. [1] https://godbolt.org/z/a9YfWdn57 [2] https://godbolt.org/z/Gh36cr7E4 libstdc++-v3/ChangeLog: * include/std/chrono: Fix + and - for months and weekdays. * testsuite/std/time/month/1.cc: Add constexpr tests against overflow. * testsuite/std/time/month/2.cc: New test for extreme values. * testsuite/std/time/weekday/1.cc: Add constexpr tests against overflow. * testsuite/std/time/weekday/2.cc: New test for extreme values. (cherry picked from commit 2cb3d42d3f3e7a5345ee7a6f3676a10c84864d72) Diff: --- libstdc++-v3/include/std/chrono | 79 +++++++++++++++++++--------- libstdc++-v3/testsuite/std/time/month/1.cc | 19 +++++++ libstdc++-v3/testsuite/std/time/month/2.cc | 32 +++++++++++ libstdc++-v3/testsuite/std/time/weekday/1.cc | 16 +++++- libstdc++-v3/testsuite/std/time/weekday/2.cc | 32 +++++++++++ 5 files changed, 151 insertions(+), 27 deletions(-) diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 3c5b425c7fe..6da08f9cf13 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -134,18 +134,47 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION namespace __detail { - // Compute the remainder of the Euclidean division of __n divided by __d. - // Euclidean division truncates toward negative infinity and always - // produces a remainder in the range of [0,__d-1] (whereas standard - // division truncates toward zero and yields a nonpositive remainder - // for negative __n). + // Helper to __add_modulo and __sub_modulo. + template + consteval auto + __modulo_offset() + { + using _Up = make_unsigned_t<_Tp>; + auto constexpr __a = _Up(-1) - _Up(255 + __d - 2); + auto constexpr __b = _Up(__d * (__a / __d) - 1); + // Notice: b <= a - 1 <= _Up(-1) - (255 + d - 1) and b % d = d - 1. + return _Up(-1) - __b; // >= 255 + d - 1 + } + + // Compute the remainder of the Euclidean division of __x + __y divided by + // __d without overflowing. Typically, __x <= 255 + d - 1 is sum of + // weekday/month with a shift in [0, d - 1] and __y is a duration count. + template constexpr unsigned - __modulo(long long __n, unsigned __d) + __add_modulo(unsigned __x, _Tp __y) { - if (__n >= 0) - return __n % __d; - else - return (__d + (__n % __d)) % __d; + using _Up = make_unsigned_t<_Tp>; + // For __y >= 0, _Up(__y) has the same mathematical value as __y and + // this function simply returns (__x + _Up(__y)) % d. Typically, this + // doesn't overflow since the range of _Up contains many more positive + // values than _Tp's. For __y < 0, _Up(__y) has a mathematical value in + // the upper-half range of _Up so that adding a positive value to it + // might overflow. Moreover, most likely, _Up(__y) != __y mod d. To + // fix both issues we subtract from _Up(__y) an __offset >= + // 255 + d - 1 to make room for the addition to __x and shift the modulo + // to the correct value. + auto const __offset = __y >= 0 ? _Up(0) : __modulo_offset<__d, _Tp>(); + return (__x + _Up(__y) - __offset) % __d; + } + + // Similar to __add_modulo but for __x - __y. + template + constexpr unsigned + __sub_modulo(unsigned __x, _Tp __y) + { + using _Up = make_unsigned_t<_Tp>; + auto const __offset = __y <= 0 ? _Up(0) : __modulo_offset<__d, _Tp>(); + return (__x - _Up(__y) - __offset) % __d; } inline constexpr unsigned __days_per_month[12] @@ -339,8 +368,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr month operator+(const month& __x, const months& __y) noexcept { - auto __n = static_cast(unsigned{__x}) + (__y.count() - 1); - return month{__detail::__modulo(__n, 12) + 1}; + // modulo(x + (y - 1), 12) = modulo(x + (y - 1) + 12, 12) + // = modulo((x + 11) + y , 12) + return month{1 + __detail::__add_modulo<12>( + unsigned{__x} + 11, __y.count())}; } friend constexpr month @@ -349,7 +380,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr month operator-(const month& __x, const months& __y) noexcept - { return __x + -__y; } + { + // modulo(x + (-y - 1), 12) = modulo(x + (-y - 1) + 12, 12) + // = modulo((x + 11) - y , 12) + return month{1 + __detail::__sub_modulo<12>( + unsigned{__x} + 11, __y.count())}; + } friend constexpr months operator-(const month& __x, const month& __y) noexcept @@ -573,15 +609,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION static constexpr weekday _S_from_days(const days& __d) { - 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); + return weekday{__detail::__add_modulo<7>(4, __d.count())}; } public: @@ -671,8 +699,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr weekday operator+(const weekday& __x, const days& __y) noexcept { - auto __n = static_cast(__x._M_wd) + __y.count(); - return weekday{__detail::__modulo(__n, 7)}; + return weekday{__detail::__add_modulo<7>(__x._M_wd, __y.count())}; } friend constexpr weekday @@ -681,7 +708,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr weekday operator-(const weekday& __x, const days& __y) noexcept - { return __x + -__y; } + { + return weekday{__detail::__sub_modulo<7>(__x._M_wd, __y.count())}; + } friend constexpr days operator-(const weekday& __x, const weekday& __y) noexcept diff --git a/libstdc++-v3/testsuite/std/time/month/1.cc b/libstdc++-v3/testsuite/std/time/month/1.cc index 8278386932c..7435cfabedd 100644 --- a/libstdc++-v3/testsuite/std/time/month/1.cc +++ b/libstdc++-v3/testsuite/std/time/month/1.cc @@ -21,6 +21,7 @@ // Class template day [time.cal.month] #include +#include constexpr void constexpr_month() @@ -35,6 +36,24 @@ constexpr_month() dm += months{3}; dm -= months{3}; + // Test for UB (overflow). + { + using rep = months::rep; + using std::numeric_limits; + + auto constexpr months_min = months{numeric_limits::min()}; + auto constexpr month_000_plus_months_min = month{ 0 } + months_min; + auto constexpr month_255_plus_months_min = month{255} + months_min; + auto constexpr month_000_minus_months_min = month{ 0 } - months_min; + auto constexpr month_255_minus_months_min = month{255} - months_min; + + auto constexpr months_max = months{numeric_limits::max()}; + auto constexpr month_000_plus_months_max = month{ 0 } + months_max; + auto constexpr month_255_plus_months_max = month{255} + months_max; + auto constexpr month_000_minus_months_max = month{ 0 } - months_max; + auto constexpr month_255_minus_months_max = month{255} - months_max; + } + static_assert(February + months{11} == January); static_assert(January + months{1200} == January); static_assert(January + months{1201} == February); diff --git a/libstdc++-v3/testsuite/std/time/month/2.cc b/libstdc++-v3/testsuite/std/time/month/2.cc new file mode 100644 index 00000000000..3bcefa60003 --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/month/2.cc @@ -0,0 +1,32 @@ +// { dg-do run { target c++20 } } + +// Class month [time.cal.month] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(months extreme) +{ + auto const count = extreme.count(); + auto const safe = count < 0 ? count + 12 : count; + auto const mod = safe - 12 * ((safe < 0 ? safe - 11 : safe) / 12); + + for (unsigned m = 0; m < 256; ++m) + { + auto const month_plus_extreme = month{m} + extreme; + VERIFY(unsigned{month_plus_extreme } == (m + 11 + mod) % 12 + 1); + + auto const month_minus_extreme = month{m} - extreme; + VERIFY(unsigned{month_minus_extreme} == (m + 11 - mod) % 12 + 1); + } +} + +int main() +{ + test_extreme_values(months{std::numeric_limits::max()}); + test_extreme_values(months{std::numeric_limits::min()}); + return 0; +} diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc b/libstdc++-v3/testsuite/std/time/weekday/1.cc index 96cd5ebc0d8..0e31a8b599f 100644 --- a/libstdc++-v3/testsuite/std/time/weekday/1.cc +++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc @@ -43,8 +43,20 @@ constexpr_weekday() { 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()}}}; + + auto constexpr days_min = days{numeric_limits::min()}; + auto constexpr weekday_from_sysdays_min = weekday{sys_days{days_min}}; + auto constexpr weekday_000_plus_days_min = weekday{ 0 } + days_min; + auto constexpr weekday_255_plus_days_min = weekday{255} + days_min; + auto constexpr weekday_000_minus_days_min = weekday{ 0 } - days_min; + auto constexpr weekday_255_minus_days_min = weekday{255} - days_min; + + auto constexpr days_max = days{numeric_limits::max()}; + auto constexpr weekday_from_sysdays_max = weekday{sys_days{days_max}}; + auto constexpr weekday_000_plus_days_max = weekday{ 0 } + days_max; + auto constexpr weekday_255_plus_days_max = weekday{255} + days_max; + auto constexpr weekday_000_minus_days_max = weekday{ 0 } - days_max; + auto constexpr weekday_255_minus_days_max = weekday{255} - days_max; } static_assert(weekday{sys_days{1900y/January/1}} == Monday); diff --git a/libstdc++-v3/testsuite/std/time/weekday/2.cc b/libstdc++-v3/testsuite/std/time/weekday/2.cc new file mode 100644 index 00000000000..924709321e5 --- /dev/null +++ b/libstdc++-v3/testsuite/std/time/weekday/2.cc @@ -0,0 +1,32 @@ +// { dg-do run { target c++20 } } + +// Class weekday [time.cal.wd] + +#include +#include +#include + +using namespace std::chrono; + +void test_extreme_values(days extreme) +{ + auto const count = extreme.count(); + auto const safe = count < 0 ? count + 7 : count; + auto const mod = safe - 7 * ((safe < 0 ? safe - 6 : safe) / 7); + + for (unsigned d = 0; d < 254; ++d) + { + auto const weekday_plus_extreme = weekday{d} + extreme; + VERIFY(weekday_plus_extreme.c_encoding() == (d + mod) % 7); + + auto const weekday_minus_extreme = weekday{d} - extreme; + VERIFY(weekday_minus_extreme.c_encoding() == (d + 7 - mod) % 7); + } +} + +int main() +{ + test_extreme_values(days{std::numeric_limits::max()}); + test_extreme_values(days{std::numeric_limits::min()}); + return 0; +}