From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-x32c.google.com (mail-wm1-x32c.google.com [IPv6:2a00:1450:4864:20::32c]) by sourceware.org (Postfix) with ESMTPS id E829F3858D1E; Sat, 18 Nov 2023 00:07:24 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org E829F3858D1E Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com ARC-Filter: OpenARC Filter v1.0.0 sourceware.org E829F3858D1E Authentication-Results: server2.sourceware.org; arc=none smtp.remote-ip=2a00:1450:4864:20::32c ARC-Seal: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1700266047; cv=none; b=GnwXUBX/Ck73HDU1WbABqtnPqKGUBDJx2jm/xrjIJfbqVXFx1eGjiEU761rlaujgDMoUXsykn9iGd9XZ9eoKorcjzLAbFqGTAdMh+X+dSPOyVSrVuLWzg7nNc0EONapKdjO7sME5ey3u8nDFNkPZTBgFLAeIqlSqVdUEDulEvoo= ARC-Message-Signature: i=1; a=rsa-sha256; d=sourceware.org; s=key; t=1700266047; c=relaxed/simple; bh=dX8Cxnaui3k725Fgv7aI7ZWgmb73/GLhgvK4+2qgRwU=; h=DKIM-Signature:From:To:Subject:Date:Message-ID:MIME-Version; b=aTpBp1p7A3uS9YGWaFa6TFcwZ/w6yL2jMBC1hQACUtU8xxvPEBYz4gCLKT+LAShWnLw4m1+2mRvbdWbLR5mFcqMfjwONpCLA9y35Nt8QxYDu1EAio5hGbbZnDrHg/6Jv8lygkMqEDa6egmIrEXF6T9VyEb7OBI5J0MGuCkPklaA= ARC-Authentication-Results: i=1; server2.sourceware.org Received: by mail-wm1-x32c.google.com with SMTP id 5b1f17b1804b1-40a48775c58so206115e9.3; Fri, 17 Nov 2023 16:07:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1700266043; x=1700870843; darn=gcc.gnu.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=KbWIZxc86qPdk//BQZv9v1wsPyK0N70BlUOGLr/zuvI=; b=GTy1Lwwi3RWzZfqTxJ8pgmXb+9iCajep627nrvR7jqN9m0AOI08DDjtif9WJhX/Uee 28sFwVAq5IjW5DuV31xWlzHvmM58G3Eqzh0sei3joGCvDmM24zTplRWCTgrsbEiPfT2Z 5qPyTm2KAoMVLdmMfzEWzcvZqq6kQsZOMcTvysNOEmwKcJcoc1Eq+tTCv88z9yNb28N8 0SZrW5g95fGS9NsXZle0fHK0rBopNCHe366UPyZyW8cCP9TJRliO0ersDKZvJ8V9hnzQ H+kgQkCeKT8CbFgh8wFWBK86Nt6MdLam2Ris9I6V1G0drlHWXKwmXz+LODjL3c6xv0r6 YCbA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1700266043; x=1700870843; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=KbWIZxc86qPdk//BQZv9v1wsPyK0N70BlUOGLr/zuvI=; b=eSBHlZpmIyG1OgnKCV7qOOdNaeV+E7yKpiiPW3OR5AMJDtyz22ZZ4FLSICAbkQSZmn AVYdZ70yLWPi+1AqyQPZOskMPr+Vmg4KVEP+oOPNt0QgVcEUDoQQfx5c1HVWacjzLIOH fo2J8iucYjsgH78LbSL1G80HrklijEGTxT4sE5t+gr5sR/Yd1AK7MmMSbCHiV4x2/uOs uaKt1zoat1SkGmRAVd4mzLS7DWMUW+mDooOS8gUVSNRdSbUTMwAm8gIyNVRNfqBsRe6x hUXlXbsoBxE4dagLdedIFgACWaImTIN9/b/agsNdTQmeurjjhp93G7IbepaIhDdAKIbL YSAQ== X-Gm-Message-State: AOJu0Yy5mzJsa3jcjEbG+yKoFw3gBn6rgpBw9SOQA4TeChO60+5qwKpn /pMG65kG3gEUy74oQK1jy5cbKaT8zIc= X-Google-Smtp-Source: AGHT+IEMr4o6gWNYxhpxJbMRDhPD8URvkU12zPujGLsFBaSrPdiZEN0K4B4dyJZt3x7jUX6uvjmJUw== X-Received: by 2002:a5d:5f94:0:b0:32f:8a45:93a8 with SMTP id dr20-20020a5d5f94000000b0032f8a4593a8mr460545wrb.0.1700266042552; Fri, 17 Nov 2023 16:07:22 -0800 (PST) Received: from othello.cust.communityfibre.co.uk ([2a02:6b64:8086:0:f9dd:28dd:c9b9:d8f4]) by smtp.gmail.com with ESMTPSA id m5-20020a056000180500b003316a32cb25sm3269883wrh.86.2023.11.17.16.07.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 17 Nov 2023 16:07:21 -0800 (PST) From: Cassio Neri To: libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org Cc: Cassio Neri Subject: [PATCH v2] The following functions invoke signed integer overflow (UB) for some extreme values of days and months [1]: Date: Sat, 18 Nov 2023 00:06:53 +0000 Message-ID: <20231118000653.6924-1-cassio.neri@gmail.com> X-Mailer: git-send-email 2.41.0 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-12.0 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP,T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: weekday operator+(const weekday& x, const days& y); // #1 month operator+(const month& x, const months& y); // #2 For #1 the problem is that in libstdc++ days::rep is int64_t. Other implementations use int32_t and cast operands to int64_t. Hence then perform arithmetic operations without fear of overflowing. For instance, #1 evaluates: modulo(static_cast(unsigned{x}._M_wd) + __y.count(), 7); For x86-64, long long is int64 so the cast is useless. For #2, casting to a larger type could help but all implementations follow the Standard's "Returns clause" and evaluate: modulo(static_cast(unsigned{__x}) + (__y.count() - 1), 12); Hence, overflow occurs when __y.count() is the minimum value of its type. When long long is larger than months::rep, this is a fix: modulo(static_cast(unsigned{__x}) + 11 + __y.count(), 12); Again, this is not possible for libstdc++. The fix uses this new function: template unsigned __add_modulo(unsigned __x, _T __y); which returns the remainder of Euclidean division of __x +__y by __d without overflowing. This function replaces constexpr unsigned __modulo(long long __n, unsigned __d); In addition to solve the UB issues, __add_modulo allows shorter branchless code on x86-64 and ARM [2]. [1] https://godbolt.org/z/WqvosbrvG [2] https://godbolt.org/z/o63794GEE libstdc++-v3/ChangeLog: * include/std/chrono: Fix operator+ 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. --- libstdc++-v3/include/std/chrono | 61 ++++++++++++-------- libstdc++-v3/testsuite/std/time/month/1.cc | 9 +++ libstdc++-v3/testsuite/std/time/weekday/1.cc | 8 +++ 3 files changed, 54 insertions(+), 24 deletions(-) Changes with respect to previous versions: v2: Replaced _T with _Tp and _U with _Up. Removed copyright+license from test. diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 10bdd1c4ede..691bb106bb9 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -497,18 +497,38 @@ _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). + // 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 and an offset in [0, d - 1] and __y is a duration count. + // For instance, [time.cal.month.nonmembers] says that given month x and + // months y, to get x + y one must calculate: + // + // modulo(static_cast(unsigned{x}) + (y.count() - 1), 12) + 1. + // + // Since y.count() is a 64-bits signed value the subtraction y.count() - 1 + // or the addition of this value with static_cast(unsigned{x}) + // might overflow. This function can be used to avoid this problem: + // __add_modulo<12>(unsigned{x} + 11, y.count()) + 1; + // (More details in the implementation of operator+(month, months).) + template constexpr unsigned - __modulo(long long __n, unsigned __d) - { - if (__n >= 0) - return __n % __d; - else - return (__d + (__n % __d)) % __d; + __add_modulo(unsigned __x, _Tp __y) + { + 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 from _Up(__y)"subtract" an __offset >= + // 255 + d - 1 to make room for the addition to __x and shift the modulo + // to the correct value. + auto constexpr __a = _Up(-1) - _Up(255 + __d - 2); + auto constexpr __b = _Up(__d * (__a / __d) - 1); + // Notice: b <= a - 1 <= _Up(-1) - _Up(255 + d - 1) and b % d = d - 1. + auto const __offset = __y >= 0 ? _Up(0) : __b - _Up(-1); + return (__x + _Up(__y) + __offset) % __d; } inline constexpr unsigned __days_per_month[12] @@ -700,8 +720,10 @@ _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 @@ -930,15 +952,7 @@ _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: @@ -1028,8 +1042,7 @@ _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 diff --git a/libstdc++-v3/testsuite/std/time/month/1.cc b/libstdc++-v3/testsuite/std/time/month/1.cc index 773f772bf07..f72a4ab9a5d 100644 --- a/libstdc++-v3/testsuite/std/time/month/1.cc +++ b/libstdc++-v3/testsuite/std/time/month/1.cc @@ -20,6 +20,7 @@ // Class template day [time.cal.month] #include +#include constexpr void constexpr_month() @@ -71,4 +72,12 @@ constexpr_month() static_assert(month{0} <=> month{1} == std::strong_ordering::less); static_assert(month{3} <=> month{3} == std::strong_ordering::equal); static_assert(month{5} <=> month{2} == std::strong_ordering::greater); + + auto constexpr months_min = months{std::numeric_limits::min()}; + auto constexpr m0_min = month{ 0 } + months_min; + auto constexpr m255_min = month{255} + months_min; + + auto constexpr months_max = months{std::numeric_limits::max()}; + auto constexpr m0_max = month{ 0 } + months_max; + auto constexpr m255_max = month{255} + months_max; } diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc b/libstdc++-v3/testsuite/std/time/weekday/1.cc index e89fca47d4b..cc8b0db78f8 100644 --- a/libstdc++-v3/testsuite/std/time/weekday/1.cc +++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc @@ -107,4 +107,12 @@ constexpr_weekday() static_assert(weekday{7} == weekday{0}); static_assert(!(weekday{0} == weekday{1})); static_assert( (weekday{0} != weekday{2})); + + auto constexpr days_min = days{std::numeric_limits::min()}; + auto constexpr wd0_min = weekday{ 0 } + days_min; + auto constexpr wd255_min = weekday{255} + days_min; + + auto constexpr days_max = days{std::numeric_limits::max()}; + auto constexpr m0_max = weekday{ 0 } + days_max; + auto constexpr m255_max = weekday{255} + days_max; } -- 2.41.0