From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757774AbbJ2QbE (ORCPT ); Thu, 29 Oct 2015 12:31:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54234 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756132AbbJ2QbA (ORCPT ); Thu, 29 Oct 2015 12:31:00 -0400 From: Vitaly Kuznetsov To: Andrew Morton Cc: Rasmus Villemoes , Andy Shevchenko , Ulf Hansson , James Bottomley , Kees Cook , linux-kernel@vger.kernel.org Subject: [PATCH v3 3/4] lib/string_helpers.c: don't lose precision in string_get_size() Date: Thu, 29 Oct 2015 17:30:49 +0100 Message-Id: <1446136250-11507-4-git-send-email-vkuznets@redhat.com> In-Reply-To: <1446136250-11507-1-git-send-email-vkuznets@redhat.com> References: <1446136250-11507-1-git-send-email-vkuznets@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org string_get_size() loses precision when there is a remainder for blk_size / divisor[units] and size is big enough. E.g string_get_size(8192, 4096, STRING_UNITS_10, ...) returns "32.7 MB" while it is supposed to return "33.5 MB". For some artificial inputs the result can be ridiculously wrong, e.g. string_get_size(3000, 1900, STRING_UNITS_10, ...) returns "3.00 MB" when "5.70 MB" is expected. The issues comes from the fact than we through away blk_size / divisor[units] remainder when size is > exp. This can be fixed by saving it and doing some non-trivial calculations later to fix the error but that would make this function even more cumbersome. Slightly re-factor the function to not lose the precision for all inputs. The overall complexity of this function comes from the fact that size can be huge and we don't want to do size * blk_size as it can overflow. Do the math in two steps: 1) Reduce size to something < U64_MAX / blk_size. 2) Multiply the result by blk_size and do final calculations. Suggested-by: Rasmus Villemoes Signed-off-by: Vitaly Kuznetsov --- Changes since v2: - Reduce size to something < U64_MAX / blk_size as a first step instead of blk_size * divisor[units], remove fixup code [Rasmus Villemoes] - Separate !blk_size check into a separate patch [Andy Shevchenko] --- lib/string_helpers.c | 31 ++++++++++++++----------------- 1 file changed, 14 insertions(+), 17 deletions(-) diff --git a/lib/string_helpers.c b/lib/string_helpers.c index ff3575b..29263c1 100644 --- a/lib/string_helpers.c +++ b/lib/string_helpers.c @@ -44,7 +44,7 @@ void string_get_size(u64 size, u32 blk_size, const enum string_size_units units, [STRING_UNITS_2] = 1024, }; int i, j; - u32 remainder = 0, sf_cap, exp; + u32 remainder = 0, sf_cap; char tmp[8]; const char *unit; @@ -58,27 +58,23 @@ void string_get_size(u64 size, u32 blk_size, const enum string_size_units units, if (!size) goto out; - while (blk_size >= divisor[units]) { - remainder = do_div(blk_size, divisor[units]); - i++; - } - - exp = divisor[units] / blk_size; /* - * size must be strictly greater than exp here to ensure that remainder - * is greater than divisor[units] coming out of the if below. + * size can be huge and doing size * blk_size right away can overflow. + * As a first step reduce huge size to something less than + * U64_MAX / blk_size. */ - if (size > exp) { - remainder = do_div(size, divisor[units]); - remainder *= blk_size; - i++; - } else { - remainder *= size; + while (size > div_u64(U64_MAX, blk_size)) { + /* + * We do not need to keep the remainder as blk_size is capped + * by U32_MAX and we'll still have enough precision after the + * loop. + */ + do_div(size, divisor[units]); + ++i; } + /* Now we're OK with doing size * blk_size, it won't overflow. */ size *= blk_size; - size += remainder / divisor[units]; - remainder %= divisor[units]; while (size >= divisor[units]) { remainder = do_div(size, divisor[units]); @@ -102,6 +98,7 @@ void string_get_size(u64 size, u32 blk_size, const enum string_size_units units, else unit = units_str[units][i]; + /* size is < divisor[units] here, (u32) is legit */ snprintf(buf, len, "%u%s %s", (u32)size, tmp, unit); } -- 2.4.3