From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 71B5CC4332F for ; Fri, 3 Nov 2023 02:09:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229820AbjKCCJC (ORCPT ); Thu, 2 Nov 2023 22:09:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46108 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229447AbjKCCJA (ORCPT ); Thu, 2 Nov 2023 22:09:00 -0400 Received: from mail.nfschina.com (unknown [42.101.60.195]) by lindbergh.monkeyblade.net (Postfix) with SMTP id 5B464196; Thu, 2 Nov 2023 19:08:57 -0700 (PDT) Received: from [172.30.11.106] (unknown [180.167.10.98]) by mail.nfschina.com (Maildata Gateway V2.8.8) with ESMTPSA id 174C76082CF9C; Fri, 3 Nov 2023 10:08:46 +0800 (CST) Message-ID: Date: Fri, 3 Nov 2023 10:08:45 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.8.0 Subject: Re: [PATCH][V2] x86/lib: Fix overflow of variable m when val >= 1410065408 Content-Language: en-US To: Colin Ian King , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H . Peter Anvin" Cc: kernel-janitors@vger.kernel.org, linux-kernel@vger.kernel.org X-MD-Sfrom: suhui@nfschina.com X-MD-SrcIP: 180.167.10.98 From: Su Hui In-Reply-To: <20231102174901.2590325-1-colin.i.king@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2023/11/3 01:49, Colin Ian King wrote: > There is an overflow in variable m in function num_digits when val > is >= 1410065408 which leads to the digit calculation loop to > iterate more times than required. This results in either more > digits being counted or in some cases (for example where val is > 1932683193) the value of m eventually overflows to zero and the > while loop spins forever). > > Currently the function num_digits is currently only being used for > small values of val in the SMP boot stage for digit counting on the > number of cpus and NUMA nodes, so the overflow is never encountered. > However it is useful to fix the overflow issue in case the function > is used for other purposes in the future. (The issue was discovered > while investigating the digit counting performance in various > kernel helper functions rather than any real-world use-case). > > The simplest fix is to make m a long long, the overhead in > multiplication speed for a long long is very minor for small values > of val less than 10000 on modern processors. The alternative > fix is to replace the multiplication with a constant division > by 10 loop (this compiles down to an multiplication and shift) > without needing to make m a long long, but this is slightly slower > than the fix in this commit when measured on a range of x86 > processors). > > Fixes: 646e29a1789a ("x86: Improve the printout of the SMP bootup CPU table") > Signed-off-by: Colin Ian King > --- > > V2: Make m long long instead of long to fix issue for i386 as well as > x86-64 > --- > arch/x86/lib/misc.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c > index 92cd8ecc3a2c..41e26e246d8f 100644 > --- a/arch/x86/lib/misc.c > +++ b/arch/x86/lib/misc.c > @@ -8,7 +8,7 @@ > */ > int num_digits(int val) Hi, num_digits() still has a problem when val = INT_MIN. 14 if (val < 0) { 15 d++; 16 val = -val; ^^^^^^^^^^^ When val = INT_MIN, -val still equal to INT_MIN. Make 'val' long long instead of int to fix this issue. Su Hui > { > - int m = 10; > + long long m = 10; > int d = 1; > > if (val < 0) {