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 95150C4332F for ; Wed, 1 Nov 2023 23:51:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230208AbjKAXva (ORCPT ); Wed, 1 Nov 2023 19:51:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49766 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231359AbjKAXv0 (ORCPT ); Wed, 1 Nov 2023 19:51:26 -0400 Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2607:7c80:54:3::133]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 27A29126; Wed, 1 Nov 2023 16:51:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Sender:Reply-To:Content-ID:Content-Description; bh=KRgbwZSg+I8F4GIWxZtVNCJODdPzeC1pCi/Dya698bY=; b=jRTV0RXOOtdPIBwTv30bAO9kVO eG7/s4QyT4OltzFZZ/ZFrZpucTGfEwluKPNcSgw0wErQqeG3pcS0NKJSg2yvrFOrMDRv3BCxstay7 24aCUC9B8ow6ha+pnJnNitqSDC4w0ZIQn0d1XWbiiwgJmke7qnsXwqmMvcZcAholaSRUT2zkzPwZm p0w/ebAS9BUM8YTPsPviPUxRQAMaELqTcp3TenRwXinrB+dHusyIlI5VjUIHNPA9esobcXjBBmgLe knyZjy8lQlvU5ed/lkSZPQP8gVT3Osavf142MA5P8BMGFqKRjTHFYRjmXJaeKLODpo0lK2vDtqcU3 1wwYzrpQ==; Received: from [50.53.46.231] (helo=[192.168.254.15]) by bombadil.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1qyKzD-008MgL-1I; Wed, 01 Nov 2023 23:50:59 +0000 Message-ID: <4db3c258-a7c8-4e92-83db-797da32dd336@infradead.org> Date: Wed, 1 Nov 2023 16:50:57 -0700 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] 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 References: <20231101153237.2214698-1-colin.i.king@gmail.com> From: Randy Dunlap In-Reply-To: <20231101153237.2214698-1-colin.i.king@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/1/23 08:32, 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 encounterd. 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 int, the overhead in > multiplication speed for a 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 int, 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 Reviewed-by: Randy Dunlap Tested-by: Randy Dunlap num_digits() now works for all int values. Thanks. > --- > 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) > { > - int m = 10; > + long m = 10; > int d = 1; > > if (val < 0) { -- ~Randy