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 D86A2C4332F for ; Thu, 24 Nov 2022 02:52:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229505AbiKXCwx (ORCPT ); Wed, 23 Nov 2022 21:52:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43686 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229499AbiKXCwt (ORCPT ); Wed, 23 Nov 2022 21:52:49 -0500 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DF1FF2D759 for ; Wed, 23 Nov 2022 18:52:46 -0800 (PST) Received: from dggpemm500020.china.huawei.com (unknown [172.30.72.55]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4NHj9r2fjfzqSXT; Thu, 24 Nov 2022 10:48:48 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500020.china.huawei.com (7.185.36.49) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Thu, 24 Nov 2022 10:52:44 +0800 Received: from [10.174.178.55] (10.174.178.55) by dggpemm500006.china.huawei.com (7.185.36.236) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Thu, 24 Nov 2022 10:52:43 +0800 Subject: Re: [PATCH] kernfs: fix potential null-ptr-deref in kernfs_path_from_node_locked() From: "Leizhen (ThunderTown)" To: Tejun Heo CC: Greg Kroah-Hartman , References: <20221123020419.1867-1-thunder.leizhen@huawei.com> Message-ID: <55041efe-7443-d576-287b-49d1221fced2@huawei.com> Date: Thu, 24 Nov 2022 10:52:43 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.178.55] X-ClientProxiedBy: dggems706-chm.china.huawei.com (10.3.19.183) To dggpemm500006.china.huawei.com (7.185.36.236) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/11/24 10:28, Leizhen (ThunderTown) wrote: > > > On 2022/11/24 10:24, Leizhen (ThunderTown) wrote: >> >> >> On 2022/11/24 0:55, Tejun Heo wrote: >>> On Wed, Nov 23, 2022 at 10:04:19AM +0800, Zhen Lei wrote: >>>> Ensure that the 'buf' is not empty before strlcpy() uses it. >>>> >>>> Commit bbe70e4e4211 ("fs: kernfs: Fix possible null-pointer dereferences >>>> in kernfs_path_from_node_locked()") first noticed this, but it didn't >>>> fix it completely. >>>> >>>> Fixes: 9f6df573a404 ("kernfs: Add API to generate relative kernfs path") >>>> Signed-off-by: Zhen Lei >>> >>> I think the right thing to do is removing that if. It makes no sense to call >>> that function with NULL buf and the fact that nobody reported crashes on >>> NULL buf indicates that we in fact never do. >> >> OK. >> >> How about I remove "buf[0] = '\0';" too? It seems to be a useless operation. >> When 'kn_from' and 'kn_to' have a common ancestor, there must be a path from >> 'kn_from' to 'kn_to', and strlcpy() always fills in the terminator correctly, >> even if the buf is too small to save the first path node. > > Sorry, I misanalyzed. The length used by "len < buflen ? buflen - len : 0" may > be zero. Ah, my brain is unstable today. The initial value of len is 0. So "buf[0] = '\0';" can still be safely removed. > >> >> static void test(void) >> { >> char buf[4]; >> int i, n, buflen; >> >> buflen = 1; >> n = strlcpy(buf, "string", buflen); >> for (i = 0; i < buflen; i++) >> printk("%d: %02x\n", i, buf[i]); >> printk("n=%d\n\n", n); >> >> buflen = sizeof(buf); >> n = strlcpy(buf, "string", buflen); >> for (i = 0; i < buflen; i++) >> printk("%d: %02x\n", i, buf[i]); >> printk("n=%d\n", n); >> } >> >> Output: >> [ 33.691497] 0: 00 >> [ 33.691569] n=6 >> >> [ 33.691595] 0: 73 >> [ 33.691622] 1: 74 >> [ 33.691630] 2: 72 >> [ 33.691637] 3: 00 >> [ 33.691650] n=6 >> >>> >>> Thanks. >>> >> > -- Regards, Zhen Lei