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 07A8CC433FE for ; Thu, 24 Nov 2022 02:24:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229697AbiKXCY2 (ORCPT ); Wed, 23 Nov 2022 21:24:28 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47812 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229448AbiKXCYZ (ORCPT ); Wed, 23 Nov 2022 21:24:25 -0500 Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 51ED2E121B for ; Wed, 23 Nov 2022 18:24:24 -0800 (PST) Received: from dggpemm500022.china.huawei.com (unknown [172.30.72.53]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4NHhYs5cfYzJnpy; Thu, 24 Nov 2022 10:21:05 +0800 (CST) Received: from dggpemm500006.china.huawei.com (7.185.36.236) by dggpemm500022.china.huawei.com (7.185.36.162) 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 10:24:22 +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 10:24:22 +0800 Subject: Re: [PATCH] kernfs: fix potential null-ptr-deref in kernfs_path_from_node_locked() To: Tejun Heo CC: Greg Kroah-Hartman , References: <20221123020419.1867-1-thunder.leizhen@huawei.com> From: "Leizhen (ThunderTown)" Message-ID: Date: Thu, 24 Nov 2022 10:24:21 +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: dggems702-chm.china.huawei.com (10.3.19.179) 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 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. 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