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Wed, 21 Jul 2021 04:38:51 +0000 Received: from BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a]) by BY5PR10MB4196.namprd10.prod.outlook.com ([fe80::8d2a:558d:ab4a:9c2a%5]) with mapi id 15.20.4331.034; Wed, 21 Jul 2021 04:38:51 +0000 Subject: Re: [Bug 213785] New: hugetlbfs scrambles mode when sticky bit is set To: Andrew Morton , bugs+kernel.org@dtnr.ch, David Howells , Al Viro Cc: bugzilla-daemon@bugzilla.kernel.org, linux-mm@kvack.org References: <20210720150704.59f78224be810a0cf9dd5f39@linux-foundation.org> From: Mike Kravetz Message-ID: <4a02aa78-c600-f2f4-c7d3-d79164c2c8a1@oracle.com> Date: Tue, 20 Jul 2021 21:38:48 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 In-Reply-To: <20210720150704.59f78224be810a0cf9dd5f39@linux-foundation.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW2PR2101CA0028.namprd21.prod.outlook.com (2603:10b6:302:1::41) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.2.112] (50.38.35.18) by MW2PR2101CA0028.namprd21.prod.outlook.com (2603:10b6:302:1::41) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4373.0 via Frontend Transport; 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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=IBePQcKy; dkim=pass header.d=oracle.com header.s=corp-2020-01-29 header.b=vFe8FIv7; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=hqr5nf0i; spf=none (imf08.hostedemail.com: domain of mike.kravetz@oracle.com has no SPF policy when checking 205.220.165.32) smtp.mailfrom=mike.kravetz@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Rspamd-Server: rspam05 X-Stat-Signature: ham7qgddf3awt3qukjuxqsuqpx1pd5a8 X-Rspamd-Queue-Id: 584053000DBB X-HE-Tag: 1626842340-181929 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Add David, Al On 7/20/21 3:07 PM, Andrew Morton wrote: >> https://bugzilla.kernel.org/show_bug.cgi?id=213785 >> >> Summary: hugetlbfs scrambles mode when sticky bit is set >> Product: Memory Management >> Version: 2.5 >> Kernel Version: >4.19 (certainly >= 5.4) >> Regression: Yes >> >> I noticed the following strange behaviour with hugetlbfs mounts for kernel >> versions something north of 4.19, but certainly since 5.4: >> >> when the mode= option of the mount has the sticky bit set, the mode of the >> mount point gets scrambled. >> >> To test the following command line can be used: >> mkdir -p /tmp/tlbtest && mount -t hugetlbfs -o mode=.... none /tmp/tlbtest && >> stat -c 'mode: %04a' /tmp/tlbtest >> >> For kernel versions <= 4.19 or if the first byte of mode is 0, stat outputs >> what was set using "-o mode". >> >> But on newer kernel versions if the first byte of mode is 1 the output of stat >> is as follows (input -> output): >> 1700 -> 1244 >> 1750 -> 1326 >> 1770 -> 1352 >> 1775 -> 1357 >> 1777 -> 1361 >> >> The behaviour is reproducible across different kernel versions and >> architectures (5.4.47-amd64, 5.9.8-amd64, 5.10.40-ppc64el, 5.12.15-amd64). I took a quick look and believe a change in behavior was caused by commit 32021982a324 "Convert the hugetlbfs to use the fs_context during mount". Prior to the commit, code processing the mode option used match_octal() to convert the command line string to a numeric value. Since match_octal expects a string representing an octal value, it does not require a leading '0'. As a result, prior to this commit the argument 'mode=1700' would result in a mode value of 01700. After the commit one must precede octal values with 0. So, mode=1700 would result in a mode value of 03244 (& 01777U) = 1244. If my analysis is correct, I am not sure how to proceed. IMO, the current behavior is 'more correct'. However, until v5.1 a preceeding 0 was not required when specifying mode for hugetlbfs. So, this was certainly a change in behavior. Suggestions? -- Mike Kravetz