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Wed, 21 Apr 2021 11:25:26 +0000 Subject: Re: [PATCH] ovl: restore vma->vm_file to old file To: Miklos Szeredi Cc: Chengguang Xu , Jason Gunthorpe , Andrew Morton , Al Viro , Linus Torvalds , overlayfs References: <20210420020738.201670-1-cgxu519@mykernel.net> <481e8c92-3084-f0bc-56ec-86099abfdc55@amd.com> From: =?UTF-8?Q?Christian_K=c3=b6nig?= Message-ID: <0a34847c-2db0-4901-2206-7df1f348e32e@amd.com> Date: Wed, 21 Apr 2021 13:25:21 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [2a02:908:1252:fb60:30c1:a1b7:433d:2c5d] X-ClientProxiedBy: AM9P192CA0001.EURP192.PROD.OUTLOOK.COM (2603:10a6:20b:21d::6) To MN2PR12MB3775.namprd12.prod.outlook.com (2603:10b6:208:159::19) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2a02:908:1252:fb60:30c1:a1b7:433d:2c5d] (2a02:908:1252:fb60:30c1:a1b7:433d:2c5d) by AM9P192CA0001.EURP192.PROD.OUTLOOK.COM (2603:10a6:20b:21d::6) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4065.20 via Frontend Transport; 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In other words why did the patch broke the >> reference counting in overlayfs? > In the error case overlayfs would put the reference on realfile (which > is vma->vm_file at that point) and mmap_region() would put the > reference to the original file (which was vma->vm_file before being > overridden). > > After your commit mmap_region() puts the ref on the override vm_file, > but not on the original file. Ah, of course. Double checking the mmap callback implementation of overlayfs that is rather obvious. >>> Changing refcounting rules in core kernel is no easy matter, a full >>> audit of ->mmap instances (>200) should have been done beforehand. >> Which is pretty much what was done, see the follow up commit: >> >> commit 295992fb815e791d14b18ef7cdbbaf1a76211a31 (able/vma_file) >> Author: Christian König >> Date: Mon Sep 14 15:09:33 2020 +0200 >> >> mm: introduce vma_set_file function v5 >> >> Add the new vma_set_file() function to allow changing >> vma->vm_file with the necessary refcount dance. >> >> It just looks like I missed the case in overlayfs while doing this. > Yes. And apparently a number of other cases where vm_file is assigned... Yeah, I wasn't aware that filesystems do that as well and only concentrated on the drivers. Just did a "grep -R 'vm_file[[:space:]]*=' on the full kernel source and it only showed one more case in fs/coda/file.c. Do you see any other occurrences I potentially missed? Otherwise I would say I provide patches for those two cases in a minute. Thanks, Christian. > > Thanks, > Miklos