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 3D18CCCA479 for ; Mon, 18 Jul 2022 15:03:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234977AbiGRPDu (ORCPT ); Mon, 18 Jul 2022 11:03:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53912 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234957AbiGRPDs (ORCPT ); Mon, 18 Jul 2022 11:03:48 -0400 X-Greylist: delayed 90 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Mon, 18 Jul 2022 08:03:47 PDT Received: from omta037.useast.a.cloudfilter.net (omta037.useast.a.cloudfilter.net [44.202.169.36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C47E719E for ; Mon, 18 Jul 2022 08:03:46 -0700 (PDT) Received: from eig-obgw-6012a.ext.cloudfilter.net ([10.0.30.238]) by cmsmtp with ESMTP id DPeeotrebeWDSDSGFoLwL7; Mon, 18 Jul 2022 15:02:16 +0000 Received: from gator3309.hostgator.com ([192.254.250.173]) by cmsmtp with ESMTP id DSGCoZbK5KBWXDSGCochdT; Mon, 18 Jul 2022 15:02:12 +0000 X-Authority-Analysis: v=2.4 cv=IoYNzZzg c=1 sm=1 tr=0 ts=62d575f5 a=dOmPygiJvdb+5OUNGItVWg==:117 a=mH/2fUS72G5xRhX6WoQJBg==:17 a=IkcTkHD0fZMA:10 a=MKtGQD3n3ToA:10 a=1oJP67jkp3AA:10 a=RgO8CyIxsXoA:10 a=1agXfLV7zN0A:10 a=0NJJaq8bbSYA:10 a=lWuoVP_hAAAA:8 a=NEAV23lmAAAA:8 a=qyu0ZQsMAABPi5KGwgUA:9 a=QEXdDO2ut3YA:10 a=Cd2r5mmCzboHVgs915Dn:22 Received: from [71.247.229.35] (port=47608 helo=[192.168.1.133]) by gator3309.hostgator.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.95) (envelope-from ) id 1oDSGB-0004cp-GT; Mon, 18 Jul 2022 10:02:11 -0500 Message-ID: <0e2af067-6b11-8e61-84c2-c9df25596ffe@spawn.link> Date: Mon, 18 Jul 2022 11:02:08 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH] [REGRESSION] ovl: Handle ENOSYS when fileattr support is missing in lower/upper fs Content-Language: en-US To: =?UTF-8?Q?Christian_Kohlsch=c3=bctter?= , Miklos Szeredi Cc: Linus Torvalds , overlayfs , linux-kernel , linux-fsdevel@vger.kernel.org References: <4B9D76D5-C794-4A49-A76F-3D4C10385EE0@kohlschutter.com> <83A29F9C-1A91-4753-953A-0C98E8A9832C@kohlschutter.com> <0B8DA307-7E1F-4534-B864-BC2632740C89@kohlschutter.com> From: Antonio SJ Musumeci In-Reply-To: <0B8DA307-7E1F-4534-B864-BC2632740C89@kohlschutter.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - gator3309.hostgator.com X-AntiAbuse: Original Domain - vger.kernel.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - spawn.link X-BWhitelist: no X-Source-IP: 71.247.229.35 X-Source-L: No X-Exim-ID: 1oDSGB-0004cp-GT X-Source: X-Source-Args: X-Source-Dir: X-Source-Sender: ([192.168.1.133]) [71.247.229.35]:47608 X-Source-Auth: trapexit@spawn.link X-Email-Count: 1 X-Org: HG=hgshared;ORG=hostgator; X-Source-Cap: YmlsZTtiaWxlO2dhdG9yMzMwOS5ob3N0Z2F0b3IuY29t X-Local-Domain: yes X-CMAE-Envelope: MS4xfH3o+rUs+9LJOtHY3F4WwzE8o8ZBzgo5hNR8l7+dMGxW9fywkQG57Uq/GS/EoEvxX/aH+fHfKL89kuivKDYFQoQamcYJoWyhSNfZzPkG7haMBVKcfUNG n+cNPfn+Nb88z7U/cSBSPhNlpLVaFWqEg7H8oRFqXvkHl2Xrvu3aLftkk/xtfuHHX+tyYCpMCHOm8NdYm9GqVSa8Eog5XQprHGJ05hETt7s0viM6KWMZaKUU Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 7/18/22 10:25, Christian Kohlschütter wrote: >> Am 18.07.2022 um 15:13 schrieb Miklos Szeredi : >> >> On Mon, 18 Jul 2022 at 15:03, Christian Kohlschütter >> wrote: >>> Am 18.07.2022 um 14:21 schrieb Miklos Szeredi : >>>> On Mon, 18 Jul 2022 at 12:56, Christian Kohlschütter >>>> wrote: >>>> >>>>> However, users of fuse that have no business with overlayfs suddenly see their ioctl return ENOTTY instead of ENOSYS. >>>> And returning ENOTTY is the correct behavior. See this comment in >>>> : >>>> >>>> /* >>>> * This error code is special: arch syscall entry code will return >>>> * -ENOSYS if users try to call a syscall that doesn't exist. To keep >>>> * failures of syscalls that really do exist distinguishable from >>>> * failures due to attempts to use a nonexistent syscall, syscall >>>> * implementations should refrain from returning -ENOSYS. >>>> */ >>>> #define ENOSYS 38 /* Invalid system call number */ >>>> >>>> Thanks, >>>> Miklos >>> That ship is sailed since ENOSYS was returned to user-space for the first time. >>> >>> It reminds me a bit of Linus' "we do not break userspace" email from 2012 [1, 2], where Linus wrote: >>>> Applications *do* care about error return values. There's no way in >>>> hell you can willy-nilly just change them. And if you do change them, >>>> and applications break, there is no way in hell you can then blame the >>>> application. >> Correct. The question is whether any application would break in this >> case. I think not, but you are free to prove otherwise. >> >> Thanks, >> Miklos > I'm not going to do that since I expect any answer I give would not change your position here. All I know is there is a non-zero chance such programs exist. > > If you're willing to go ahead with the fuse change you proposed, I see no purpose in debating with you further since you're the kernel maintainer of both file systems. > That change "fixes" the problem that I had seen in my setup; I do not know the extent of side effects, but I expect some could surface eventually. > > Once you're done fixing fuse, please also talk to the folks over at https://github.com/trapexit/mergerfs who explicitly return ENOSYS upon request. Who knows, maybe someone is audacious enough to try mergerfs as a lower filesystem for overlay? > > Alas, I think this a clash between the philosophies of writing robust code versus writing against a personal interpretation of some specification. > You refer to "asm-generic/errno.h" as the specification and rationale for treating ENOSYS as sacrosanct. Note that the comment says "should refrain from", it doesn't say "must not", and that's why we're in this mess. > > It therefore wouldn't hurt to be lenient when a lower filesystem returns an error code known to refer to "unsupported operation", and that's what my original patch to ovl does. > > I thought this approach would resonate with you, since you must have been following the same logic when you added the special-case check for "EINVAL" as an exception for ntfs-3g in the commit that most likely triggered the regression ("ovl: fix filattr copy-up failure") 9 months ago. > > I honestly wonder why you're risking further breakage, having introduced that regression only recently. > > So long, > Christian Author of mergerfs here. What are you referring to exactly? It's possible I'm forgetting something but  I should only be returning ENOSYS in similar cases to libfuse where some function is not supported or when wishing to disable xattr calls.