Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935254AbcKKR1Y (ORCPT ); Fri, 11 Nov 2016 12:27:24 -0500 Received: from out4-smtp.messagingengine.com ([66.111.4.28]:57730 "EHLO out4-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933164AbcKKR1X (ORCPT ); Fri, 11 Nov 2016 12:27:23 -0500 X-ME-Sender: X-Sasl-enc: +OXugP3ov+y3wyVNpIwLIIiWZjNBqp8wVtCUibwDaa1C 1478885241 From: Nikolaus Rath To: Mike Marshall Cc: Miklos Szeredi , Andrew Gallagher , lkml , linux-fsdevel Subject: Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN_SUPPORT flag to INIT References: <87lgwrufuk.fsf@thinkpad.rath.org> <87mvh6likl.fsf@vostro.rath.org> <87mvh6j803.fsf@thinkpad.rath.org> Date: Fri, 11 Nov 2016 09:27:20 -0800 In-Reply-To: (Mike Marshall's message of "Fri, 11 Nov 2016 11:53:04 -0500") Message-ID: <87d1i2j5af.fsf@thinkpad.rath.org> User-Agent: Gnus/5.130014 (Ma Gnus v0.14) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id uABHRSnH031600 Content-Length: 3001 Lines: 64 On Nov 11 2016, Mike Marshall wrote: > On Fri, Nov 11, 2016 at 11:28 AM, Nikolaus Rath wrote: >> On Nov 11 2016, Miklos Szeredi wrote: >>> On Fri, Nov 11, 2016 at 5:57 AM, Nikolaus Rath wrote: >>>> On Nov 11 2016, Miklos Szeredi wrote: >>>>> On Thu, Nov 10, 2016 at 11:31 PM, Nikolaus Rath wrote: >>>>>> Hi Andrew, >>>>>> >>>>>> In commit d7afaec0b564f0609e116f5 you added a new FUSE_NO_OPEN_SUPPORT >>>>>> flag. But as far as I can tell, the flag is simply accepted without >>>>>> having any effect (including in libfuse). >>>>>> >>>>>> I tried to find related later commits, but did not find anything either. >>>>>> >>>>>> Am I missing something? >>>>> >>>>> Hmm, if fuse fs detects this flag, then it can return ENOSYS from open >>>>> resulting in this and subsequent opens succeeding without further >>>>> calls to userspace. If fuse fs doesn't detect this flag, it should >>>>> not return -ENOSYS, as that will result in the open failing, it should >>>>> instead implement a no-op open method. >>>> >>>> That doesn't sound like a good approach to me. That way, the file system >>>> has to *know* that this flag has been introduced in order to behave >>>> correctly, i.e. filesystems that predate the introduction of the flag >>>> will suddenly behave differently. >>>> >>>> I think the correct behavior would be to for the kernel to check if >>>> userspace passed the flag, and treat ENOSYS specially if and only if the >>>> flag was passed. >>> >>> ENOSYS is not a valid return value for any existing syscall. Fuse >>> uses that fact to attach this special meaning to ENOSYS. So >>> compatibility is not an issue here, old filesystems should never >>> return ENOSYS from open. >> >> Oh, I was under the impression that any FUSE handler is free to return >> any error it wants. Where does one have to look to determine which >> return values are valid? >> > I try to choose error codes from the appropriate man > page when vfs calls into Orangefs with > whatever_operations.action... there's probably better > ways, like reading the vfs code and seeing what it > expects ... Yeah, I'd expect most people to do that. But FUSE file systems are often a little more exotic and produce error conditions that don't match well with any of the codes documented in the manpages. If there is no good fit, I'd expect that most people would (as I have done so far) simply pick something more appropriate from errno(3). If some of these codes are forbidden (or only a subset allowed) I'd really like to document this. It's not reasonable to expect every libfuse user to start browsing the Linux VFS code to determine if they can use a particular error code. Best, -Nikolaus -- GPG encrypted emails preferred. Key id: 0xD113FCAC3C4E599F Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F »Time flies like an arrow, fruit flies like a Banana.«