Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN_SUPPORT flag to INIT

From: Nikolaus Rath <Nikolaus@rath.org>
To: Miklos Szeredi <mszeredi@redhat.com>
Cc: Andrew Gallagher <andrewjcg@fb.com>,
	lkml <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN_SUPPORT flag to INIT
Date: Fri, 11 Nov 2016 08:28:44 -0800	[thread overview]
Message-ID: <87mvh6j803.fsf@thinkpad.rath.org> (raw)
In-Reply-To: <CAOssrKc3RPbJuwTRpkykAvrvgjM0i0J=X8WWrGCpeX67AwGcGA@mail.gmail.com> (Miklos Szeredi's message of "Fri, 11 Nov 2016 09:43:12 +0100")

On Nov 11 2016, Miklos Szeredi <mszeredi@redhat.com> wrote:
> On Fri, Nov 11, 2016 at 5:57 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>> On Nov 11 2016, Miklos Szeredi <mszeredi@redhat.com> wrote:
>>> On Thu, Nov 10, 2016 at 11:31 PM, Nikolaus Rath <Nikolaus@rath.org> 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?

Best,
-Nikolaus

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