linux-kernel - Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN

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Date:   Fri, 11 Nov 2016 11:53:04 -0500
From:   Mike Marshall <hubcap@...ibond.com>
To:     Nikolaus Rath <Nikolaus@...h.org>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        Andrew Gallagher <andrewjcg@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: commit d7afaec0b564f0609e116f5: fuse: add FUSE_NO_OPEN_SUPPORT
 flag to INIT

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 <g>...

-Mike

On Fri, Nov 11, 2016 at 11:28 AM, Nikolaus Rath <Nikolaus@...h.org> wrote:
> On Nov 11 2016, Miklos Szeredi <mszeredi@...hat.com> wrote:
>> On Fri, Nov 11, 2016 at 5:57 AM, Nikolaus Rath <Nikolaus@...h.org> wrote:
>>> On Nov 11 2016, Miklos Szeredi <mszeredi@...hat.com> wrote:
>>>> On Thu, Nov 10, 2016 at 11:31 PM, Nikolaus Rath <Nikolaus@...h.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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