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Subject: Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in
 lower/upper fs
From:   =?utf-8?Q?Christian_Kohlsch=C3=BCtter?= 
        <christian@kohlschutter.com>
In-Reply-To: <CAJfpegv5W0CycWCc2-kcn4=UVqk1hP7KrvBpzXHwW-Nmkjx8zA@mail.gmail.com>
Date:   Mon, 18 Jul 2022 12:56:19 +0200
Cc:     overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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To:     Miklos Szeredi <miklos@szeredi.hu>
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--=20
Dr. Christian Kohlsch=C3=BCtter


> Am 18.07.2022 um 12:31 schrieb Miklos Szeredi <miklos@szeredi.hu>:
>=20
> On Mon, 18 Jul 2022 at 12:10, Christian Kohlsch=C3=BCtter
> <christian@kohlschutter.com> wrote:
>>=20
>>> Am 18.07.2022 um 11:14 schrieb Miklos Szeredi <miklos@szeredi.hu>:
>>>=20
>>> On Mon, 4 Jul 2022 at 20:36, Christian Kohlsch=C3=BCtter
>>> <christian@kohlschutter.com> wrote:
>>>>=20
>>>> overlayfs may fail to complete updates when a filesystem lacks
>>>> fileattr/xattr syscall support and responds with an ENOSYS error =
code,
>>>> resulting in an unexpected "Function not implemented" error.
>>>=20
>>> Issue seems to be with fuse: nothing should be returning ENOSYS to
>>> userspace except the syscall lookup code itself.  ENOSYS means that
>>> the syscall does not exist.
>>>=20
>>> Fuse uses ENOSYS in the protocol to indicate that the filesystem =
does
>>> not support that operation, but that's not the value that the
>>> filesystem should be returning to userspace.
>>>=20
>>> The getxattr/setxattr implementations already translate ENOSYS to
>>> EOPNOTSUPP, but ioctl doesn't.
>>>=20
>>> The attached patch (untested) should do this.   Can you please give =
it a try?
>>>=20
>>> Thanks,
>>> Miklos
>>> <fuse-ioctl-translate-enosys.patch>
>>=20
>> Yes, that change basically has the same effect for the demo use =
case,.
>>=20
>> However: it will change (and potentially) break assumptions in user =
space. We should never break user space.
>>=20
>> Example: lsattr /test/lower
>> Currently, fuse returns ENOSYS, e.g.
>>> lsattr: reading ./lost+found: Function not implemented
>> With your change, it would return ENOTTY
>>> lsattr: reading ./lost+found: Not a tty
>=20
> No, it would return success.

I'm referring to /test/lower (powered by fuse davfs2), not /test/mnt =
(overlayfs).

>=20
>> I also tried the setup (without patches) on a very old 4.4.176 =
system, and everything works fine. ovl introduced the regression, so it =
should also be fixed there.
>> It may affect other filing systems as well (I see some other fs also =
return ENOSYS on occasion).
>>=20
>> It's safe to say that adding the ENOSYS to the ovl code is probably =
the best move. Besides, you already have a workaround for ntfs-3g there =
as well.
>=20
> Flawed arguments.  The change in overlayfs just made the preexisting
> bug in fuse visible.  The bug should still be fixed in fuse.

I understand your point from ovl's perspective, however you are =
proposing a fix in fuse, and so we have to see it from fuse's =
perspective (and its users).

=46rom ovl's point of view, your patch fixes it because the behavior in =
ovl will now become how it was before the regression was introduced.
However, users of fuse that have no business with overlayfs suddenly see =
their ioctl return ENOTTY instead of ENOSYS.