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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: <CAJfpegs1Kta-HcikDGFt4=fa_LDttCeRmffKhUjWLr=DxzXg-A@mail.gmail.com>
Date:   Mon, 18 Jul 2022 12:10:09 +0200
Cc:     overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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Message-Id: <83A29F9C-1A91-4753-953A-0C98E8A9832C@kohlschutter.com>
References: <4B9D76D5-C794-4A49-A76F-3D4C10385EE0@kohlschutter.com>
 <CAJfpegs1Kta-HcikDGFt4=fa_LDttCeRmffKhUjWLr=DxzXg-A@mail.gmail.com>
To:     Miklos Szeredi <miklos@szeredi.hu>
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> 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>

Yes, that change basically has the same effect for the demo use case,.

However: it will change (and potentially) break assumptions in user =
space. We should never break user space.

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


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).

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.

Best,
Christian