linux-kernel - Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in lower/upper fs

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Message-ID: <CAJfpegv5W0CycWCc2-kcn4=UVqk1hP7KrvBpzXHwW-Nmkjx8zA@mail.gmail.com>
Date:   Mon, 18 Jul 2022 12:31:35 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Christian Kohlschütter 
        <christian@...lschutter.com>
Cc:     overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ovl: Handle ENOSYS when fileattr support is missing in
 lower/upper fs

On Mon, 18 Jul 2022 at 12:10, Christian Kohlschütter
<christian@...lschutter.com> wrote:
>
> > Am 18.07.2022 um 11:14 schrieb Miklos Szeredi <miklos@...redi.hu>:
> >
> > On Mon, 4 Jul 2022 at 20:36, Christian Kohlschütter
> > <christian@...lschutter.com> wrote:
> >>
> >> 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.
> >
> > 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.
> >
> > 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.
> >
> > The getxattr/setxattr implementations already translate ENOSYS to
> > EOPNOTSUPP, but ioctl doesn't.
> >
> > The attached patch (untested) should do this.   Can you please give it a try?
> >
> > 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

No, it would return success.

> 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.

Flawed arguments.  The change in overlayfs just made the preexisting
bug in fuse visible.  The bug should still be fixed in fuse.

Thanks,
Miklos