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

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Message-Id: <83A29F9C-1A91-4753-953A-0C98E8A9832C@kohlschutter.com>
Date:   Mon, 18 Jul 2022 12:10:09 +0200
From:   Christian Kohlschütter 
        <christian@...lschutter.com>
To:     Miklos Szeredi <miklos@...redi.hu>
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

> 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


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