linux-kernel - Re: [PATCH] ovl: Allow mount options to be parsed on remount

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Message-ID: <CAOQ4uxg6RCJf6OBzKgaWbOKn3JhtgWhD6t=yOfufHuJ7jwxKmw@mail.gmail.com>
Date: Thu, 22 May 2025 11:52:35 +0200
From: Amir Goldstein <amir73il@...il.com>
To: André Almeida <andrealmeid@...lia.com>
Cc: Christian Brauner <brauner@...nel.org>, Miklos Szeredi <miklos@...redi.hu>, linux-unionfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kernel-dev@...lia.com, 
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ovl: Allow mount options to be parsed on remount

On Thu, May 22, 2025 at 8:20 AM André Almeida <andrealmeid@...lia.com> wrote:
>
> Hi Christian, Amir,
>
> Thanks for the feedback :)
>
> Em 21/05/2025 08:20, Christian Brauner escreveu:
> > On Wed, May 21, 2025 at 12:35:57PM +0200, Amir Goldstein wrote:
> >> On Wed, May 21, 2025 at 8:45 AM André Almeida <andrealmeid@...lia.com> wrote:
> >>>
>
> [...]
>
> >>
> >> I see the test generic/623 failure - this test needs to be fixed for overlay
> >> or not run on overlayfs.
> >>
> >> I do not see those other 5 failures although before running the test I did:
> >> export LIBMOUNT_FORCE_MOUNT2=always
> >>
> >> Not sure what I am doing differently.
> >>
>
> I have created a smaller reproducer for this, have a look:
>
>   mkdir -p ovl/lower ovl/upper ovl/merge ovl/work ovl/mnt
>   sudo mount -t overlay overlay -o lowerdir=ovl/lower,upperdir=ovl/
> upper,workdir=ovl/work ovl/mnt
>   sudo mount ovl/mnt -o remount,ro
>

Why would you use this command?
Why would you want to re-specify the lower/upperdir when remounting ro?
And more specifically, fstests does not use this command in the tests
that you mention that they fail, so what am I missing?

> And this returns:
>
>   mount: /tmp/ovl/mnt: fsconfig() failed: overlay: No changes allowed in
>   reconfigure.
>         dmesg(1) may have more information after failed mount system call.
>
> However, when I use mount like this:
>
>   sudo mount -t overlay overlay -o remount,ro ovl/mnt
>
> mount succeeds. Having a look at strace, I found out that the first
> mount command tries to set lowerdir to "ovl/lower" again, which will to
> return -EINVAL from ovl_parse_param():
>
>     fspick(3, "", FSPICK_NO_AUTOMOUNT|FSPICK_EMPTY_PATH) = 4
>     fsconfig(4, FSCONFIG_SET_STRING, "lowerdir", "/tmp/ovl/lower", 0) =
> -1 EINVAL (Invalid argument)
>
> Now, the second mount command sets just the "ro" flag, which will return
> after vfs_parse_sb_flag(), before getting to ovl_parse_param():
>
>     fspick(3, "", FSPICK_NO_AUTOMOUNT|FSPICK_EMPTY_PATH) = 4
>     fsconfig(4, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
>
> After applying my patch and running the first mount command again, we
> can set that this flag is set only after setting all the strings:
>
>     fsconfig(4, FSCONFIG_SET_STRING, "lowerdir", "/tmp/ovl/lower", 0) = 0
>     fsconfig(4, FSCONFIG_SET_STRING, "upperdir", "/tmp/ovl/upper", 0) = 0
>     fsconfig(4, FSCONFIG_SET_STRING, "workdir", "/tmp/ovl/work", 0) = 0
>     fsconfig(4, FSCONFIG_SET_STRING, "uuid", "on", 0) = 0
>     fsconfig(4, FSCONFIG_SET_FLAG, "ro", NULL, 0) = 0
>
> I understood that the patch that I proposed is wrong, and now I wonder
> if the kernel needs to be fixed at all, or if the bug is how mount is
> using fsconfig() in the first mount command?

Maybe I am not reading your report correctly, but as this commands works:

mount -t overlay overlay -o remount,ro ovl/mnt

and the fstests that call _scratch_remount() work
I don't think there is anything to fix and I do not understand
what is the complaint.

Thanks,
Amir.