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

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Message-ID: <CAOQ4uxihs3ORNu7aVijO0_GUKbacA65Y6btcrhdL_A-rH0TkAA@mail.gmail.com>
Date: Thu, 22 May 2025 17:13:06 +0200
From: Amir Goldstein <amir73il@...il.com>
To: André Almeida <andrealmeid@...lia.com>, 
	Karel Zak <kzak@...hat.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

cc libfuse maintainer

On Thu, May 22, 2025 at 4:30 PM André Almeida <andrealmeid@...lia.com> wrote:
>
> Em 22/05/2025 06:52, Amir Goldstein escreveu:
> > 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?
> >
>
> I've added "set -x" to tests/generic/294 to see exactly which mount
> parameters were being used and I got this from the output:
>
> + _try_scratch_mount -o remount,ro
> + local mount_ret
> + '[' overlay == overlay ']'
> + _overlay_scratch_mount -o remount,ro
> + echo '-o remount,ro'
> + grep -q remount
> + /usr/bin/mount /tmp/dir2/ovl-mnt -o remount,ro
> mount: /tmp/dir2/ovl-mnt: fsconfig() failed: ...
>
> So, from what I can see, fstests is using this command. Not sure if I
> did something wrong when setting up fstests.
>

No you are right, I misread your reproducer.
The problem is that my test machine has older libmount 2.38.1
without the new mount API.


> >> 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?
> >

If you ask me, when a user does:
/usr/bin/mount /tmp/dir2/ovl-mnt -o remount,ro

The library only needs to do the FSCONFIG_SET_FLAG command and has no
business re-sending the other config commands, but that's just me.

BTW, which version of libmount (mount --version) are you using?
I think there were a few problematic versions when the new mount api
was first introduced.

Thanks,
Amir.