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

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Message-Id: <20250521-ovl_ro-v1-1-2350b1493d94@igalia.com>
Date: Wed, 21 May 2025 03:42:12 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Miklos Szeredi <miklos@...redi.hu>, Amir Goldstein <amir73il@...il.com>, 
 Christian Brauner <brauner@...nel.org>
Cc: linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel-dev@...lia.com, linux-fsdevel@...r.kernel.org, 
 André Almeida <andrealmeid@...lia.com>
Subject: [PATCH] ovl: Allow mount options to be parsed on remount

Allow mount options to be parsed on remount when using the new mount(8)
API. This allows to give a precise error code to userspace when the
remount is using wrong arguments instead of a generic -EINVAL error.

Signed-off-by: André Almeida <andrealmeid@...lia.com>
---
Hi folks,

I was playing with xfstest with overlayfs and I got those fails:

$ sudo TEST_DIR=/tmp/dir1 TEST_DEV=/dev/vdb SCRATCH_DEV=/dev/vdc SCRATCH_MNT=/tmp/dir2 ./check -overlay
...
Failures: generic/294 generic/306 generic/452 generic/599 generic/623 overlay/035
Failed 6 of 859 tests

5 of those 6 fails were related to the same issue, where fsconfig
syscall returns EINVAL instead of EROFS:

-mount: cannot remount device read-write, is write-protected
+mount: /tmp/dir2/ovl-mnt: fsconfig() failed: overlay: No changes allowed in reconfigure

I tracked down the origin of this issue being commit ceecc2d87f00 ("ovl:
reserve ability to reconfigure mount options with new mount api"), where
ovl_parse_param() was modified to reject any reconfiguration when using
the new mount API, returning -EINVAL. This was done to avoid non-sense
parameters being accepted by the new API, as exemplified in the commit
message: 

	mount -t overlay overlay -o lowerdir=/mnt/a:/mnt/b,upperdir=/mnt/upper,workdir=/mnt/work /mnt/merged

    and then issue a remount via:

            # force mount(8) to use mount(2)
            export LIBMOUNT_FORCE_MOUNT2=always
            mount -t overlay overlay -o remount,WOOTWOOT,lowerdir=/DOESNT-EXIST /mnt/merged

    with completely nonsensical mount options whatsoever it will succeed
    nonetheless. 

However, after manually reverting such commit, I found out that
currently those nonsensical mount options are being reject by the
kernel:

$ mount -t overlay overlay -o remount,WOOTWOOT,lowerdir=/DOESNT-EXIST /mnt/merged
mount: /mnt/merged: fsconfig() failed: overlay: Unknown parameter 'WOOTWOOT'.

$ mount -t overlay overlay -o remount,lowerdir=/DOESNT-EXIST /mnt/merged
mount: /mnt/merged: special device overlay does not exist.
       dmesg(1) may have more information after failed mount system call

And now 5 tests are passing because the code can now returns EROFS:
Failures: generic/623
Failed 1 of 1 tests

So this patch basically allows for the parameters to be parsed and to
return an appropriated error message instead of a generic EINVAL one.

Please let me know if this looks like going in the right direction.

Thanks!
---
 fs/overlayfs/params.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index f42488c019572479d8fdcfc1efd62b21d2995875..f6b7acc0fee6174c48fcc8b87481fbcb60e6d421 100644
--- a/fs/overlayfs/params.c
+++ b/fs/overlayfs/params.c
@@ -600,15 +600,6 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
 		 */
 		if (fc->oldapi)
 			return 0;
-
-		/*
-		 * Give us the freedom to allow changing mount options
-		 * with the new mount api in the future. So instead of
-		 * silently ignoring everything we report a proper
-		 * error. This is only visible for users of the new
-		 * mount api.
-		 */
-		return invalfc(fc, "No changes allowed in reconfigure");
 	}

 	opt = fs_parse(fc, ovl_parameter_spec, param, &result);

---
base-commit: b87e2318cdaa14024b62ab428b3471d81eafaf1a
change-id: 20250521-ovl_ro-b38a57d2984d

Best regards,
-- 
André Almeida <andrealmeid@...lia.com>