[PATCH v2 1/6] overlay: workaround libmount failure to remount,ro

[Amir Goldstein <amir73il@gmail.com>]

libmount >= v1.39 calls several unneeded fsconfig() calls to reconfigure
lowerdir/upperdir when user requests only -o remount,ro.

Those calls fail because overlayfs does not allow making any config
changes with new mount api, besides MS_RDONLY.

We workaround this problem with --options-mode ignore.

Reported-by: André Almeida <andrealmeid@igalia.com>
Suggested-by: Karel Zak <kzak@redhat.com>
Link: https://lore.kernel.org/linux-fsdevel/20250521-ovl_ro-v1-1-2350b1493d94@igalia.com/
Link: https://lore.kernel.org/fstests/CAJfpegtJ3SDKmC80B4AfWiC3JmtWdW2+78fRZVtsuhe-wSRPvg@mail.gmail.com/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Changes since v1 [1]:
- Change workaround from LIBMOUNT_FORCE_MOUNT2 to --options-mode=ignore

[1] https://lore.kernel.org/fstests/20250526143500.1520660-1-amir73il@gmail.com/

 common/overlay    | 5 ++++-
 tests/overlay/035 | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/common/overlay b/common/overlay
index 01b6622f..0fad6e70 100644
--- a/common/overlay
+++ b/common/overlay
@@ -127,7 +127,10 @@ _overlay_base_scratch_mount()
 _overlay_scratch_mount()
 {
 	if echo "$*" | grep -q remount; then
-		$MOUNT_PROG $SCRATCH_MNT $*
+		# By default, libmount merges remount options with old mount options.
+		# overlayfs does not support re-configuring the same mount options.
+		# We workaround this problem with --options-mode ignore.
+		$MOUNT_PROG $SCRATCH_MNT --options-mode ignore $*
 		return
 	fi
 
diff --git a/tests/overlay/035 b/tests/overlay/035
index 0b3257c4..2a4df99a 100755
--- a/tests/overlay/035
+++ b/tests/overlay/035
@@ -42,7 +42,7 @@ mkdir -p $lowerdir1 $lowerdir2 $upperdir $workdir
 # Verify that overlay is mounted read-only and that it cannot be remounted rw.
 _overlay_scratch_mount_opts -o"lowerdir=$lowerdir2:$lowerdir1"
 touch $SCRATCH_MNT/foo 2>&1 | _filter_scratch
-$MOUNT_PROG -o remount,rw $SCRATCH_MNT 2>&1 | _filter_ro_mount
+_scratch_remount rw 2>&1 | _filter_ro_mount
 $UMOUNT_PROG $SCRATCH_MNT
 
 # Make workdir immutable to prevent workdir re-create on mount
-- 
2.34.1