[PATCH v3] btrfs: test POSIX ACL changes for RO btrfs property

[Sun YangKai <sunk67188@gmail.com>]

From: Sun YangKai <sunyangkai@fygo.io>

Test creation, modification and deletion of POSIX ACLs on a btrfs
filesystem that has the read-only property set to true.

This exercises a bug fixed by the kernel patch with subject:
"btrfs: check if root is readonly when setting posix acl"

Signed-off-by: Sun YangKai <sunyangkai@fygo.io>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
---
Changes since v2:

- updated commit message
- proper format for fixed_by_kernel_commit
- lower case variable name
- handle mkfs failure

Suggested by Filipe Manana
---
 tests/btrfs/353     | 95 +++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/353.out | 39 +++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100755 tests/btrfs/353
 create mode 100644 tests/btrfs/353.out

diff --git a/tests/btrfs/353 b/tests/btrfs/353
new file mode 100755
index 00000000..63ba4681
--- /dev/null
+++ b/tests/btrfs/353
@@ -0,0 +1,95 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2026 Fygo OS. All Rights Reserved.
+#
+# FS QA Test No. 353
+#
+# Test that POSIX ACLs cannot be changed once a btrfs subvolume has the
+# read-only property set.
+#
+# Setting or removing a POSIX ACL goes through the ->set_acl inode
+# operation, which is a different code path from the generic ->setxattr
+# one covered by btrfs/275.  It used to be allowed on a read-only
+# subvolume and thus bypassed the RO protection.  Such modifications must
+# fail with EROFS, just like any other xattr.
+#
+. ./common/preamble
+_begin_fstest auto quick acl attr
+
+. ./common/filter
+. ./common/attr
+
+_fixed_by_kernel_commit xxxxxxxxxxxx \
+	"btrfs: check if root is readonly when setting posix acl"
+
+_require_acls
+_require_btrfs_command "property"
+_require_scratch
+
+_scratch_mkfs >> $seqres.full 2>&1 || _fail "mkfs failed"
+_scratch_mount
+
+filename=$SCRATCH_MNT/foo
+
+set_acl()
+{
+	local perm=$1
+
+	# Use -n so setfacl does not recalculate the mask, keeping the
+	# golden output deterministic regardless of the named user's
+	# permissions.
+	setfacl -n -m u:$acl2:$perm,m::rwx $filename 2>&1 | _filter_scratch
+}
+
+get_acl()
+{
+	getfacl --absolute-names -n $filename | _filter_scratch | _getfacl_filter_id
+}
+
+del_acl()
+{
+	setfacl -b $filename 2>&1 | _filter_scratch
+}
+
+_acl_setup_ids
+
+# Create a test file.
+echo "hello world" > $filename
+
+# Set an initial ACL while the subvolume is writable.
+set_acl rwx
+
+# Attempt to change the ACL once the subvolume is read-only.  This must
+# fail with EROFS.
+$BTRFS_UTIL_PROG property set $SCRATCH_MNT ro true
+$BTRFS_UTIL_PROG property get $SCRATCH_MNT ro
+
+set_acl r--
+
+# The ACL must not have changed.
+get_acl
+
+# Attempt to remove the ACL from the read-only subvolume.  This must
+# fail with EROFS as well.
+del_acl
+
+# The ACL must still be present.
+get_acl
+
+# Make the subvolume writable again.
+$BTRFS_UTIL_PROG property set $SCRATCH_MNT ro false
+$BTRFS_UTIL_PROG property get $SCRATCH_MNT ro
+
+# Now changing the ACL must succeed.
+set_acl r--
+
+get_acl
+
+# And removing it must succeed too.
+del_acl
+
+# Check the ACL is really gone.
+get_acl
+
+status=0
+exit
diff --git a/tests/btrfs/353.out b/tests/btrfs/353.out
new file mode 100644
index 00000000..66f4a0e5
--- /dev/null
+++ b/tests/btrfs/353.out
@@ -0,0 +1,39 @@
+QA output created by 353
+ro=true
+setfacl: SCRATCH_MNT/foo: Read-only file system
+# file: SCRATCH_MNT/foo
+# owner: 0
+# group: 0
+user::rw-
+user:id2:rwx
+group::r--
+mask::rwx
+other::r--
+
+setfacl: SCRATCH_MNT/foo: Read-only file system
+# file: SCRATCH_MNT/foo
+# owner: 0
+# group: 0
+user::rw-
+user:id2:rwx
+group::r--
+mask::rwx
+other::r--
+
+ro=false
+# file: SCRATCH_MNT/foo
+# owner: 0
+# group: 0
+user::rw-
+user:id2:r--
+group::r--
+mask::rwx
+other::r--
+
+# file: SCRATCH_MNT/foo
+# owner: 0
+# group: 0
+user::rw-
+group::r--
+other::r--
+
-- 
2.54.0