[PATCH 4/4] ovl: Expose the filesystem encoding through sysfs

[Gabriel Krisman Bertazi <krisman@suse.de>]

This patch reproduces the same interface from ext4/f2fs to have feature
parity, so users don't need to rely on parsing dmesg or trying +F on a
temporary directory just to check for support.  Note there is no
encoding_flags file, as none are supported by ovl.

Assisted-by: LLM
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
---
 Documentation/ABI/testing/sysfs-fs-overlay |  7 +++++++
 fs/overlayfs/sysfs.c                       | 15 +++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-fs-overlay b/Documentation/ABI/testing/sysfs-fs-overlay
index 9060c39679ae..7f3c6403c4f8 100644
--- a/Documentation/ABI/testing/sysfs-fs-overlay
+++ b/Documentation/ABI/testing/sysfs-fs-overlay
@@ -4,3 +4,10 @@ Contact:	linux-unionfs@vger.kernel.org
 Description:
 		A directory is created for every mounted overlayfs instance.
 		<volume> is set to the device major:minor.
+
+What:		/sys/fs/overlay/<volume>/encoding
+Date:		September 2026
+Contact:	linux-unionfs@vger.kernel.org
+Description:
+		Displays name and version of the encoding set for the
+		overlay.  If no encoding is set, displays (none)
diff --git a/fs/overlayfs/sysfs.c b/fs/overlayfs/sysfs.c
index be83b57f637c..2144cbb418bc 100644
--- a/fs/overlayfs/sysfs.c
+++ b/fs/overlayfs/sysfs.c
@@ -22,7 +22,22 @@ struct ovl_sysfs_attr {
 		.show	= ovl_##_name##_show,				\
 	}
 
+static ssize_t ovl_encoding_show(struct ovl_fs *ofs, char *buf)
+{
+	struct unicode_map *um = sb_encoding(ofs->sb);
+
+	if (!um)
+		return sysfs_emit(buf, "(none)\n");
+
+	return sysfs_emit(buf, "UTF-8 (%d.%d.%d)\n",
+			  unicode_major(um->version),
+			  unicode_minor(um->version),
+			  unicode_rev(um->version));
+}
+OVL_SYSFS_ATTR_RO(encoding);
+
 static struct attribute *ovl_sysfs_attrs[] = {
+	&ovl_sysfs_attr_encoding.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(ovl_sysfs);
-- 
2.55.0