linux-kernel - [PATCH v4 5/9] ovl: Ensure that all layers have the same encoding

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Message-Id: <20250813-tonyk-overlayfs-v4-5-357ccf2e12ad@igalia.com>
Date: Wed, 13 Aug 2025 19:36:41 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Miklos Szeredi <miklos@...redi.hu>, Amir Goldstein <amir73il@...il.com>, 
 Theodore Tso <tytso@....edu>, Gabriel Krisman Bertazi <krisman@...nel.org>
Cc: linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-fsdevel@...r.kernel.org, Alexander Viro <viro@...iv.linux.org.uk>, 
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
 kernel-dev@...lia.com, 
 André Almeida <andrealmeid@...lia.com>
Subject: [PATCH v4 5/9] ovl: Ensure that all layers have the same encoding

When merging layers from different filesystems with casefold enabled,
all layers should use the same encoding version and have the same flags
to avoid any kind of incompatibility issues.

Also, set the encoding and the encoding flags for the ovl super block as
the same as used by the first valid layer.

Signed-off-by: André Almeida <andrealmeid@...lia.com>
---
Changes from v3:
- Check this restriction just when casefold is enabled
- Create new helper ovl_set_encoding() and change the logic a bit
---
 fs/overlayfs/super.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index df85a76597e910d00323018f1d2cd720c5db921d..b1dbd3c79961094d00c7f99cc622e515d544d22f 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -991,6 +991,18 @@ static int ovl_get_data_fsid(struct ovl_fs *ofs)
 	return ofs->numfs;
 }
 
+/*
+ * Set the ovl sb encoding as the same one used by the first layer
+ */
+static void ovl_set_encoding(struct super_block *sb, struct super_block *fs_sb)
+{
+#if IS_ENABLED(CONFIG_UNICODE)
+	if (sb_has_encoding(fs_sb)) {
+		sb->s_encoding = fs_sb->s_encoding;
+		sb->s_encoding_flags = fs_sb->s_encoding_flags;
+	}
+#endif
+}
 
 static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
 			  struct ovl_fs_context *ctx, struct ovl_layer *layers)
@@ -1024,6 +1036,9 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
 	if (ovl_upper_mnt(ofs)) {
 		ofs->fs[0].sb = ovl_upper_mnt(ofs)->mnt_sb;
 		ofs->fs[0].is_lower = false;
+
+		if (ofs->casefold)
+			ovl_set_encoding(sb, ofs->fs[0].sb);
 	}
 
 	nr_merged_lower = ctx->nr - ctx->nr_data;
@@ -1083,6 +1098,16 @@ static int ovl_get_layers(struct super_block *sb, struct ovl_fs *ofs,
 		l->name = NULL;
 		ofs->numlayer++;
 		ofs->fs[fsid].is_lower = true;
+
+		if (ofs->casefold) {
+			if (!ovl_upper_mnt(ofs) && !sb_has_encoding(sb))
+				ovl_set_encoding(sb, ofs->fs[fsid].sb);
+
+			if (!sb_has_encoding(sb) || !sb_same_encoding(sb, mnt->mnt_sb)) {
+				pr_err("all layers must have the same encoding\n");
+				return -EINVAL;
+			}
+		}
 	}
 
 	/*

-- 
2.50.1