ovl: Prepare for mounting case-insensitive enabled layers

Prepare for mounting layers with case-insensitive dentries in order to
supporting such layers in overlayfs, while enforcing uniform casefold
layers.

Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: André Almeida <[e-mail]>
Reviewed-by: Gabriel Krisman Bertazi <[e-mail]>
Signed-off-by: Amir Goldstein <[e-mail]>

fs/overlayfs/ovl_entry.h

···
 	struct mutex whiteout_lock;
 	/* r/o snapshot of upperdir sb's only taken on volatile mounts */
 	errseq_t errseq;
+	bool casefold;
 };
 
 /* Number of lower layers, not including data-only layers */

fs/overlayfs/params.c

···
 static int ovl_mount_dir_check(struct fs_context *fc, const struct path *path,
 			       enum ovl_opt layer, const char *name, bool upper)
 {
+	bool is_casefolded = ovl_dentry_casefolded(path->dentry);
 	struct ovl_fs_context *ctx = fc->fs_private;
+	struct ovl_fs *ofs = fc->s_fs_info;
 
 	if (!d_is_dir(path->dentry))
 		return invalfc(fc, "%s is not a directory", name);
 
 	/*
 	 * Allow filesystems that are case-folding capable but deny composing
-	 * ovl stack from case-folded directories.
+	 * ovl stack from inconsistent case-folded directories.
 	 */
-	if (ovl_dentry_casefolded(path->dentry))
-		return invalfc(fc, "case-insensitive directory on %s not supported", name);
+	if (!ctx->casefold_set) {
+		ofs->casefold = is_casefolded;
+		ctx->casefold_set = true;
+	}
+
+	if (ofs->casefold != is_casefolded) {
+		return invalfc(fc, "case-%ssensitive directory on %s is inconsistent",
+			       is_casefolded ? "in" : "", name);
+	}
 
 	if (ovl_dentry_weird(path->dentry))
 		return invalfc(fc, "filesystem on %s not supported", name);

fs/overlayfs/params.h

···
 	struct ovl_opt_set set;
 	struct ovl_fs_context_layer *lower;
 	char *lowerdir_all; /* user provided lowerdir string */
+	bool casefold_set;
 };
 
 int ovl_init_fs_context(struct fs_context *fc);