ovl: fix wrong lowerdir number check for parameter Opt_lowerdir

The max count of lowerdir is OVL_MAX_STACK[500], which is broken by
commit 37f32f5("ovl: fix memory leak in ovl_parse_param()") for
parameter Opt_lowerdir. Since commit 819829f("ovl: refactor layer
parsing helpers") and commit 24e16e3("ovl: add support for
appending lowerdirs one by one") added check ovl_mount_dir_check() in
function ovl_parse_param_lowerdir(), the 'ctx->nr' should be smaller
than OVL_MAX_STACK, after commit 37f32f5("ovl: fix memory leak in
ovl_parse_param()") is applied, the 'ctx->nr' is updated before the
check ovl_mount_dir_check(), which leads the max count of lowerdir
to become 499 for parameter Opt_lowerdir.
Fix it by replacing lower layers parsing code with the existing helper
function ovl_parse_layer().

Fixes: 37f32f5 ("ovl: fix memory leak in ovl_parse_param()")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://lore.kernel.org/r/20240705011510.794025-3-chengzhihao1@huawei.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Zhihao Cheng

fs/overlayfs/params.c

@@ -353,6 +353,8 @@ static void ovl_add_layer(struct fs_context *fc, enum ovl_opt layer,
 	case Opt_datadir_add:
 		ctx->nr_data++;
 		fallthrough;
+	case Opt_lowerdir:
+		fallthrough;
 	case Opt_lowerdir_add:
 		WARN_ON(ctx->nr >= ctx->capacity);
 		l = &ctx->lower[ctx->nr++];
@@ -375,7 +377,7 @@ static int ovl_parse_layer(struct fs_context *fc, const char *layer_name, enum o
 	if (!name)
 		return -ENOMEM;
 
-	if (upper)
+	if (upper || layer == Opt_lowerdir)
 		err = ovl_mount_dir(name, &path);
 	else
 		err = ovl_mount_dir_noesc(name, &path);
@@ -431,7 +433,6 @@ static int ovl_parse_param_lowerdir(const char *name, struct fs_context *fc)
 {
 	int err;
 	struct ovl_fs_context *ctx = fc->fs_private;
-	struct ovl_fs_context_layer *l;
 	char *dup = NULL, *iter;
 	ssize_t nr_lower, nr;
 	bool data_layer = false;
@@ -471,35 +472,11 @@ static int ovl_parse_param_lowerdir(const char *name, struct fs_context *fc)
 		goto out_err;
 	}
 
-	if (nr_lower > ctx->capacity) {
-		err = -ENOMEM;
-		l = krealloc_array(ctx->lower, nr_lower, sizeof(*ctx->lower),
-				   GFP_KERNEL_ACCOUNT);
-		if (!l)
-			goto out_err;
-
-		ctx->lower = l;
-		ctx->capacity = nr_lower;
-	}
-
 	iter = dup;
-	l = ctx->lower;
-	for (nr = 0; nr < nr_lower; nr++, l++) {
-		ctx->nr++;
-		memset(l, 0, sizeof(*l));
-
-		err = ovl_mount_dir(iter, &l->path);
+	for (nr = 0; nr < nr_lower; nr++) {
+		err = ovl_parse_layer(fc, iter, Opt_lowerdir);
 		if (err)
-			goto out_put;
-
-		err = ovl_mount_dir_check(fc, &l->path, Opt_lowerdir, iter, false);
-		if (err)
-			goto out_put;
-
-		err = -ENOMEM;
-		l->name = kstrdup(iter, GFP_KERNEL_ACCOUNT);
-		if (!l->name)
-			goto out_put;
+			goto out_err;
 
 		if (data_layer)
 			ctx->nr_data++;
@@ -517,7 +494,7 @@ static int ovl_parse_param_lowerdir(const char *name, struct fs_context *fc)
 			 */
 			if (ctx->nr_data > 0) {
 				pr_err("regular lower layers cannot follow data lower layers");
-				goto out_put;
+				goto out_err;
 			}
 
 			data_layer = false;
@@ -531,9 +508,6 @@ static int ovl_parse_param_lowerdir(const char *name, struct fs_context *fc)
 	kfree(dup);
 	return 0;
 
-out_put:
-	ovl_reset_lowerdirs(ctx);
-
 out_err:
 	kfree(dup);