[comboot] Allow for tail recursion of COMBOOT images

Multi-level menus via COMBOOT rely on the COMBOOT program being able
to exit and invoke a new COMBOOT program (the next menu).  This works,
but rapidly (within about five iterations) runs out of space in gPXE's
internal stack, since each new image is executed in a new function
context.

Fix by allowing tail recursion between images; an image can now
specify a replacement image for itself, and image_exec() will perform
the necessary tail recursion.

1 files changed, 27 insertions, 3 deletions

diff --git a/src/core/image.c b/src/core/image.c
index 440a68c9..741b0547 100644
--- a/src/core/image.c
+++ b/src/core/image.c
@@ -53,6 +53,7 @@ static void free_image ( struct refcnt *refcnt ) {
 
 	uri_put ( image->uri );
 	ufree ( image->data );
+	image_put ( image->replacement );
 	free ( image );
 	DBGC ( image, "IMAGE %p freed\n", image );
 }
@@ -142,9 +143,9 @@ int register_image ( struct image *image ) {
  * @v image		Executable/loadable image
  */
 void unregister_image ( struct image *image ) {
+	DBGC ( image, "IMAGE %p unregistered\n", image );
 	list_del ( &image->list );
 	image_put ( image );
-	DBGC ( image, "IMAGE %p unregistered\n", image );
 }
 
 /**
@@ -237,6 +238,7 @@ int image_autoload ( struct image *image ) {
  * @ret rc		Return status code
  */
 int image_exec ( struct image *image ) {
+	struct image *replacement;
 	struct uri *old_cwuri;
 	int rc;
 
@@ -257,18 +259,40 @@ int image_exec ( struct image *image ) {
 	old_cwuri = uri_get ( cwuri );
 	churi ( image->uri );
 
+	/* Take out a temporary reference to the image.  This allows
+	 * the image to unregister itself if necessary, without
+	 * automatically freeing itself.
+	 */
+	image_get ( image );
+
 	/* Try executing the image */
 	if ( ( rc = image->type->exec ( image ) ) != 0 ) {
 		DBGC ( image, "IMAGE %p could not execute: %s\n",
 		       image, strerror ( rc ) );
-		goto done;
+		/* Do not return yet; we still have clean-up to do */
 	}
 
- done:
+	/* Pick up replacement image before we drop the original
+	 * image's temporary reference.
+	 */
+	if ( ( replacement = image->replacement ) != NULL )
+		image_get ( replacement );
+
+	/* Drop temporary reference to the original image */
+	image_put ( image );
+
 	/* Reset current working directory */
 	churi ( old_cwuri );
 	uri_put ( old_cwuri );
 
+	/* Tail-recurse into replacement image, if one exists */
+	if ( replacement ) {
+		DBGC ( image, "IMAGE %p replacing self with IMAGE %p\n",
+		       image, replacement );
+		if ( ( rc = image_exec ( replacement ) ) != 0 )
+			return rc;
+	}
+
 	return rc;
 }