[image] Generalise concept of selected image

Most image flags are independent values: any combination of flags may
be set for any image, and the flags for one image are independent of
the flags for any other image.  The "selected" flag does not follow
this pattern: at most one image may be marked as selected at any time.

When invoking a kernel via the UEFI shim, there will be multiple
"special" images: the selected kernel itself, the shim image, and
potentially a shim-signed GRUB binary to be used as a crutch to assist
shim in loading the kernel (since current versions of the UEFI shim
are not capable of directly loading a Linux kernel).

Remove the "selected" image flag and replace it with a general concept
of an image tag with the same semantics: a given tag may be assigned
to at most one image, an image may be found by its tag only while the
image is currently registered, and a tag will survive unregistration
and reregistration of an image (if it has not already been assigned to
a new image).  For visual consistency, also replace the current image
pointer with a current image tag.

The image pointer stored within the image tag holds only a weak
reference to the image, since the selection of an image should not
prevent that image from being freed.  (The strong reference to the
currently executing image is held locally within the execution scope
of image_exec(), and is logically separate from the current image
pointer.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 36 insertions, 33 deletions

diff --git a/src/core/image.c b/src/core/image.c
index b280eb4d..3e65b5ed 100644
--- a/src/core/image.c
+++ b/src/core/image.c
@@ -56,8 +56,15 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /** List of registered images */
 struct list_head images = LIST_HEAD_INIT ( images );
 
+/** Image selected for execution */
+struct image_tag selected_image __image_tag = {
+	.name = "SELECTED",
+};
+
 /** Currently-executing image */
-struct image *current_image;
+struct image_tag current_image __image_tag = {
+	.name = "CURRENT",
+};
 
 /** Current image trust requirement */
 static int require_trusted_images = 0;
@@ -72,8 +79,13 @@ static int require_trusted_images_permanent = 0;
  */
 static void free_image ( struct refcnt *refcnt ) {
 	struct image *image = container_of ( refcnt, struct image, refcnt );
+	struct image_tag *tag;
 
 	DBGC ( image, "IMAGE %s freed\n", image->name );
+	for_each_table_entry ( tag, IMAGE_TAGS ) {
+		if ( tag->image == image )
+			tag->image = NULL;
+	}
 	free ( image->name );
 	free ( image->cmdline );
 	uri_put ( image->uri );
@@ -261,12 +273,6 @@ int register_image ( struct image *image ) {
 			return rc;
 	}
 
-	/* Avoid ending up with multiple "selected" images on
-	 * re-registration
-	 */
-	if ( image_find_selected() )
-		image->flags &= ~IMAGE_SELECTED;
-
 	/* Add to image list */
 	image_get ( image );
 	image->flags |= IMAGE_REGISTERED;
@@ -321,6 +327,23 @@ struct image * find_image ( const char *name ) {
 }
 
 /**
+ * Find image by tag
+ *
+ * @v tag		Image tag
+ * @ret image		Executable image, or NULL
+ */
+struct image * find_image_tag ( struct image_tag *tag ) {
+	struct image *image;
+
+	for_each_image ( image ) {
+		if ( tag->image == image )
+			return image;
+	}
+
+	return NULL;
+}
+
+/**
  * Execute image
  *
  * @v image		Executable image
@@ -346,13 +369,13 @@ int image_exec ( struct image *image ) {
 	if ( image->uri )
 		churi ( image->uri );
 
-	/* Preserve record of any currently-running image */
-	saved_current_image = current_image;
+	/* Set as currently running image */
+	saved_current_image = image_tag ( image, &current_image );
 
 	/* Take out a temporary reference to the image, so that it
 	 * does not get freed when temporarily unregistered.
 	 */
-	current_image = image_get ( image );
+	image_get ( image );
 
 	/* Check that this image can be executed */
 	if ( ! ( image->type && image->type->exec ) ) {
@@ -419,7 +442,7 @@ int image_exec ( struct image *image ) {
 	image_put ( image );
 
 	/* Restore previous currently-running image */
-	current_image = saved_current_image;
+	image_tag ( saved_current_image, &current_image );
 
 	/* Reset current working directory */
 	churi ( old_cwuri );
@@ -442,7 +465,7 @@ int image_exec ( struct image *image ) {
  * registered until the currently-executing image returns.
  */
 int image_replace ( struct image *replacement ) {
-	struct image *image = current_image;
+	struct image *image = current_image.image;
 	int rc;
 
 	/* Sanity check */
@@ -478,38 +501,18 @@ int image_replace ( struct image *replacement ) {
  * @ret rc		Return status code
  */
 int image_select ( struct image *image ) {
-	struct image *tmp;
-
-	/* Unselect all other images */
-	for_each_image ( tmp )
-		tmp->flags &= ~IMAGE_SELECTED;
 
 	/* Check that this image can be executed */
 	if ( ! ( image->type && image->type->exec ) )
 		return -ENOEXEC;
 
 	/* Mark image as selected */
-	image->flags |= IMAGE_SELECTED;
+	image_tag ( image, &selected_image );
 
 	return 0;
 }
 
 /**
- * Find selected image
- *
- * @ret image		Executable image, or NULL
- */
-struct image * image_find_selected ( void ) {
-	struct image *image;
-
-	for_each_image ( image ) {
-		if ( image->flags & IMAGE_SELECTED )
-			return image;
-	}
-	return NULL;
-}
-
-/**
  * Change image trust requirement
  *
  * @v require_trusted	Require trusted images