[vlc-commits] lib: media: rewrite recursion into list exhaustion

[Alexandre Janniaux]

vlc | branch: master | Alexandre Janniaux <ajanni at videolabs.io> | Tue Jul 14 22:18:05 2020 +0200| [b015ac27b40022c7711b608d638b897118b1e10a] | committer: Alexandre Janniaux

lib: media: rewrite recursion into list exhaustion

As the comment described it, recursion could lead to a stack overflow
when parsing very long recursive playlist. Instead, use temporary
dynamic allocation to chain the addition requests and process them in
the same order.

The out-of-memory errors are only returned to the user as a log and the
preparsed playlist item is truncated, which is non-optimal.

> http://git.videolan.org/gitweb.cgi/vlc.git/?a=commit;h=b015ac27b40022c7711b608d638b897118b1e10a
---

 lib/media.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 78 insertions(+), 11 deletions(-)

diff --git a/lib/media.c b/lib/media.c
index d46371afa9..fd7405a7f8 100644
--- a/lib/media.c
+++ b/lib/media.c
@@ -181,19 +181,90 @@ static libvlc_media_t *input_item_add_subitem( libvlc_media_t *p_md,
     return p_md_child;
 }
 
+struct vlc_item_list
+{
+    struct vlc_list node;
+    input_item_node_t *item;
+    libvlc_media_t *media;
+};
+
+static struct vlc_item_list *
+wrap_item_in_list( libvlc_media_t *media, input_item_node_t *item )
+{
+    struct vlc_item_list *node = malloc( sizeof *node );
+    if( node == NULL )
+        return NULL;
+    node->item = item;
+    node->media = media;
+    return node;
+}
+
 static void input_item_add_subnode( libvlc_media_t *md,
-                                    input_item_node_t *node )
+                                    input_item_node_t *root )
 {
-    for( int i = 0; i < node->i_children; i++ )
+    struct vlc_list list;
+    vlc_list_init( &list );
+
+    /* Retain the media because we don't want the search algorithm to release
+     * it when its subitems get parsed. */
+    libvlc_media_retain(md);
+
+    struct vlc_item_list *node_root = wrap_item_in_list( md, root );
+    if( node_root == NULL )
     {
-        input_item_node_t *child = node->pp_children[i];
-        libvlc_media_t *md_child = input_item_add_subitem( md, child->p_item );
+        libvlc_media_release(md);
+        goto error;
+    }
+
+    /* This is a depth-first search algorithm, so stash the root of the tree
+     * first, then stash its children and loop back on the last item in the
+     * list until the full subtree is parsed, and eventually the full tree is
+     * parsed. */
+    vlc_list_append( &node_root->node, &list );
 
-        if( md_child != NULL )
+    while( !vlc_list_is_empty( &list ) )
+    {
+        /* Pop last item in the list. */
+        struct vlc_item_list *node =
+            vlc_list_last_entry_or_null( &list, struct vlc_item_list, node );
+        vlc_list_remove(&node->node);
+
+        for( int i = 0; i < node->item->i_children; i++ )
         {
-            input_item_add_subnode( md_child, child );
-            libvlc_media_release( md_child );
+            input_item_node_t *child = node->item->pp_children[i];
+
+            /* The media will be released when its children will be added to
+             * the list. */
+            libvlc_media_t *md_child = input_item_add_subitem( node->media, child->p_item );
+            if( md_child == NULL )
+                goto error;
+
+            struct vlc_item_list *submedia =
+                wrap_item_in_list( md_child, child );
+            if (submedia == NULL)
+            {
+                libvlc_media_release( md_child );
+                goto error;
+            }
+
+            /* Stash a request to parse this subtree. */
+            vlc_list_append( &submedia->node, &list );
         }
+
+        libvlc_media_release( node->media );
+        free( node );
+    }
+    return;
+
+error:
+    libvlc_printerr( "Not enough memory" );
+
+    struct vlc_item_list *node;
+    vlc_list_foreach( node, &list, node )
+    {
+        if( node->media != NULL )
+            libvlc_media_release( node->media );
+        free( node );
     }
 }
 
@@ -212,10 +283,6 @@ static void input_item_subtree_added(input_item_t *item,
 
 void libvlc_media_add_subtree(libvlc_media_t *p_md, input_item_node_t *node)
 {
-    /* FIXME FIXME FIXME
-     * Recursive function calls seem much simpler for this. But playlists are
-     * untrusted and can be arbitrarily deep (e.g. with XSPF). So recursion can
-     * potentially lead to plain old stack overflow. */
     input_item_add_subnode( p_md, node );
 
     /* Construct the event */