dri3: check for ::get_drawable_modifiers failure

Currently if the function fails, we'll fall into two false assumptions:
 - the the count is zero
 - that the storage pointer is safe for free()

I've just fixed the former (in glamor + xwayland) and have no
plans on adding yet another workaround for the latter.

Simply zero both variables. Regardless if the implementation is missing
the callback or it foobars with output variables (normally a bad idea).

Bonus points - this fixes a bug where we feed garbage to free() further
down ;-)

Fixes: cef12efc15c ("glamor: Implement GetSupportedModifiers")
Cc: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Cc: Daniel Stone <daniels@collabora.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/dri3/dri3_screen.c b/dri3/dri3_screen.c
index 1257c0ef4..a9d2e01db 100644
--- a/dri3/dri3_screen.c
+++ b/dri3/dri3_screen.c
@@ -253,10 +253,13 @@ dri3_get_supported_modifiers(ScreenPtr screen, DrawablePtr drawable,
         return Success;
     }
 
-    if (info->get_drawable_modifiers)
-        (*info->get_drawable_modifiers) (drawable, format,
-                                         &num_drawable_mods,
-                                         &drawable_mods);
+    if (!info->get_drawable_modifiers ||
+        !info->get_drawable_modifiers(drawable, format,
+                                      &num_drawable_mods,
+                                      &drawable_mods)) {
+        num_drawable_mods = 0;
+        drawable_mods = NULL;
+    }
 
     /* We're allocating slightly more memory than necessary but it reduces
      * the complexity of finding the intersection set.