xwayland: clear pixmaps after creation in rootless mode

When a pixmap is created with a backing FBO, the FBO should be cleared
to avoid rendering uninitialized memory. This could happen when the
pixmap is rendered without being filled in its entirety.

One example is when a top-level window without a background is
resized. The pixmap would be reallocated to prepare for more pixels,
but uninitialized memory would be rendered in the resize offset until
the client sends a frame that fills these additional pixels.

Another example is when a new top-level window is created without a
background. Uninitialized memory would be rendered after the pixmap is
allocated and before the client sends its first frame.

This issue is only apparent in OpenGL implementations that don't zero
the VRAM of allocated buffers by default, such as RadeonSI.

Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Closes: https://gitlab.freedesktop.org/xorg/xserver/issues/636
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
(cherry picked from commit 0e9a0c203c)

[ Michel Dänzer:
* Squashed in commit ebf549db2d
* Dropped code related to glamor_format, which only exists on master ]

glamor/glamor.c

@@ -128,6 +128,21 @@ glamor_set_pixmap_texture(PixmapPtr pixmap, unsigned int tex)
     glamor_pixmap_attach_fbo(pixmap, fbo);
 }
 
+_X_EXPORT void
+glamor_clear_pixmap(PixmapPtr pixmap)
+{
+    ScreenPtr screen = pixmap->drawable.pScreen;
+    glamor_screen_private *glamor_priv;
+    glamor_pixmap_private *pixmap_priv;
+
+    glamor_priv = glamor_get_screen_private(screen);
+    pixmap_priv = glamor_get_pixmap_private(pixmap);
+
+    assert(pixmap_priv->fbo != NULL);
+
+    glamor_pixmap_clear_fbo(glamor_priv, pixmap_priv->fbo);
+}
+
 uint32_t
 glamor_get_pixmap_texture(PixmapPtr pixmap)
 {

glamor/glamor.h

@@ -115,6 +115,9 @@ extern _X_EXPORT void glamor_set_pixmap_texture(PixmapPtr pixmap,
 
 extern _X_EXPORT void glamor_set_pixmap_type(PixmapPtr pixmap,
                                              glamor_pixmap_type_t type);
+
+extern _X_EXPORT void glamor_clear_pixmap(PixmapPtr pixmap);
+
 extern _X_EXPORT void glamor_block_handler(ScreenPtr screen);
 
 extern _X_EXPORT PixmapPtr glamor_create_pixmap(ScreenPtr screen, int w, int h,

glamor/glamor_fbo.c

@@ -239,6 +239,18 @@ glamor_create_fbo_array(glamor_screen_private *glamor_priv,
     return NULL;
 }
 
+void
+glamor_pixmap_clear_fbo(glamor_screen_private *glamor_priv, glamor_pixmap_fbo *fbo)
+{
+    glamor_make_current(glamor_priv);
+
+    assert(fbo->fb != 0 && fbo->tex != 0);
+
+    glamor_set_destination_pixmap_fbo(glamor_priv, fbo, 0, 0, fbo->width, fbo->height);
+    glClearColor(0.0, 0.0, 0.0, 0.0);
+    glClear(GL_COLOR_BUFFER_BIT);
+}
+
 glamor_pixmap_fbo *
 glamor_pixmap_detach_fbo(glamor_pixmap_private *pixmap_priv)
 {

glamor/glamor_priv.h

@@ -539,6 +539,7 @@ void glamor_destroy_fbo(glamor_screen_private *glamor_priv,
                         glamor_pixmap_fbo *fbo);
 void glamor_pixmap_destroy_fbo(PixmapPtr pixmap);
 Bool glamor_pixmap_fbo_fixup(ScreenPtr screen, PixmapPtr pixmap);
+void glamor_pixmap_clear_fbo(glamor_screen_private *glamor_priv, glamor_pixmap_fbo *fbo);
 
 /* Return whether 'picture' is alpha-only */
 static inline Bool glamor_picture_is_alpha(PicturePtr picture)

hw/xwayland/xwayland-glamor-gbm.c

@@ -242,8 +242,12 @@ xwl_glamor_gbm_create_pixmap(ScreenPtr screen,
         if (bo) {
             pixmap = xwl_glamor_gbm_create_pixmap_for_bo(screen, bo, depth);
 
-            if (!pixmap)
+            if (!pixmap) {
                 gbm_bo_destroy(bo);
+            }
+            else if (xwl_screen->rootless && hint == CREATE_PIXMAP_USAGE_BACKING_PIXMAP) {
+                glamor_clear_pixmap(pixmap);
+            }
         }
     }