ui/gtk: Check if fence_fd is equal to or greater than 0

'fence_fd' needs to be validated always before being referenced
And the passing condition should include '== 0' as 0 is a valid
value for the file descriptor.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Message-Id: <20240508175403.3399895-2-dongwon.kim@intel.com>

ui/gtk-egl.c

@@ -99,7 +99,7 @@ void gd_egl_draw(VirtualConsole *vc)
 #ifdef CONFIG_GBM
         if (dmabuf) {
             egl_dmabuf_create_fence(dmabuf);
-            if (dmabuf->fence_fd > 0) {
+            if (dmabuf->fence_fd >= 0) {
                 qemu_set_fd_handler(dmabuf->fence_fd, gd_hw_gl_flushed, NULL, vc);
                 return;
             }

ui/gtk-gl-area.c

@@ -86,7 +86,7 @@ void gd_gl_area_draw(VirtualConsole *vc)
 #ifdef CONFIG_GBM
         if (dmabuf) {
             egl_dmabuf_create_fence(dmabuf);
-            if (dmabuf->fence_fd > 0) {
+            if (dmabuf->fence_fd >= 0) {
                 qemu_set_fd_handler(dmabuf->fence_fd, gd_hw_gl_flushed, NULL, vc);
                 return;
             }

ui/gtk.c

@@ -597,10 +597,12 @@ void gd_hw_gl_flushed(void *vcon)
     VirtualConsole *vc = vcon;
     QemuDmaBuf *dmabuf = vc->gfx.guest_fb.dmabuf;
 
-    qemu_set_fd_handler(dmabuf->fence_fd, NULL, NULL, NULL);
-    close(dmabuf->fence_fd);
-    dmabuf->fence_fd = -1;
-    graphic_hw_gl_block(vc->gfx.dcl.con, false);
+    if (dmabuf->fence_fd >= 0) {
+        qemu_set_fd_handler(dmabuf->fence_fd, NULL, NULL, NULL);
+        close(dmabuf->fence_fd);
+        dmabuf->fence_fd = -1;
+        graphic_hw_gl_block(vc->gfx.dcl.con, false);
+    }
 }
 
 /** DisplayState Callbacks (opengl version) **/