[PATCH] fbdev: modedb: Fix kernel crash in fb_videomode_to_var()

[Helge Deller <deller@gmx.de>]

Fix a kernel crash in the fbdev modedb code which can happen if you boot
a system without any graphic card driver, in which case the dummycon
driver takes the console. If you then load a fbdev graphics driver and
start a the X11-fbdev the kernel will crash with a backtrace:

  IAOQ[0]: fb_videomode_to_var+0xc/0x88
 Backtrace:
  [<10582ff8>] display_to_var+0x28/0xe8
  [<1058584c>] fbcon_switch+0x15c/0x55c
  [<105a8a1c>] redraw_screen+0xdc/0x228
  [<1059d6f8>] complete_change_console+0x50/0x140
  [<1059eae0>] change_console+0x6c/0xdc
  [<105ab4f4>] console_callback+0x1a0/0x1a8
  [<101cb5e8>] process_one_work+0x1c4/0x3cc
  [<101cb978>] worker_thread+0x188/0x4b4
  [<101d5a94>] kthread+0xec/0xf4
  [<1018801c>] ret_from_kernel_thread+0x1c/0x24

The problem is, that the dummycon driver may not have a valid monitor
mode defined (which is ok for that driver), so the mode field in
fbcon_display can be NULL.

Fix the crash by simply returning from fb_var_to_videomode()
if the video mode field is NULL.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable <stable@kernel.org>

---

diff --git a/drivers/video/fbdev/core/modedb.c b/drivers/video/fbdev/core/modedb.c
index 6473e0dfe146..6a670b9dbcb4 100644
--- a/drivers/video/fbdev/core/modedb.c
+++ b/drivers/video/fbdev/core/modedb.c
@@ -893,6 +893,8 @@ void fb_var_to_videomode(struct fb_videomode *mode,
 void fb_videomode_to_var(struct fb_var_screeninfo *var,
 			 const struct fb_videomode *mode)
 {
+	if (!mode)
+		return;
 	var->xres = mode->xres;
 	var->yres = mode->yres;
 	var->xres_virtual = mode->xres;