[PATCH] media: bt878: avoid schedule tasklet when it is not setup

[Tong Zhang]

There is a problem with the tasklet in bt878. bt->tasklet is set by
dvb-bt8xx.ko, and bt878.ko can be loaded independetly.
In this case if interrupt comes it may cause null-ptr-dereference.
To solve this issue, we check if the tasklet is actually set before
calling tasklet_schedule.

[    1.750438] bt878(0): irq FDSR FBUS risc_pc=
[    1.750728] BUG: kernel NULL pointer dereference, address: 0000000000000000
[    1.752969] RIP: 0010:0x0
[    1.757526] Call Trace:
[    1.757659]  <IRQ>
[    1.757770]  tasklet_action_common.isra.0+0x107/0x110
[    1.758041]  tasklet_action+0x22/0x30
[    1.758237]  __do_softirq+0xe0/0x29b
[    1.758430]  irq_exit_rcu+0xa4/0xb0
[    1.758618]  common_interrupt+0x8d/0xa0
[    1.758824]  </IRQ>

Signed-off-by: Tong Zhang <ztong0001@xxxxxxxxx>
---
 drivers/media/pci/bt8xx/bt878.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/media/pci/bt8xx/bt878.c b/drivers/media/pci/bt8xx/bt878.c
index 78dd35c9b65d..6fc89649e320 100644
--- a/drivers/media/pci/bt8xx/bt878.c
+++ b/drivers/media/pci/bt8xx/bt878.c
@@ -300,7 +300,9 @@ static irqreturn_t bt878_irq(int irq, void *dev_id)
 		}
 		if (astat & BT878_ARISCI) {
 			bt->finished_block = (stat & BT878_ARISCS) >> 28;
-			tasklet_schedule(&bt->tasklet);
+			if (bt->tasklet.callback) {
+				tasklet_schedule(&bt->tasklet);
+			}
 			break;
 		}
 		count++;
-- 
2.25.1