[PATCH] fbdev: fbcon: Fix unregister crash when more than one framebuffer

[Noralf Trønnes]

When unregistering fbdev using unregister_framebuffer(), any bound
console will unbind automatically. This is working fine if this is the
only framebuffer, resulting in a switch to the dummy console. However if
there is a fb0 and I unregister fb1 having a bound console, I eventually
get a crash. The fastest way for me to trigger the crash is to do a
reboot, resulting in this splat:

[   76.478825] WARNING: CPU: 0 PID: 527 at linux/kernel/workqueue.c:1442 __queue_work+0x2d4/0x41c
[   76.478849] Modules linked in: raspberrypi_hwmon gpio_backlight backlight bcm2835_rng rng_core [last unloaded: tinydrm]
[   76.478916] CPU: 0 PID: 527 Comm: systemd-udevd Not tainted 4.20.0-rc4+ #4
[   76.478933] Hardware name: BCM2835
[   76.478949] Backtrace:
[   76.478995] [<c010d388>] (dump_backtrace) from [<c010d670>] (show_stack+0x20/0x24)
[   76.479022]  r6:00000000 r5:c0bc73be r4:00000000 r3:6fb5bf81
[   76.479060] [<c010d650>] (show_stack) from [<c08e82f4>] (dump_stack+0x20/0x28)
[   76.479102] [<c08e82d4>] (dump_stack) from [<c0120070>] (__warn+0xec/0x12c)
[   76.479134] [<c011ff84>] (__warn) from [<c01201e4>] (warn_slowpath_null+0x4c/0x58)
[   76.479165]  r9:c0eb6944 r8:00000001 r7:c0e927f8 r6:c0bc73be r5:000005a2 r4:c0139e84
[   76.479197] [<c0120198>] (warn_slowpath_null) from [<c0139e84>] (__queue_work+0x2d4/0x41c)
[   76.479222]  r6:d7666a00 r5:c0e918ee r4:dbc4e700
[   76.479251] [<c0139bb0>] (__queue_work) from [<c013a02c>] (queue_work_on+0x60/0x88)
[   76.479281]  r10:c0496bf8 r9:00000100 r8:c0e92ae0 r7:00000001 r6:d9403700 r5:d7666a00
[   76.479298]  r4:20000113
[   76.479348] [<c0139fcc>] (queue_work_on) from [<c0496c28>] (cursor_timer_handler+0x30/0x54)
[   76.479374]  r7:d8a8fabc r6:c0e08088 r5:d8afdc5c r4:d8a8fabc
[   76.479413] [<c0496bf8>] (cursor_timer_handler) from [<c0178744>] (call_timer_fn+0x100/0x230)
[   76.479435]  r4:c0e9192f r3:d758a340
[   76.479465] [<c0178644>] (call_timer_fn) from [<c0178980>] (expire_timers+0x10c/0x12c)
[   76.479495]  r10:40000000 r9:c0e9192f r8:c0e92ae0 r7:d8afdccc r6:c0e19280 r5:c0496bf8
[   76.479513]  r4:d8a8fabc
[   76.479541] [<c0178874>] (expire_timers) from [<c0179630>] (run_timer_softirq+0xa8/0x184)
[   76.479570]  r9:00000001 r8:c0e19280 r7:00000000 r6:c0e08088 r5:c0e1a3e0 r4:c0e19280
[   76.479603] [<c0179588>] (run_timer_softirq) from [<c0102404>] (__do_softirq+0x1ac/0x3fc)
[   76.479632]  r10:c0e91680 r9:d8afc020 r8:0000000a r7:00000100 r6:00000001 r5:00000002
[   76.479650]  r4:c0eb65ec
[   76.479686] [<c0102258>] (__do_softirq) from [<c0124d10>] (irq_exit+0xe8/0x168)
[   76.479716]  r10:d8d1a9b0 r9:d8afc000 r8:00000001 r7:d949c000 r6:00000000 r5:c0e8b3f0
[   76.479734]  r4:00000000
[   76.479764] [<c0124c28>] (irq_exit) from [<c016b72c>] (__handle_domain_irq+0x94/0xb0)
[   76.479793] [<c016b698>] (__handle_domain_irq) from [<c01021dc>] (bcm2835_handle_irq+0x3c/0x48)
[   76.479823]  r8:d8afdebc r7:d8afddfc r6:ffffffff r5:c0e089f8 r4:d8afddc8 r3:d8afddc8
[   76.479851] [<c01021a0>] (bcm2835_handle_irq) from [<c01019f0>] (__irq_svc+0x70/0x98)

The problem is in the console rebinding in fbcon_fb_unbind(). It uses the
virtual console index as the new framebuffer index to bind the console(s)
to. The correct way is to use the con2fb_map lookup table to find the
framebuffer index.

Fixes: cfafca8067c6 ("fbdev: fbcon: console unregistration from unregister_framebuffer")
Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
Reviewed-by: Mikulas Patocka <mpatocka at redhat.com>
---

Mikulas, 
If you have forgotten about this, here's where you gave your r-b:
https://lists.freedesktop.org/archives/dri-devel/2018-July/182728.html

 drivers/video/fbdev/core/fbcon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 8958ccc8b1ac..8976190b6c1f 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -3064,7 +3064,7 @@ static int fbcon_fb_unbind(int idx)
 	for (i = first_fb_vc; i <= last_fb_vc; i++) {
 		if (con2fb_map[i] != idx &&
 		    con2fb_map[i] != -1) {
-			new_idx = i;
+			new_idx = con2fb_map[i];
 			break;
 		}
 	}
-- 
2.19.2