[PATCH] drm: qxl: Don't clean up uninitialized fbdev framebuffer

[Gabriel Krisman Bertazi]

Daniel Vetter <daniel at ffwll.ch> writes:

Hi Daniel, thanks again for your review.

> Hm, I bit silly that we need #ifdef here, all this stuff is meant to Just
> Work. Can't we just fix the oops with suitable checks in the fini paths
> instead?

Heh.  I expected someone would bring up this objection.  My rationale is
that on the driver side, and for many drivers, fbdev emulation code is
always executed, even if it is only calling a bunch of NOP functions on
the drm core, which is a bit silly.  I wonder if struct drm_driver could
have a new callback for fbdev_initialization, which we could avoid
calling in drm_core (maybe at registering time) if FBDEV_EMULATION is
disabled. (well, this would go a bit in the opposite direction of having
open coded probe sequences).

Anyway, for the Oops I mentioned, can you consider the patch below,
instead?

>
> Also 0day has hit some oops in bochs, it might suffer from similar bugs.

I can take a look at that too.

-- >8 --
Subject: [PATCH] drm: qxl: Don't clean up uninitialized fbdev framebuffer

If fbdev emulation is disabled, the QXL shutdown path still tries to
clean the framebuffer, which wasn't initialized, hitting the Oops below.
We can check for the qfb->obj element to ensure the fb was initialized,
because the qfb always has a bo object associated with it since
initialization time.

[   24.284684] BUG: unable to handle kernel NULL pointer dereference at 00000000000002e0
[   24.285627] IP: mutex_lock+0x18/0x30
[   24.286049] PGD 78cdf067
[   24.286050] PUD 7940f067
[   24.286344] PMD 0
[   24.286649]
[   24.287072] Oops: 0002 [#1] SMP
[   24.287422] Modules linked in: qxl
[   24.287806] CPU: 0 PID: 2328 Comm: bash Not tainted 4.10.0-rc5+ #97
[   24.288515] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-20161025_171302-gandalf 04/01/2014
[   24.289681] task: ffff88007c4c0000 task.stack: ffffc90001b58000
[   24.290354] RIP: 0010:mutex_lock+0x18/0x30
[   24.290812] RSP: 0018:ffffc90001b5bcb0 EFLAGS: 00010246
[   24.291401] RAX: 0000000000000000 RBX: 00000000000002e0 RCX: 0000000000000000
[   24.292209] RDX: ffff88007c4c0000 RSI: 0000000000000001 RDI: 00000000000002e0
[   24.292987] RBP: ffffc90001b5bcb8 R08: fffffffffffffffe R09: 0000000000000001
[   24.293797] R10: ffff880078d80b80 R11: 0000000000011400 R12: 0000000000000000
[   24.294601] R13: 00000000000002e0 R14: ffffffffa0009c28 R15: 0000000000000060
[   24.295439] FS:  00007f30e3acbb40(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
[   24.296364] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   24.296997] CR2: 00000000000002e0 CR3: 0000000078c7b000 CR4: 00000000000006f0
[   24.297813] Call Trace:
[   24.298097]  drm_framebuffer_cleanup+0x1f/0x70
[   24.298612]  qxl_fbdev_fini+0x68/0x90 [qxl]
[   24.299074]  qxl_modeset_fini+0xd/0x30 [qxl]
[   24.299562]  qxl_pci_remove+0x22/0x50 [qxl]
[   24.300025]  pci_device_remove+0x34/0xb0
[   24.300507]  device_release_driver_internal+0x150/0x200
[   24.301082]  device_release_driver+0xd/0x10
[   24.301587]  unbind_store+0x108/0x150
[   24.301993]  drv_attr_store+0x20/0x30
[   24.302402]  sysfs_kf_write+0x32/0x40
[   24.302827]  kernfs_fop_write+0x108/0x190
[   24.303269]  __vfs_write+0x23/0x120
[   24.303678]  ? security_file_permission+0x36/0xb0
[   24.304193]  ? rw_verify_area+0x49/0xb0
[   24.304636]  vfs_write+0xb0/0x190
[   24.305004]  SyS_write+0x41/0xa0
[   24.305362]  entry_SYSCALL_64_fastpath+0x1a/0xa9
[   24.305887] RIP: 0033:0x7f30e31d9620
[   24.306285] RSP: 002b:00007ffc54b47e68 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   24.307128] RAX: ffffffffffffffda RBX: 00007f30e3497600 RCX: 00007f30e31d9620
[   24.307928] RDX: 000000000000000d RSI: 0000000000da2008 RDI: 0000000000000001
[   24.308727] RBP: 000000000070bc60 R08: 00007f30e3498760 R09: 00007f30e3acbb40
[   24.309504] R10: 0000000000000073 R11: 0000000000000246 R12: 0000000000000001
[   24.310295] R13: 0000000000000000 R14: 0000000000000000 R15: 00007ffc54b47f34
[   24.311095] Code: 0e 01 e9 7b fe ff ff 66 90 66 2e 0f 1f 84 00 00 00 00 00
55 48 89 e5 53 48 89 fb e8 83 e8 ff ff 65 48 8b 14 25 40 c4 00 00 31 c0 <3e>
48 0f b1 13 48 85 c0 74 08 48 89 df e8 66 fd ff ff 5b 5d c3
[   24.313182] RIP: mutex_lock+0x18/0x30 RSP: ffffc90001b5bcb0
[   24.313811] CR2: 00000000000002e0
[   24.314208] ---[ end trace 29669c1593cae14b ]---

Signed-off-by: Gabriel Krisman Bertazi <krisman at collabora.co.uk>
---
 drivers/gpu/drm/qxl/qxl_fb.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/qxl/qxl_fb.c b/drivers/gpu/drm/qxl/qxl_fb.c
index 35124737666e..4d6c311e8e5e 100644
--- a/drivers/gpu/drm/qxl/qxl_fb.c
+++ b/drivers/gpu/drm/qxl/qxl_fb.c
@@ -351,13 +351,16 @@ static int qxl_fbdev_destroy(struct drm_device *dev, struct qxl_fbdev *qfbdev)
 
 	drm_fb_helper_unregister_fbi(&qfbdev->helper);
 
-	if (qfb->obj) {
+	if (qfb->obj)
 		qxlfb_destroy_pinned_object(qfb->obj);
-		qfb->obj = NULL;
-	}
+
 	drm_fb_helper_fini(&qfbdev->helper);
 	vfree(qfbdev->shadow);
-	drm_framebuffer_cleanup(&qfb->base);
+
+	if (qfb->obj)
+		drm_framebuffer_cleanup(&qfb->base);
+
+	qfb->obj = NULL;
 
 	return 0;
 }
-- 
2.11.0