Re: a bochs-drm (?) oops on head

From: Thomas Zimmermann
Date: Tue Dec 17 2024 - 06:43:27 EST

(cc'ing Hans, who implemented deferred console takeover)

Hi

Am 16.12.24 um 18:35 schrieb Dr. David Alan Gilbert:
* Thomas Zimmermann (tzimmermann@xxxxxxx) wrote:
Hi


Am 16.12.24 um 14:46 schrieb Dr. David Alan Gilbert:
[...]
The attached patch fixes the problem for me. Could you please test and
report back the results.
That gets me a different oops; this was run with:
qemu-system-x86_64 -M pc -cpu host --enable-kvm -smp 4 -m 2G -kernel /discs/fast/kernel/arch/x86/boot/bzImage -append "console=tty0 console=ttyS0 root=/dev/vdb1 single" -drive if=virtio,file=/discs/more/images/debian12-64scan.qcow2

It looks to me if it made the mistake of trying to print something in the middle of being removed:
From the stack trace below, I'd say it is the one at [1]. But I fail to
reproduce the problem.

Could you please send me the complete output of dmesg after the system
finished booting?
Sure; this is as far as it gets until it hits the vga oops that stops it:

I was able to reproduce it a single time. My setup is

- CONFIG_DEBUG_TEST_DRIVER_REMOVE=y
- CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER=y
- startup with vgacon
- 'quiet' on kernel command line (important!)

and then I got the segfault you report.

With the provided stack trace and log, I think I got some idea what is happened


[ 0.000000][ T0] Linux version 6.13.0-rc2+ (dg@dalek) (gcc (GCC) 14.2.1 20240912 (Red Hat 14.2.1-3), GNU ld version 2.43.1-4.fc41) #373 SMP PREEMPT_DYNAMIC Mon Dec 16 13:25:32 GMT 2024
[ 0.000000][ T0] Command line: console=tty0 console=ttyS0 root=/dev/vdb1 single
[...]
[ 74.077481][ T1] SPI driver abt-y030xx067a has no spi_device_id for abt,y030xx067a
[ 74.088805][ T1] SPI driver panel-ilitek-ili9322 has no spi_device_id for dlink,dir-685-panel
[ 74.090492][ T1] SPI driver panel-ilitek-ili9322 has no spi_device_id for ilitek,ili9322
[ 74.094556][ T1] SPI driver panel-innolux-ej030na has no spi_device_id for innolux,ej030na
[ 74.106367][ T1] SPI driver nt39016 has no spi_device_id for kingdisplay,kd035g6-54nt
[ 74.116623][ T1] SPI driver s6d27a1-panel has no spi_device_id for samsung,s6d27a1
[ 74.120701][ T1] SPI driver panel-samsung-s6e63m0 has no spi_device_id for samsung,s6e63m0

[ 74.177273][ T1] bochs-drm 0000:00:02.0: vgaarb: deactivate vga console

This comes from the first iteration in really_probe() [1]. This is the bochs instance that will be removed. It should install fbcon, but that gets deferred.

[1] https://elixir.bootlin.com/linux/v6.12.5/source/drivers/base/dd.c#L631

[ 74.179388][ T1] [drm] Found bochs VGA, ID 0xb0c5.
[ 74.180931][ T1] [drm] Framebuffer size 16384 kB @ 0xfd000000, mmio @ 0xfebf0000.
[ 74.199314][ T1] [drm] Initialized bochs-drm 1.0.0 for 0000:00:02.0 on minor 2
[ 74.265834][ T1] fbcon: bochs-drmdrmfb (fb1) is primary device
[ 74.265882][ T1] fbcon: Remapping primary device, fb1, to tty 1-63
[ 79.736367][ T1] bochs-drm 0000:00:02.0: [drm] fb1: bochs-drmdrmfb frame buffer device

End of first bochs instance here..

[ 79.800872][ T1] bochs-drm 0000:00:02.0: vgaarb: deactivate vga console

The second instance of bochs starts here and tries to deactivate the console a second time. Notice that we didn't have any "Console is " or "Taking over console" messages.

[ 79.802400][ T1] BUG: kernel NULL pointer dereference, address: 000000000000020c

I've not been able to figure out what is a offset 0x20c (524 decimal). None of the structs involved appears to have any fields starting at this offset. The nearest case is vc_hi_font_mask, [2] which is at +520. Could be related to aligned memory access. get_color() would read that field. [3] vc_num at +512 is another candidate.

[2] https://elixir.bootlin.com/linux/v6.13-rc2/source/include/linux/console_struct.h#L124
[3] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/video/fbdev/core/fbcon.c#L302

[ 79.802448][ T1] #PF: supervisor write access in kernel mode
[ 79.802498][ T1] #PF: error_code(0x0002) - not-present page
[ 79.802545][ T1] PGD 0 P4D 0
[ 79.802622][ T1] Oops: Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC NOPTI
[ 79.802669][ T1] CPU: 2 UID: 0 PID: 1 Comm: swapper/0 Tainted: G W N 6.13.0-rc2+ #373 5a5c0ce8f09b0b72067981f01985e201a0118bb6
[ 79.802669][ T1] Tainted: [W]=WARN, [N]=TEST
[ 79.802669][ T1] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-3.fc41 04/01/2014
[ 79.802669][ T1] RIP: 0010:fbcon_cursor+0xa9/0x3c0
[ 79.802669][ T1] Code: c0 05 00 00 66 89 44 24 06 e8 f3 35 2a fd 0f b7 bb c0 05 00 00 e8 27 b8 e9 fc 49 8d bc 24 0c 02 00 00 49 89 c7 e8 d7 3d 2a fd <45> 89 bc 24 0c 02 00 00 48 8d bd e0 05 00 00 e8 c3 3b 2a fd 44 8b
[ 79.802669][ T1] RSP: 0018:ffffb70f800136c0 EFLAGS: 00010046
[ 79.802669][ T1] RAX: 0000000000000000 RBX: ffff9d4fc10a8800 RCX: 0000000000000000
[ 79.802669][ T1] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[ 79.802669][ T1] RBP: ffff9d4fc54ba800 R08: 0000000000000000 R09: 0000000000000000
[ 79.802669][ T1] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 79.802669][ T1] R13: 0000000000000000 R14: ffff9d4fc54bade8 R15: 0000000000000032
[ 79.802669][ T1] FS: 0000000000000000(0000) GS:ffff9d503d200000(0000) knlGS:0000000000000000
[ 79.802669][ T1] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 79.802669][ T1] CR2: 000000000000020c CR3: 0000000056520000 CR4: 0000000000350ef0
[ 79.802669][ T1] Call Trace:
[ 79.802669][ T1] <TASK>
[ 79.802669][ T1] ? __die+0x23/0x80
[ 79.802669][ T1] ? page_fault_oops+0x21c/0x240
[ 79.802669][ T1] ? do_user_addr_fault+0x893/0x1180
[ 79.802669][ T1] ? srso_return_thunk+0x5/0x7f
[ 79.802669][ T1] ? exc_page_fault+0x3f/0x180
[ 79.802669][ T1] ? exc_page_fault+0x87/0x180
[ 79.802669][ T1] ? asm_exc_page_fault+0x26/0x40

[ 79.802669][ T1] ? fbcon_cursor+0xa9/0x3c0
[ 79.802669][ T1] hide_cursor+0x66/0x1c0
[ 79.802669][ T1] vt_console_print+0x9b1/0xa40

I think we get here via steps [4] to [8].

[4] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/pci/vgaarb.c#L173
[5] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/tty/vt/vt.c#L3287
[6] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/tty/vt/vt.c#L861
[7] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/tty/vt/vt.c#L846
[8] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/video/fbdev/core/fbcon.c#L1322

When vt_console_print() invokes the call at [9], it apparently replaces the deferred console implementation (maybe ?) and
then the next line [5] operates on a NULL pointer somewhere.

[9] https://elixir.bootlin.com/linux/v6.13-rc2/source/drivers/tty/vt/vt.c#L3286

Best regards
Thomas


--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)