[PATCH] fbdev: Fix unregistering of framebuffers without device

[Javier Martinez Canillas]

Hello Thomas,

On 4/4/22 21:44, Thomas Zimmermann wrote:
> OF framebuffers do not have an underlying device in the Linux
> device hierarchy. Do a regular unregister call instead of hot
> unplugging such a non-existing device. Fixes a NULL dereference.
> An example error message on ppc64le is shown below.
> 
>   BUG: Kernel NULL pointer dereference on read at 0x00000060
>   Faulting instruction address: 0xc00000000080dfa4
>   Oops: Kernel access of bad area, sig: 11 [#1]
>   LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
>   [...]
>   CPU: 2 PID: 139 Comm: systemd-udevd Not tainted 5.17.0-ae085d7f9365 #1
>   NIP:  c00000000080dfa4 LR: c00000000080df9c CTR: c000000000797430
>   REGS: c000000004132fe0 TRAP: 0300   Not tainted  (5.17.0-ae085d7f9365)
>   MSR:  8000000002009033 <SF,VEC,EE,ME,IR,DR,RI,LE>  CR: 28228282  XER: 20000000
>   CFAR: c00000000000c80c DAR: 0000000000000060 DSISR: 40000000 IRQMASK: 0
>   GPR00: c00000000080df9c c000000004133280 c00000000169d200 0000000000000029
>   GPR04: 00000000ffffefff c000000004132f90 c000000004132f88 0000000000000000
>   GPR08: c0000000015658f8 c0000000015cd200 c0000000014f57d0 0000000048228283
>   GPR12: 0000000000000000 c00000003fffe300 0000000020000000 0000000000000000
>   GPR16: 0000000000000000 0000000113fc4a40 0000000000000005 0000000113fcfb80
>   GPR20: 000001000f7283b0 0000000000000000 c000000000e4a588 c000000000e4a5b0
>   GPR24: 0000000000000001 00000000000a0000 c008000000db0168 c0000000021f6ec0
>   GPR28: c0000000016d65a8 c000000004b36460 0000000000000000 c0000000016d64b0
>   NIP [c00000000080dfa4] do_remove_conflicting_framebuffers+0x184/0x1d0
>   [c000000004133280] [c00000000080df9c] do_remove_conflicting_framebuffers+0x17c/0x1d0 (unreliable)
>   [c000000004133350] [c00000000080e4d0] remove_conflicting_framebuffers+0x60/0x150
>   [c0000000041333a0] [c00000000080e6f4] remove_conflicting_pci_framebuffers+0x134/0x1b0
>   [c000000004133450] [c008000000e70438] drm_aperture_remove_conflicting_pci_framebuffers+0x90/0x100 [drm]
>   [c000000004133490] [c008000000da0ce4] bochs_pci_probe+0x6c/0xa64 [bochs]
>   [...]
>   [c000000004133db0] [c00000000002aaa0] system_call_exception+0x170/0x2d0
>   [c000000004133e10] [c00000000000c3cc] system_call_common+0xec/0x250
> 
> The bug [1] was introduced by commit 27599aacbaef ("fbdev: Hot-unplug
> firmware fb devices on forced removal"). Most firmware framebuffers
> have an underlying platform device, which can be hot-unplugged
> before loading the native graphics driver. OF framebuffers do not
> (yet) have that device. Fix the code by unregistering the framebuffer
> as before without a hot unplug.
>

I believe the assumption that all firmware fb would have an underlying
device was a reasonable one and it's a pity that offb doesn't...

But that is how things are and your patch is the least intrusive fix.
 
Reviewed-by: Javier Martinez Canillas <javierm at redhat.com>

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat