Re: acpi_get_devices() crash when acpi_disabled==true (was [PATCH v2] drm/privacy-screen: honor acpi=off in detect_thinkpad_privacy_screen)

[Hans de Goede]

Hi,

On 1/26/22 18:11, Rafael J. Wysocki wrote:
> On Wed, Jan 26, 2022 at 5:41 PM Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>>
>> Hi,
>>
>> On 1/26/22 16:54, Rafael J. Wysocki wrote:
>>> On Wed, Jan 26, 2022 at 2:47 PM Hans de Goede <hdegoede@xxxxxxxxxx> wrote:
>>>>
>>>> Hi All,
>>>>
>>>> On 1/23/22 10:10, Tong Zhang wrote:
>>>>> when acpi=off is provided in bootarg, kernel crash with
>>>>>
>>>>> [    1.252739] BUG: kernel NULL pointer dereference, address: 0000000000000018
>>>>> [    1.258308] Call Trace:
>>>>> [    1.258490]  ? acpi_walk_namespace+0x147/0x147
>>>>> [    1.258770]  acpi_get_devices+0xe4/0x137
>>>>> [    1.258921]  ? drm_core_init+0xc0/0xc0 [drm]
>>>>> [    1.259108]  detect_thinkpad_privacy_screen+0x5e/0xa8 [drm]
>>>>> [    1.259337]  drm_privacy_screen_lookup_init+0xe/0xe85 [drm]
>>>>>
>>>>> The reason is that acpi_walk_namespace expects acpi related stuff
>>>>> initialized but in fact it wouldn't when acpi is set to off. In this case
>>>>> we should honor acpi=off in detect_thinkpad_privacy_screen().
>>>>>
>>>>> Signed-off-by: Tong Zhang <ztong0001@xxxxxxxxx>
>>>>
>>>> Thank you for catching this and thank you for your patch. I was about to merge
>>>> this, but then I realized that this might not be the best way to fix this.
>>>>
>>>> A quick grep shows 10 acpi_get_devices() calls outside of drivers/acpi,
>>>> and at a first glance about half of those are missing an acpi_disabled
>>>> check. IMHO it would be better to simply add an acpi_disabled check to
>>>> acpi_get_devices() itself.
>>>>
>>>> Rafael, do you agree ?
>>>
>>> Yes, I do.
>>
>> Did you see my follow-up that that is not going to work because
>> acpi_get_devices() is an acpica function ?
> 
> No, I didn't, but it is possible to add a wrapper doing the check
> around it and convert all of the users.

Yes I did think about that. Note that I've gone ahead and pushed
the fix which started this to drm-misc-fixes, to resolve the crash
for now.

If we add such a wrapper we can remove a bunch of acpi_disabled checks
from various callers.

> Alternatively, the ACPICA function can check acpi_gbl_root_node
> against NULL, like in the attached (untested) patch.

That is probably an even better idea, as that avoids the need
for a wrapper altogether. So I believe that that is the best
solution.

Regards,

Hans