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

[Rafael J. Wysocki]

On Wed, Jan 26, 2022 at 5:41 PM Hans de Goede <hdegoede at redhat.com> 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 at redhat.com> 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 at gmail.com>
> >>
> >> 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.

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