Re: 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" <rafael@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 1868 bytes --]

On Wed, Jan 26, 2022 at 5:41 PM Hans de Goede <hdegoede@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@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@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.

[-- Attachment #2: acpica-check-root-in-walks.patch --]
[-- Type: text/x-patch, Size: 535 bytes --]

---
 drivers/acpi/acpica/nswalk.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux-pm/drivers/acpi/acpica/nswalk.c
===================================================================
--- linux-pm.orig/drivers/acpi/acpica/nswalk.c
+++ linux-pm/drivers/acpi/acpica/nswalk.c
@@ -169,6 +169,9 @@ acpi_ns_walk_namespace(acpi_object_type
 
 	if (start_node == ACPI_ROOT_OBJECT) {
 		start_node = acpi_gbl_root_node;
+		if (!start_node) {
+			return_ACPI_STATUS(AE_NO_NAMESPACE);
+		}
 	}
 
 	/* Null child means "get first node" */