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

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Message-ID: <CAJZ5v0hXKJhCSKki8JHs+Q=3BWYygDNz9LLAaiVKpDvLPr6-ZA@mail.gmail.com>
Date:   Thu, 27 Jan 2022 14:33:52 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Tong Zhang <ztong0001@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-acpi <linux-acpi@...r.kernel.org>
Subject: Re: acpi_get_devices() crash when acpi_disabled==true (was [PATCH v2]
 drm/privacy-screen: honor acpi=off in detect_thinkpad_privacy_screen)

On Thu, Jan 27, 2022 at 2:05 PM Hans de Goede <hdegoede@...hat.com> wrote:
>
> Hi,
>
> On 1/26/22 18:11, Rafael J. Wysocki wrote:
> > On Wed, Jan 26, 2022 at 5:41 PM Hans de Goede <hdegoede@...hat.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@...hat.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@...il.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.
>
> 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.

OK

> 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.

Allright, let me cut an analogous patch for the upstream ACPICA, then.