linux-kernel - Re: [PATCH] qxl: fix null-pointer crash during suspend

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Date:   Mon, 1 Oct 2018 22:33:22 +0200
From:   Peter Wu <peter@...ensteyn.nl>
To:     Fubo Chen <fubo.chen@...il.com>
Cc:     airlied@...hat.com, kraxel@...hat.com,
        alan.christopher.jenkins@...il.com,
        virtualization@...ts.linux-foundation.org,
        dri-devel@...ts.freedesktop.org,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] qxl: fix null-pointer crash during suspend

On Mon, Oct 01, 2018 at 01:13:59PM -0700, Fubo Chen wrote:
> On Tue, Sep 4, 2018 at 2:10 PM Peter Wu <peter@...ensteyn.nl> wrote:
> >
> > "crtc->helper_private" is not initialized by the QXL driver and thus the
> > "crtc_funcs->disable" call would crash (resulting in suspend failure).
> > Fix this by converting the suspend/resume functions to use the
> > drm_mode_config_helper_* helpers.
> >
> > Tested system sleep with QEMU 3.0 using "echo mem > /sys/power/state".
> > During suspend the following message is visible from QEMU:
> >
> >     spice/server/display-channel.c:2425:display_channel_validate_surface: canvas address is 0x7fd05da68308 for 0 (and is NULL)
> >     spice/server/display-channel.c:2426:display_channel_validate_surface: failed on 0
> >
> > This seems to be triggered by QXL_IO_NOTIFY_CMD after
> > QXL_IO_DESTROY_PRIMARY_ASYNC, but aside from the warning things still
> > seem to work (tested with both the GTK and -spice options).
> >
> > Signed-off-by: Peter Wu <peter@...ensteyn.nl>
> 
> Is this a new issue or something that was introduced a long time ago?
> In the latter case, please consider adding a "Cc:
> <stable@...r.kernel.org>" tag to this patch.

I am not sure exactly when the issue was introduced, but the original
code was added in v3.10-rc7-800-gd84300bf7934 while the new
drm_mode_config_helper_suspend API was added in 4.16.

The intended call chain to initialize the private object seems to be:
drm_crtc_helper_add
<- qdev_crtc_init
<- qxl_modeset_init
<- qxl_pci_probe

If any error occurs along the callchain, then the helper_private pointer
will remain NULL. Or if the crtc is obtained in a different way (not
sure how).

Not sure if it is worth backporting, suspend/resume does not seem an
important use case for VMs using QXL and the fix was not validated for
older kernels.
-- 
Kind regards,
Peter Wu
https://lekensteyn.nl