linux-kernel - Re: [PATCH v6 2/2] drm: Prevent release fb after cleanup mode config

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Date:   Tue, 11 Apr 2017 11:04:39 +0800
From:   jeffy <jeffy.chen@...k-chips.com>
To:     Sean Paul <seanpaul@...omium.org>
CC:     linux-kernel@...r.kernel.org, briannorris@...omium.org,
        dianders@...omium.org, tfiga@...omium.org, zyw@...k-chips.com,
        marcheu@...omium.org, mark.yao@...k-chips.com, hshi@...omium.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH v6 2/2] drm: Prevent release fb after cleanup mode config

Hi Sean,

On 04/11/2017 04:31 AM, Sean Paul wrote:
> On Mon, Apr 10, 2017 at 06:00:45PM +0800, Jeffy Chen wrote:
>> After unbinding drm, the user space may still owns the drm dev fd,
>> and may trigger fb release after cleanup mode config.
>>
>> Add a sanity check to prevent that.
>>
>> Signed-off-by: Jeffy Chen <jeffy.chen@...k-chips.com>
>> ---
>>
>> Changes in v6: None
>> Changes in v5: None
>> Changes in v2: None
>>
>>   drivers/gpu/drm/drm_framebuffer.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
>> index e8f9c13..03c1632 100644
>> --- a/drivers/gpu/drm/drm_framebuffer.c
>> +++ b/drivers/gpu/drm/drm_framebuffer.c
>> @@ -583,6 +583,11 @@ void drm_fb_release(struct drm_file *priv)
>>   {
>>   	struct drm_framebuffer *fb, *tfb;
>>   	struct drm_mode_rmfb_work arg;
>> +	struct drm_minor *minor = priv->minor;
>> +	struct drm_device *dev = minor->dev;
>> +
>> +	if (WARN_ON(!dev->mode_config.num_fb && !list_empty(&priv->fbs)))
>
> Have you actually seen this happen? num_fb should be tightly couple to
> priv->fbs, so it seems like this could only result from a driver bug (or I'm not
> reading the code correctly).
yes, 100% repro by:
1/ start display server
2/ unbind drm
3/ stop display server

the num_fb would be decreased(with a warning in 
drm_mode_config_cleanup's fb_list check) in 
drm_mode_config_cleanup->drm_framebuffer_free->rockchip_drm_fb_destroy->drm_framebuffer_cleanup

this flow would not modify the priv->fbs at the same time. so it would 
still remains the pointer of those freed fb.

>
> Sean
>
>> +		return;
>>
>>   	INIT_LIST_HEAD(&arg.fbs);
>>
>> --
>> 2.1.4
>>
>