linux-kernel - Re: [PATCH] drm: udl: Destroy framebuffer only if it was initialized

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Message-ID: <CAKMK7uE_0FSfF694xKfhtRbdfkYP_DbT+KcBNM2LbG53tY1J2w@mail.gmail.com>
Date:   Thu, 26 Apr 2018 11:50:01 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Stéphane Marchesin <stephane.marchesin@...il.com>
Cc:     Sean Paul <seanpaul@...omium.org>,
        Emil Lundmark <lndmrk@...omium.org>,
        Dave Airlie <airlied@...hat.com>,
        Linux Kernel list <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] drm: udl: Destroy framebuffer only if it was initialized

On Thu, Apr 26, 2018 at 12:56 AM, Stéphane Marchesin
<stephane.marchesin@...il.com> wrote:
> On Tue, Apr 24, 2018 at 6:04 AM, Daniel Vetter <daniel@...ll.ch> wrote:
>> On Fri, Apr 20, 2018 at 09:55:32AM -0400, Sean Paul wrote:
>>> On Fri, Apr 20, 2018 at 01:50:01PM +0200, Emil Lundmark wrote:
>>> > This fixes a NULL pointer dereference that can happen if the UDL
>>> > driver is unloaded before the framebuffer is initialized. This can
>>> > happen e.g. if the USB device is unplugged right after it was plugged
>>> > in.
>>> >
>>>
>>> JFYI, in future, if someone makes a suggestion on how to fix a bug, it's good
>>> practice to add a Suggested-by tag to give credit.
>>>
>>> Reviewed-by: Sean Paul <seanpaul@...omium.org>
>>
>> I think a bit more detail in the commit message on why this is even
>> possible would be good. I think it can only happen when you only plug in
>> the udl, without anything connected.
>
> Hehe, I was just reviewing this code internally, so I can answer that one :)
>
> It happens when fbdev is disabled (which is the case for Chrome OS).
> Even though intialization of the fbdev part is optional (it's done in
> udlfb_create which is the callback for fb_probe()), the teardown isn't
> optional (udl_driver_unload -> udl_fbdev_cleanup ->
> udl_fbdev_destroy).
>
> Note that udl_fbdev_cleanup *tries* to be conditional (you can see it
> does if (!udl->fbdev)) but that doesn't work, because udl->fbdev is
> always set during udl_fbdev_init.

Ah right, that's another scenario where fbdev never gets fully
initialized, totally forgot about it :-)

It's a bit unfortunately that we don't have a simple fix for this,
i915 has a bunch of messy such checks too. Hopefully Noralf's work to
make fbdev emulation look more like a normal kms client (which just
happens to run within the kernel) will help here in the long run.
-Daniel


>
> Stéphane
>
>
>>
>> In that case fbdev setup will be delayed until something shows up (so we
>> don't pin a too small fb for it, a feature requested by soc people). This
>> can easily be tested:
>> First:
>> - plug in udl device
>> - wait a minute or so (to make it clear it's not a race)
>> - unplug
>>
>> And then:
>> - plug in an udl device, but with something connected.
>> - unplug right away.
>>
>> I expect that in the first case you'll always blow up, but in the 2nd one
>> you'll never blow up (no matter how fast you unplug).
>>
>> Cheers, Daniel
>>
>>
>>
>>>
>>> > Signed-off-by: Emil Lundmark <lndmrk@...omium.org>
>>> > ---
>>> >  drivers/gpu/drm/udl/udl_fb.c | 8 +++++---
>>> >  1 file changed, 5 insertions(+), 3 deletions(-)
>>> >
>>> > diff --git a/drivers/gpu/drm/udl/udl_fb.c b/drivers/gpu/drm/udl/udl_fb.c
>>> > index 2ebdc6d5a76e..5754e37f741b 100644
>>> > --- a/drivers/gpu/drm/udl/udl_fb.c
>>> > +++ b/drivers/gpu/drm/udl/udl_fb.c
>>> > @@ -426,9 +426,11 @@ static void udl_fbdev_destroy(struct drm_device *dev,
>>> >  {
>>> >     drm_fb_helper_unregister_fbi(&ufbdev->helper);
>>> >     drm_fb_helper_fini(&ufbdev->helper);
>>> > -   drm_framebuffer_unregister_private(&ufbdev->ufb.base);
>>> > -   drm_framebuffer_cleanup(&ufbdev->ufb.base);
>>> > -   drm_gem_object_put_unlocked(&ufbdev->ufb.obj->base);
>>> > +   if (ufbdev->ufb.obj) {
>>> > +           drm_framebuffer_unregister_private(&ufbdev->ufb.base);
>>> > +           drm_framebuffer_cleanup(&ufbdev->ufb.base);
>>> > +           drm_gem_object_put_unlocked(&ufbdev->ufb.obj->base);
>>> > +   }
>>> >  }
>>> >
>>> >  int udl_fbdev_init(struct drm_device *dev)
>>> > --
>>> > 2.17.0.484.g0c8726318c-goog
>>> >
>>>
>>> --
>>> Sean Paul, Software Engineer, Google / Chromium OS
>>> _______________________________________________
>>> dri-devel mailing list
>>> dri-devel@...ts.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>
>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
> _______________________________________________
> dri-devel mailing list
> dri-devel@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel


-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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