linux-kernel - Re: [PATCH] HID: wacom: check for wacom->shared before following the pointer

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Date:	Fri, 6 Mar 2015 14:10:20 -0500
From:	Benjamin Tissoires <benjamin.tissoires@...il.com>
To:	Ping Cheng <pinglinux@...il.com>
Cc:	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Jason Gerecke <killertofu@...il.com>,
	linux-input <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: wacom: check for wacom->shared before following the pointer

On Thu, Mar 5, 2015 at 8:54 PM, Ping Cheng <pinglinux@...il.com> wrote:
> On Thu, Mar 5, 2015 at 2:36 PM, Benjamin Tissoires
> <benjamin.tissoires@...hat.com> wrote:
>> 486b908 (HID: wacom: do not send pen events before touch is up/forced out)
>> introduces a kernel oops when plugging a tablet without touch.
>
> Thank you for the catch. You must have tried an Intuos 4 or earlier
> model. WACOM_QUIRK_MULTI_INPUT is only set for I5 and later.
>
>> wacom->shared is null for these devices so this leads to a null pointer
>> exception.
>
> The root cause is "if (wacom->shared->touch_down)" was added outside
> of "if (features->quirks & WACOM_QUIRK_MULTI_INPUT)". A consistent fix
> would be:
> ----
> @@ -555,8 +555,11 @@ static int wacom_intuos_inout(struct wacom_wac *wacom)
>            (features->type == CINTIQ && !(data[1] & 0x40)))
>                 return 1;
>
> -       if (features->quirks & WACOM_QUIRK_MULTI_INPUT)
> +       if (features->quirks & WACOM_QUIRK_MULTI_INPUT) {
>                 wacom->shared->stylus_in_proximity = true;
> +               if (wacom->shared->touch_down)
> +                       return 1;
> +       }

Yes, no, maybe :)

that was my first approach. But then I switched to "if
(wacom->shared)" to actually remember the hackers that wacom->shared
is not always valid and that we should not assume it is.
I don't have any preference except that this needs to be fixed in 4.0 final.

Jiri, do you prefer that I resend the patch? or do you amend it? or do
you keep my first version?

Cheers,
Benjamin

>
>         /* in Range while exiting */
>         if (((data[1] & 0xfe) == 0x20) && wacom->reporting_data) {
> ----
>
> Cheers,
>
> Ping
>
>> Change the condition to make it clear that what we need is wacom->shared
>> not NULL.
>>
>> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
>> ---
>>
>> Hi Jiri,
>>
>> this one should be pulled in 4.0. Or we will have surprised users :)
>>
>> Cheers,
>> Benjamin
>>
>>  drivers/hid/wacom_wac.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
>> index ff9a7ab..9739a4c 100644
>> --- a/drivers/hid/wacom_wac.c
>> +++ b/drivers/hid/wacom_wac.c
>> @@ -564,11 +564,12 @@ static int wacom_intuos_inout(struct wacom_wac *wacom)
>>            (features->type == CINTIQ && !(data[1] & 0x40)))
>>                 return 1;
>>
>> -       if (features->quirks & WACOM_QUIRK_MULTI_INPUT)
>> +       if (wacom->shared) {
>>                 wacom->shared->stylus_in_proximity = true;
>>
>> -       if (wacom->shared->touch_down)
>> -               return 1;
>> +               if (wacom->shared->touch_down)
>> +                       return 1;
>> +       }
>>
>>         /* in Range while exiting */
>>         if (((data[1] & 0xfe) == 0x20) && wacom->reporting_data) {
>> --
>> 2.1.0
>>
> --
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