Re: [PATCH v1] drm/nouveau/clk: avoid potential null-dereference

[Vince Hsu <vinceh@nvidia.com>]

On 01/08/2015 01:52 PM, Ilia Mirkin wrote:
> On Thu, Jan 8, 2015 at 12:40 AM, Vince Hsu <vinceh@nvidia.com> wrote:
>> On 01/08/2015 12:57 PM, Ilia Mirkin wrote:
>>> On Wed, Jan 7, 2015 at 11:32 PM, Vince Hsu <vinceh@nvidia.com> wrote:
>>>> On 01/08/2015 10:45 AM, Ilia Mirkin wrote:
>>>>> On Wed, Jan 7, 2015 at 5:29 PM, Andy Shevchenko
>>>>> <andy.shevchenko@gmail.com> wrote:
>>>>>> We have to check pointer before usage.
>>>>>>
>>>>>> Reported-by: Andrey Karpov <karpov@viva64.com>
>>>>>> Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
>>>>>> ---
>>>>>>     drivers/gpu/drm/nouveau/core/subdev/clock/base.c | 4 +++-
>>>>>>     1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/nouveau/core/subdev/clock/base.c
>>>>>> b/drivers/gpu/drm/nouveau/core/subdev/clock/base.c
>>>>>> index e51b72d..2e84436 100644
>>>>>> --- a/drivers/gpu/drm/nouveau/core/subdev/clock/base.c
>>>>>> +++ b/drivers/gpu/drm/nouveau/core/subdev/clock/base.c
>>>>>> @@ -322,7 +322,6 @@ nouveau_pstate_new(struct nouveau_clock *clk, int
>>>>>> idx)
>>>>>>                    return 0;
>>>>>>
>>>>>>            pstate = kzalloc(sizeof(*pstate), GFP_KERNEL);
>>>>>> -       cstate = &pstate->base;
>>>>> What's wrong with this line? If pstate == NULL, &pstate->base == NULL
>>>>> as well and we return.
>>>> If pstate == NULL (kzalloc returned NULL), pstate->base triggers a null
>>>> pointer
>>>> deference error?
>>> Where do you see "pstate->base"? I only see "&pstate->base" which
>>> merely computes an offset into a structure... No reason to dereference
>>> pstate.
>> Sorry that I don't quite understand. We do need deference pstate to get
>> the member base which has type nouveau_cstate, and then assign
>> the address of base to cstate for later use.
>>
>> struct nouveau_pstate {
>>      struct list_head head;
>>      struct list_head list; /* c-states */
>>      struct nouveau_cstate base;
>>      u8 pstate;
>>      u8 fanspeed;
>> };
> &pstate->base is the same thing as
>
> (void *)pstate + offsetof(struct nouveau_pstate, base)
>
> At no point is pstate dereferenced. In fact, take a look at
>
> http://en.wikipedia.org/wiki/Offsetof
>
> which says that the traditional implementation of offsetof is
>
> #define offsetof(st, m) ((size_t)(&((st *)0)->m))
>
Oh I got your point. You're absolutely right. Thanks for the patience. :)

Vince


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