Re: [RFC patch] vsprintf: Allow %pe to print non PTR_ERR %pe uses as decimal

[Rasmus Villemoes]

On 24/03/2021 18.20, Joe Perches wrote:
> On Wed, 2021-03-24 at 09:52 -0700, Joe Perches wrote:
>> On Wed, 2021-03-24 at 17:42 +0100, Arnd Bergmann wrote:
>>> On Wed, Mar 24, 2021 at 3:20 PM Joe Perches <joe@xxxxxxxxxxx> wrote:
>> []
>>>>> diff --git a/drivers/gpu/drm/imx/imx-ldb.c b/drivers/gpu/drm/imx/imx-ldb.c
>>>> []
>>>>> @@ -197,6 +197,12 @@ static void imx_ldb_encoder_enable(struct drm_encoder *encoder)
>>>>>       int dual = ldb->ldb_ctrl & LDB_SPLIT_MODE_EN;
>>>>>       int mux = drm_of_encoder_active_port_id(imx_ldb_ch->child, encoder);
>>>>>
>>>>> +     if (mux < 0 || mux >= ARRAY_SIZE(ldb->clk_sel)) {
>>>>> +             dev_warn(ldb->dev, "%s: invalid mux %d\n",
>>>>> +                      __func__, ERR_PTR(mux));
>>>>
>>>> This does not compile without warnings.
>>>>
>>>> drivers/gpu/drm/imx/imx-ldb.c: In function ‘imx_ldb_encoder_enable’:
>>>> drivers/gpu/drm/imx/imx-ldb.c:201:22: warning: format ‘%d’ expects argument of type ‘int’, but argument 4 has type ‘void *’ [-Wformat=]
>>>>   201 |   dev_warn(ldb->dev, "%s: invalid mux %d\n",
>>>>       |                      ^~~~~~~~~~~~~~~~~~~~~~
>>>>
>>>> If you want to use ERR_PTR, the %d should be %pe as ERR_PTR
>>>> is converting an int a void * to decode the error type and
>>>> emit it as a string.
>>>
>>> Sorry about that.
>>>
>>> I decided against using ERR_PTR() in order to also check for
>>> positive array overflow, but the version I tested was different from
>>> the version I sent.
>>>
>>> v3 coming.
>>
>> Thanks.  No worries.
>>
>> Up to you, vsprintf would emit the positive mux as a funky hashed
>> hex value by default if you use ERR_PTR with mux > ARRAY_SIZE so
>> perhaps %d without the ERR_PTR use makes the most sense.
>>

> 
> Maybe it's better to output non PTR_ERR %pe uses as decimal so this
> sort of code would work.

No, because that would leak the pointer value when somebody has
accidentally passed a real kernel pointer to %pe.

If the code wants a cute -EFOO string explaining what's wrong, what
about "%pe", ERR_PTR(mux < 0 : mux : -ERANGE)? Or two separate error
messages

if (mux < 0)
  ...
else if (mux >= ARRAY_SIZE())
  ...

Rasmus