linux-kernel - Re: [PATCH] ps3-vuart: BUG_ON on null drv before dereferencing it on dev

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Date:	Tue, 1 Sep 2015 13:28:20 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Colin Ian King <colin.king@...onical.com>
Cc:	Geoff Levand <geoff@...radead.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ps3-vuart: BUG_ON on null drv before dereferencing it on dev_dbg

Hi Colin,

On Tue, Sep 1, 2015 at 12:38 PM, Colin Ian King
<colin.king@...onical.com> wrote:
> On 01/09/15 11:35, Geert Uytterhoeven wrote:
>> On Tue, Sep 1, 2015 at 12:21 PM, Colin King <colin.king@...onical.com> wrote:
>>> On the unlikely event that drv is null, the current code will
>>> perform a null pointer dereference with it when printing a dev_dbg
>>> message.  Instead, the BUG_ON check on drv should be performed
>>> before we emit the dev_dbg message.
>>
>> What about just removing the BUG_ON()?
>>
>> The system will crash anyway, providing a backtrace.
>
> I personally think a BUG_ON() shows intention to try to catch an
> unexpected issue in a standard way as opposed to just removing it and
> then hitting an issue with a null ptr deference.

I think this BUG_ON() was purely used during development of the ps3_vuart
framework.

Note that the corresponding platform_drv_probe() also just dereferences its
driver pointer.

>>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>>> ---
>>>  drivers/ps3/ps3-vuart.c | 3 +--
>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/ps3/ps3-vuart.c b/drivers/ps3/ps3-vuart.c
>>> index d6db822..632701a 100644
>>> --- a/drivers/ps3/ps3-vuart.c
>>> +++ b/drivers/ps3/ps3-vuart.c
>>> @@ -1000,12 +1000,11 @@ static int ps3_vuart_probe(struct ps3_system_bus_device *dev)
>>>         dev_dbg(&dev->core, "%s:%d\n", __func__, __LINE__);
>>>
>>>         drv = ps3_system_bus_dev_to_vuart_drv(dev);
>>> +       BUG_ON(!drv);
>>>
>>>         dev_dbg(&dev->core, "%s:%d: (%s)\n", __func__, __LINE__,
>>>                 drv->core.core.name);
>>>
>>> -       BUG_ON(!drv);
>>> -
>>>         if (dev->port_number >= PORT_COUNT) {
>>>                 BUG();
>>>                 return -EINVAL;

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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