linux-kernel - Re: [PATCH] i2c: mux: demux-pinctrl: handle failure case of devm

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Message-ID: <ddd13632-2d1f-f87c-f0cc-1bd3d1d09452@axentia.se>
Date:   Tue, 4 Dec 2018 14:29:39 +0000
From:   Peter Rosin <peda@...ntia.se>
To:     Nicholas Mc Guire <der.herr@...r.at>
CC:     Nicholas Mc Guire <hofrat@...dl.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] i2c: mux: demux-pinctrl: handle failure case of
 devm_kstrdup()

On 2018-12-04 15:25, Nicholas Mc Guire wrote:
> On Tue, Dec 04, 2018 at 01:49:11PM +0000, Peter Rosin wrote:
>> On 2018-12-04 13:13, Nicholas Mc Guire wrote:
>>> On Tue, Dec 04, 2018 at 11:16:59AM +0000, Peter Rosin wrote:
>>>> Hi!
>>>>
>>>> This patch looks like a good idea. However, a nitpick below.
>>>>
>>>> On 2018-12-01 11:01, Nicholas Mc Guire wrote:
>>>>> devm_kstrdup() may return NULL if internal allocation failed.
>>>>> Thus using  name, value  is unsafe without being checked. As
>>>>> i2c_demux_pinctrl_probe() can return -ENOMEM in other cases
>>>>> a dev_err() message is included to make the failure location
>>>>> clear.
>>>>>
>>>>> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
>>>>> Fixes: e35478eac030 ("i2c: mux: demux-pinctrl: run properly with multiple instances")
>>>>> ---
>>>>>
>>>>> Problem located with experimental coccinelle script
>>>>>
>>>>> Q: The use of devm_kstrdup() seems a bit odd while technically not wrong,
>>>>>    personally I think devm_kasprintf() would be more suitable here.
>>>>>
>>>>> Patch was compile tested with: multi_v7_defconfig 
>>>>> (implies I2C_DEMUX_PINCTRL=y)
>>>>>
>>>>> Patch is against 4.20-rc4 (localversion-next is next-20181130)
>>>>>
>>>>>  drivers/i2c/muxes/i2c-demux-pinctrl.c | 6 ++++++
>>>>>  1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/drivers/i2c/muxes/i2c-demux-pinctrl.c b/drivers/i2c/muxes/i2c-demux-pinctrl.c
>>>>> index 035032e..c466999 100644
>>>>> --- a/drivers/i2c/muxes/i2c-demux-pinctrl.c
>>>>> +++ b/drivers/i2c/muxes/i2c-demux-pinctrl.c
>>>>> @@ -244,6 +244,12 @@ static int i2c_demux_pinctrl_probe(struct platform_device *pdev)
>>>>>  
>>>>>  		props[i].name = devm_kstrdup(&pdev->dev, "status", GFP_KERNEL);
>>>>>  		props[i].value = devm_kstrdup(&pdev->dev, "ok", GFP_KERNEL);
>>>>> +		if (!props[i].name || !props[i].value) {
>>>>> +			dev_err(&pdev->dev,
>>>>> +				"chan %d name, value allocation failed\n", i);
>>>>
>>>> Please drop this memory allocation failure message. You should get such a
>>>> message from devm_kstrdup.
>>>>
>>>
>>> hm...tried to figure out where that message would be comming
>>> from - but I could not find any point in the call tree that
>>> would issue such a message ?
>>>
>>>  devm_kstrdup() 
>>>    -> devm_kmalloc()
>>>         -> alloc_dr()
>>>              --> kmalloc_track_caller() (non-NUMA)
>>>              |     -> __kmalloc_node()
>>>              |        -> __do_kmalloc_node()
>>>              `-> __kmalloc_node_track_caller() (NUMA)
>>>                    -> __do_kmalloc_node()
>>>
>>>  __do_kmalloc_node() seems like it simply returns NULL but
>>>  issues no failure message.
>>>  Am I overlooking something ? 
>>
>> Well, I don't know the details, but checkpatch will warn about simple
>> error messages on devm_kstrdup failure (if I read the checkpatch source
>> correctly). But in this case there are two parallel conditions in the
>> if and hence checkpatch stumbles, but gist is the same, you should not
>> sprinkle messages on memory allocation failure.
>>
> not in this case - atleast checkpatch --strict on the original patch
> did not issue any complaint to that ends. But yes - you
> are right that the intent in checkpatch is clear and this should not
> be carrying a failure message.

Yes, this is exactly what I said, checkpatch stumbles since there are
two conflated tests in one if statement and checkpatch is not smart
so does not pick up on that.

Cheers,
Peter