Re: [RFT PATCH v3 10/10] iio: Don't silently expect attribute types

[Matti Vaittinen <mazziesaccount@gmail.com>]

On 10/3/22 11:58, Matti Vaittinen wrote:
> Hi Andy,
> 
> Thanks for taking the time to review :) Much appreciated.
> 
> On 10/3/22 11:43, Andy Shevchenko wrote:
>> On Mon, Oct 03, 2022 at 11:13:53AM +0300, Matti Vaittinen wrote:
>>> The iio_triggered_buffer_setup_ext() and the
>>> devm_iio_kfifo_buffer_setup_ext() were changed by
>>> commit 15097c7a1adc ("iio: buffer: wrap all buffer attributes into 
>>> iio_dev_attr")
>>> to silently expect that all attributes given in buffer_attrs array are
>>> device-attributes. This expectation was not forced by the API - and some
>>> drivers did register attributes created by IIO_CONST_ATTR().
>>>
>>> When using IIO_CONST_ATTRs the added attribute "wrapping" does not copy
>>> the pointer to stored string constant and when the sysfs file is read 
>>> the
>>> kernel will access to invalid location.
>>>
>>> Change the function signatures to expect an array of iio_dev_attrs to
>>> avoid similar errors in the future.
>>
>> ...
>> 
>>> +            attr[ARRAY_SIZE(iio_buffer_attrs) + i] =
>>> +                (struct attribute *)&id_attr->dev_attr.attr;
>>
>> ...and explicit casting here. Isn't attr is already of a struct 
>> attribute?
> 
> I am glad you asked :)
> This is one of the "things" I was not really happy about. Here we hide 
> the fact that our array is full of pointers to _const_ data. If we don't 
> cast the compiler points this out. Old code did the same thing but it 
> did this by just doing a memcpy for the pointers - which I personally 
> consider even worse as it gets really easy to miss this. The cast at 
> least hints there is something slightly "fishy" going on.
> 
> My "gut feeling" about the correct fix is we should check if some 
> attributes in the array (stored to the struct here) actually need to be 
> modified later (which I doubt). If I was keen on betting I'd bet we 
> could switch the struct definition to also contain pointers to const 
> attributes. I am afraid this would mean quite a few more changes to the 
> function signatures (changing struct attribute * to const struct 
> attribute *) here and there - and possibly also require some changes to 
> drivers. Thus I didn't even look at that option in the scope of this 
> fix. It should probably be a separate refactoring series. But yes - this 
> cast should catch attention as it did.
> 

Actually, now that you pointed it out - do you think this would warrant 
a FIXME comment?

> Yours,
>      -- Matti Vaittinen
> 

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~