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

[Matti Vaittinen]

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

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