Re: [PATCH v6 14/24] iio: buffer: wrap all buffer attributes into iio_dev_attr

[Vaittinen, Matti]

Hi dee Ho peeps!

Disclaimer - I have no HW to test this using real in-tree drivers. If 
someone has a device with a variant of bmc150 or adxl372 or  - it'd be 
nice to see if reading hwfifo_watermark_max or hwfifo_watermark_min 
works with the v6.0-rc4. Maybe I am misreading code and have my own 
issues - in which case I apologize already now and go to the corner 
while being deeply ashamed :)

On 2/15/21 12:40, Alexandru Ardelean wrote:
> This change wraps all buffer attributes into iio_dev_attr objects, and
> assigns a reference to the IIO buffer they belong to.
> 
> With the addition of multiple IIO buffers per one IIO device, we need a way
> to know which IIO buffer is being enabled/disabled/controlled.
> 
> We know that all buffer attributes are device_attributes. 

I think this assumption is slightly unsafe. I see few drivers adding 
IIO_CONST_ATTRs in attribute groups. For example the bmc150 and adxl372 
add the hwfifo_watermark_min and hwfifo_watermark_max.

Long story short (or the other way around?)

I was developing a driver for ROHM/Kionix KX022A accelerometer. I did a 
random pick and chose the bmc150-core as a reference how others have 
implemented the accel drivers. During the testing I noticed that using 
IIO_CONST_ATTRs for triggered buffers seem to cause access to somewhere 
it shouldn't... Oops.

Reading the code allows me to assume the problem is wrapping the 
attributes to IIO_DEV_ATTRs.

static struct attribute *iio_buffer_wrap_attr(struct iio_buffer *buffer,
+					      struct attribute *attr)
+{
+	struct device_attribute *dattr = to_dev_attr(attr);
+	struct iio_dev_attr *iio_attr;
+
+	iio_attr = kzalloc(sizeof(*iio_attr), GFP_KERNEL);
+	if (!iio_attr)
+		return NULL;
+
+	iio_attr->buffer = buffer;
+	memcpy(&iio_attr->dev_attr, dattr, sizeof(iio_attr->dev_attr));

This copy does assume all attributes are device_attrs, and does not take 
into account that IIO_CONST_ATTRS have the string stored in a struct 
iio_const_attr which is containing the dev_attr. Eg, copying in the 
iio_buffer_wrap_attr() does not copy the string - and later invoking the 
'show' callback goes reading something else than the mentioned string 
because the pointer is not copied.

I believe problem has been there for a while now - introduced by this:
15097c7a1adc ("iio: buffer: wrap all buffer attributes into 
iio_dev_attr") at Feb 2021.

I don't have a fix for you as I am not sure what would be the correct 
fix. We _could_ go through all the IIO drivers and ensure none used 
IIO_CONST_ATTRs for triggered buffers. This would IMHO be just a 
workaround to the real problem - which is assumption that all attributes 
in the attribute group are dev_attrs. Such silent assumptions are 
fragile as we see :)

If we opt treating all attributes in the group as device_attrs also in 
the future, then (in my not always so humble opinion) the APIs should 
force this - but I'm not sure how to do that.

Best regards
	-- Matti

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

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