Re: [PATCH v5 05/11] i2c: rtl9300: check if xfer length is valid

[Jonas Jelonek <jelonek.jonas@gmail.com>]

On 10.08.2025 09:01, Sven Eckelmann wrote:
> On Sunday, 10 August 2025 07:51:12 CEST Wolfram Sang wrote:
>> On Sat, Aug 09, 2025 at 10:07:06PM +0000, Jonas Jelonek wrote:
>>> Add an explicit check for the xfer length to 'rtl9300_i2c_config_xfer'
>>> to make sure a length < 1 or > 16 isn't accepted. While there shouldn't
>>> be a length > 16 because this is specified in the i2c_adapter_quirks, a
>>> length of 0 may be passed.
>> There is another quirk for this: I2C_AQ_NO_ZERO_LEN
>>
>> With that, you shouldn't need the code here.
> I am a little bit lost here. Let us assume that i2c_smbus_write_byte_data() is 
> called - for example by an in-kernel driver. We would then have following call 
> chain:
>
> * i2c_smbus_write_byte_data
> * i2c_smbus_xfer
> * __i2c_smbus_xfer
> * adapter->algo->smbus_xfer (aka rtl9300_i2c_smbus_xfer)
>
> But the quirk is only checked in i2c_check_for_quirks - and then on 
> `struct i2c_msg` and not `union i2c_smbus_data`. And this is only called by 
> __i2c_transfer (which is called by i2c_transfer, i2c_smbus_xfer_emulated, 
> ...). But on first glance, it didn't look like it will be called when using 
> i2c_smbus_write_byte_data - unless __i2c_smbus_xfer fails and must fall back 
> to i2c_smbus_xfer_emulated. I most likely missed something when doing a quick 
> check of the source code. Maybe you can point it out.

Thanks Sven.
I came to the same conclusion for now. The mentioned quirk doesn't seem to
prevent this for smbus_xfer. However, it doesn't harm to add it. This probably
applies to the existing quirks too, that they are not checked for.

So I think this check is necessary. It also ensures that [1] is kept in its purpose
more or less. To prevent any invalid length passed from everywhere. The
implementation of Quick in this driver is also problematic because it passes a
length of 0 internally. Thus, the next patch actually removes that completely.

> And I might have to point out that I am currently not next to the actual HW to 
> check if my statement that adapter->algo->smbus_xfer == rtl9300_i2c_smbus_xfer 
> is really true.

It's true, yes. See [2].

> Kind regards,
> 	Sven

Best regards,
Jonas

[1] https://lore.kernel.org/linux-i2c/20250809-i2c-rtl9300-multi-byte-v4-1-d71dd5eb6121@narfation.org/