Re: [PATCH 2/3] HID: steam: add serial number information.

On Fri, Feb 16, 2018 at 09:44:34AM +0100, Benjamin Tissoires wrote:

quoted

I have an issue with this one. The problem is that using
hid_report_len() on the feature report returns 64. But I must call
hid_hw_raw_request() with 65 or it will fail with EOVERFLOW.

Currently I'm allocating a buffer of 65 bytes and all is well.
If I change to hid_alloc_report_buf(), the current implementation
allocates (64+7), so I'm still safe. But I'm worried that the extra
bytes are not guaranteed and a future implementation could return
exactly 64 bytes, leaving me 1 byte short.

About why an array of 65 is required for a report of size 64, I think it
is related to hid_report->id == 0 (so hid_report_enum->numbered == 0).

That's the other way around actually. If you are just using the output
of hid_report_len(), it will take into account the extra byte for the
report ID.
*But*, given the way implement() is working (see the comment in the
implementation of hid_alloc_report()), you need to have up to 7 extra
bytes to not have the EOVERFLOW.

So if we ever change the implement() function (which is *really*
unlikely), we will have to make sure hid_alloc_report() still works,
so you are on the safe side if you use hid_alloc_report().

Ok, I'll do that. The weird thing, however, is that:

	hid_hw_raw_request(steam->hid_dev, 0x00,
		buf, hid_report_len(r), /* 64 */
		HID_FEATURE_REPORT, HID_REQ_GET_REPORT);

fails with EOVERFLOW. I have to use:

	hid_hw_raw_request(steam->hid_dev, 0x00,
		buf, 65
		HID_FEATURE_REPORT, HID_REQ_GET_REPORT);

which just feels wrong to me.

And looking around drivers/hid/*.c I see that most calls to
hid_hw_raw_request(..., HID_REQ_GET_REPORT) use a buffer allocated with
{devm_,}kzalloc() and a constant length, never using
hid_alloc_report_buf() or hid_report_len().

Maybe there is a bug in hid_hw_raw_request() and it should add 1 to the
given buffer len? But then, custom buffer allocations will overflow by
one!

Rodrigo.