linux-kernel - Re: [PATCH 1/2] fs: configfs: make qw

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Date:   Wed, 01 Feb 2017 08:59:18 -0800
From:   Stefan Agner <stefan@...er.ch>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     balbi@...nel.org, andrzej.p@...sung.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: configfs: make qw_sign attribute symmetric

On 2017-02-01 00:06, Greg KH wrote:
> On Tue, Jan 31, 2017 at 06:19:16PM -0800, Stefan Agner wrote:
>> Currently qw_sign requires UTF-8 character to set, but returns UTF-16
>> when read. This isn't obvious when simply using cat since the null
>> characters are not visible, but hexdump unveils the true string:
>>
>>   # echo MSFT100 > os_desc/qw_sign
>>   # hexdump -C os_desc/qw_sign
>>   00000000  4d 00 53 00 46 00 54 00  31 00 30 00 30 00        |M.S.F.T.1.0.0.|
>>
>> Make qw_sign symmetric by returning an UTF-8 string too. Also follow
>> common convention and add a new line at the end.
> 
> Doesn't USB require that strings be in UTF-16?  So why have the kernel
> convert them?

That is a discussion we should have had when the write side of this has
been added:

static ssize_t os_desc_qw_sign_store(struct config_item *item, const
char *page,
				     size_t len)
{
	struct gadget_info *gi = os_desc_item_to_gadget_info(item);
	int res, l;

	l = min((int)len, OS_STRING_QW_SIGN_LEN >> 1);
	if (page[l - 1] == '\n')
		--l;

	mutex_lock(&gi->lock);
	res = utf8s_to_utf16s(page, l,
			      UTF16_LITTLE_ENDIAN, (wchar_t *) gi->qw_sign,
			      OS_STRING_QW_SIGN_LEN);
	if (res > 0)
		res = len;
	mutex_unlock(&gi->lock);

	return res;
}


The store function is definitely already in use today, e.g. this script
used for ev3dev:
https://github.com/ev3dev/ev3-systemd/blob/ev3dev-jessie/scripts/ev3-usb.sh

Changing it to UTF-16 would break that script... So changing the store
part is the lesser of two evils.

Regarding new line: Just following what other attributes are doing by
using the GS_STRINGS_R macro.

--
Stefan