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

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Date:   Fri, 10 Feb 2017 14:30:01 +0200
From:   Felipe Balbi <balbi@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Stefan Agner <stefan@...er.ch>
Cc:     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


Hi,

Greg KH <gregkh@...uxfoundation.org> writes:
> On Thu, Feb 09, 2017 at 10:04:43AM -0800, Stefan Agner wrote:
>> On 2017-02-01 08:59, Stefan Agner wrote:
>> > 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.
>> 
>> Any comment on this? In my opinion especially this first patch really
>> fixes a bug and should get applied... I can remove the newline if
>> preferred.
>
> It's up to Felipe, give him a chance to catch up on patches...

I really don't know what to do here :-) Either way have the potential of
breaking userspace. Maybe returning UTF8 as in write is the lesser of
two evils.

-- 
balbi

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