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Date: Wed, 10 Jul 2024 15:40:57 +0200
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: John Keeping <jkeeping@inmusicbrands.com>
Cc: linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] usb: gadget: f_uac2: fix non-newline-terminated function
 name
Message-ID: <2024071048-science-footprint-ee8e@gregkh>
References: <20240708142553.3995022-1-jkeeping@inmusicbrands.com>
 <2024071022-exemplary-zipping-1f34@gregkh>
 <Zo6M2RntMo6Qnx3B-jkeeping@inmusicbrands.com>
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In-Reply-To: <Zo6M2RntMo6Qnx3B-jkeeping@inmusicbrands.com>

On Wed, Jul 10, 2024 at 02:30:01PM +0100, John Keeping wrote:
> On Wed, Jul 10, 2024 at 01:53:22PM +0200, Greg Kroah-Hartman wrote:
> > On Mon, Jul 08, 2024 at 03:25:53PM +0100, John Keeping wrote:
> > > Most writes to configfs handle an optional newline, but do not require
> > > it.  By using the number of bytes written as the limit for scnprintf()
> > > it is guaranteed that the final character in the buffer will be
> > > overwritten.
> > > 
> > > This is expected if it is a newline but is undesirable when a string is
> > > written "as-is" (as libusbgx does, for example).
> > 
> > So we are changing kernel functionality because a userspace program does
> > not work?  Why not fix the userspace program?
> 
> This file behaves differently from every other sysfs/debugfs/configfs
> file AFAICT.  In most places the behaviour of the following two commands
> is equivalent:
> 
> 	$ echo foo >file
> 
> 	$ printf foo >file
> 
> But for this function_name the result is that the final character is
> dropped unconditionally, so the name reported in the USB descriptors
> will be "fo" in the second case.
> 
> > > Update the store function to strip an optional newline, matching the
> > > behaviour of usb_string_copy().
> > 
> > This changes the behaviour of a lot of configfs files right?  What will
> > break if this happens?
> 
> No, this is just one file in f_uac2.
> 
> I can't see any scenario where a newline in a USB string descriptor
> makes sense so it's unlikely to break any existing use cases.
> 
> This brings the audio function name more in line with other string
> descriptors for the device manufacturer/product or configuration name
> which use usb_string_copy() and strip a trailing newline if it's
> present.

Ok, thanks for the added info, now applied.

greg k-h