linux-kernel - Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to remotely connect USB devices over IP.

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Date:	Thu, 02 Jul 2015 10:45:19 +0200
From:	Oliver Neukum <oneukum@...e.com>
To:	"Daniel P. Berrange" <berrange@...hat.com>
Cc:	Jeremy White <jwhite@...eweavers.com>, hdegoede@...hat.com,
	spice-devel@...ts.freedesktop.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [Spice-devel] [RFC PATCH 1/1] Add a usbredir kernel module to
 remotely connect USB devices over IP.

On Wed, 2015-07-01 at 10:06 +0100, Daniel P. Berrange wrote:

> I don't really think it is sensible to be defining & implementing new
> network services which can't support strong encryption and authentication.
> Rather than passing the file descriptor to the kernel and having it do
> the I/O directly, I think it would be better to dissassociate the kernel
> from the network transport, and thus leave all sockets layer data I/O
> to userspace daemons so they can layer in TLS or SASL or whatever else
> is appropriate for the security need.

Hi,

this hits a fundamental limit. Block IO must be done entirely in kernel
space or the system will deadlock. The USB stack is part of the block
layer and the SCSI error handling. Thus if you involve user space you
cannot honor memory allocation with GFP_NOFS and you break all APIs
where we pass GFP_NOIO in the USB stack.

Supposed you need to reset a storage device for error handling.
Your user space programm does a syscall, which allocates memory
and needs to launder pages. It proceeds to write to the storage device
you wish to reset.

It is the same problem FUSE has with writable mmap. You cannot do
block devices in user space sanely.

	Sorry
		Oliver

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