linux-kernel - Re: [PATCH 03/10] usb: catch attempts to submit urbs with a vmalloc'd transfer buffer

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Date:	Thu, 8 May 2014 09:32:22 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	David Laight <David.Laight@...lab.com>
Cc:	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"sarah.a.sharp@...ux.intel.com" <sarah.a.sharp@...ux.intel.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 03/10] usb: catch attempts to submit urbs with a vmalloc'd
 transfer buffer

On Thu, May 8, 2014 at 9:22 AM, David Laight <David.Laight@...lab.com> wrote:
> From: Mathias Nyman
>> From: Dan Williams <dan.j.williams@...el.com>
>>
>> Save someone else the debug cycles of figuring out why a driver's
>> transfer request is failing or causing undefined system behavior.
>> Buffers submitted for dma must come from GFP allocated / DMA-able
>> memory.
>>
>> Return -EAGAIN matching the return value for dma_mapping_error() cases.
>
> Won't that just cause the request to be resubmitted a few clock
> cycles later?
> Surely you either need to error the request, or panic.

No, panic() is too drastic for this.

> In any case is this the right place for this sort of test?

Yes.

The system is already compromised because the driver is broken.  The
expectation is that this is a clue bat for a driver developer to fix
their code before it goes upstream.  I found this while debugging an
interaction with a new driver that was causing the xhci controller to
lock up.  Had this warning been there I likely never would have
received the xhci bug report.
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