Date:   Mon, 4 May 2020 08:21:31 -0500
From:   Jeremy Linton <jeremy.linton@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-usb@...r.kernel.org, stern@...land.harvard.edu,
        git@...gavinli.com, jarkko.sakkinen@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

Hi,

On 5/4/20 2:13 AM, Greg KH wrote:
> On Fri, May 01, 2020 at 10:47:22AM -0500, Jeremy Linton wrote:
>> Hi,
>>
>> Thanks for taking a look at this.
>>
>> On 5/1/20 2:05 AM, Greg KH wrote:
>>> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
>>>> On arm64, and possibly other architectures, requesting
>>>> IO coherent memory may return Normal-NC if the underlying
>>>> hardware isn't coherent. If these pages are then
>>>> remapped into userspace as Normal, that defeats the
>>>> purpose of getting Normal-NC, as well as resulting in
>>>> mappings with differing cache attributes.
>>>
>>> What is "Normal-NC"?
>>
>> A non-cacheable attribute on arm64 pages. I think Mark R & Marc Z elaborated
>> while I was asleep (thanks!).
>>    .
>>
>>
>>>
>>>> In particular this happens with libusb, when it attempts
>>>> to create zero-copy buffers as is used by rtl-sdr, and
>>>
>>> What is "rtl-sdr"
>>
>> Its the realtek software defined radio (SDR), a really inexpensive TV dongle
>> that was discovered could be used as a general purpose SDR a decade or so
>> ago. In particular, this project
>> https://github.com/osmocom/rtl-sdr/
>> which is packaged by fedora/etc.
>>
>>>
>>>> maybe other applications. The result is usually
>>>> application death.
>>>
>>> So is this a new problem?  Old problem?  Old problem only showing up on
>>> future devices?  On current devices?  I need a hint here as to know if
>>> this is a bugfix or just work to make future devices work properly.
>>
>> This has been a problem on arm devices without IO coherent USB apparently
>> for years. The rtl-sdr project itself has a disable zero-copy mode that
>> people have been using on rpi/etc specific builds. Fedora OTOH, is building
>> it with the same flags on x86 & arm64 which means that people report
>> problems. This happened a few days ago (on a pinebook), and I duplicated it
>> on an NXP platform just running the `rtl_test` artifact with a nooelec from
>> my junk box. Guessing that it was a page mismatch I went looking for that,
>> rather than disabling the zero copy since punishing arm machine that have IO
>> coherent USB adapters for the sins of these low end devices isn't ideal. I
>> found this, and this patch allows the rtl_test app to run without issues on
>> my NXP/solidrun.
>>
>> Plus, given that its actually a kernel/libusb problem its likely there are
>> other applications having similar problems.
>>
>>>
>>>>
>>>> If dma_mmap_attr() is used instead of remap_pfn_range,
>>>> the page cache/etc attributes can be matched between the
>>>> kernel and userspace.
>>>>
>>>> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
>>>> ---
>>>>    drivers/usb/core/devio.c | 5 ++---
>>>>    1 file changed, 2 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
>>>> index 6833c918abce..1e7458dd6e5d 100644
>>>> --- a/drivers/usb/core/devio.c
>>>> +++ b/drivers/usb/core/devio.c
>>>> @@ -217,6 +217,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>>>    {
>>>>    	struct usb_memory *usbm = NULL;
>>>>    	struct usb_dev_state *ps = file->private_data;
>>>> +	struct usb_hcd *hcd = bus_to_hcd(ps->dev->bus);
>>>>    	size_t size = vma->vm_end - vma->vm_start;
>>>>    	void *mem;
>>>>    	unsigned long flags;
>>>> @@ -250,9 +251,7 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
>>>>    	usbm->vma_use_count = 1;
>>>>    	INIT_LIST_HEAD(&usbm->memlist);
>>>> -	if (remap_pfn_range(vma, vma->vm_start,
>>>> -			virt_to_phys(usbm->mem) >> PAGE_SHIFT,
>>>> -			size, vma->vm_page_prot) < 0) {
>>>> +	if (dma_mmap_attrs(hcd->self.sysdev, vma, mem, dma_handle, size, 0)) {
>>>
>>> Given that this code has not changed since 2016, how has no one noticed
>>> this issue before?
>> They have there are a lot of reports of sdr failures, but the general use
>> case is rare?
>>
>>>
>>> And have you tested this change out on other systems (i.e. x86) to
>>> ensure that this still works properly?
>>
>> Yes and no, I did some basic libusb tests on an x86 machine, but its a bit
>> tricky at the moment for me to get the rtl plugged into a x86 test machine.
>> (its a work in progress).
>>
>>
>>>
>>> And why isn't this call used more by drivers if this is a real issue?
>> The particulars of asking for iocoherent memory and then mapping it to
>> userspace is rarer than just asking for kmalloc()/remap() and then
>> performing the dma ops?
>>
>> Then there are all the softer issues around arm64 testing/availability and
>> vendors carrying "fixes" for particular issues (like rtl-sdr disabling zero
>> copy).
>>
>>> And will this cause issues with how the userspace mapping is handled as
>>> now we rely on userspace to do things differently?  Or am I reading the
>>> dma_mmap_attrs() documentation wrong?
>> I don't think userspace is doing anything differently here, and AFAIK, on
>> systems with IO coherent adapters this ends up with the same page mapping as
>> just doing the remap_pfn_rage() with the same attributes as before. I've
>> looked at dma_map_attrs() a bit, but i'm also trusting it does what it says
>> on the tin.
>>
>>
>> Thanks again for looking at this.
> 
> Ok, can I get a lot better written changelog text for this patch based
> on this thread, so that it makes more sense when we merge this patch?

Yes, sure. I also plan on changing it to dma_mmap_coherent() which is 
the same as the dma_mmap_attrs() with the attrs as above.


I will re-post later this afternoon.

Thanks,

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