Re: [PATCH v3 1/2] drivers: usb/core/urb: Add URB_FREE_COHERENT

[Shuah Khan <skhan@linuxfoundation.org>]

On 6/24/22 12:07 PM, Alan Stern wrote:
> On Fri, Jun 24, 2022 at 10:31:06AM -0600, Shuah Khan wrote:
>> On 6/24/22 8:43 AM, Alan Stern wrote:
>>>> It doesn't sound like a there is a compelling reason other than
>>>> "it would be nice to keep the flag values in numerical order".
>>>>
>>>> I would not recommend this option. I am not seeing any value to adding
>>>> change URB_* to USBIP_FLAGS_* layer without some serious techinical
>>>> concerns.
>>>>
>>>>>
>>>>> Another way is to use 0x0400 for FREE_COHERENT.
>>>>> usbip will not take care of this bit as
>>>>> it would be masked.
>>>>>
>>>>
>>>> I would go with this option adding a clear comment with link to this
>>>> discussion.
>>>>
>>>>> Cc Shuah Khan here since she is the maintainer
>>>>> on usbip.
>>>>>
>>>>
>>>> Thank you adding me to the discussion.
>>>
>>> I can see this causing more problems in the future.  There's no hint in
>>> include/linux/usb.h that any of the values it defines are part of a user
>>> API.  If they are, they should be moved to include/uapi/linux/usb/.
>>>
>>
>> Please elaborate on more problems in the future.
> 
> In the future people will want to make other changes to
> include/linux/usb.h and they will not be aware that those changes will
> adversely affect usbip, because there is no documentation saying that
> the values defined in usb.h are part of a user API.  That will be a
> problem, because those changes may be serious and important ones, not
> just decorative or stylistic as in this case.
> 

How often do these values change based on our past experience with these
fields?

>>> In general, if a user program depends on kernel details that are not
>>> designed to be part of a user API, you should expect that the program
>>> will sometimes break from one kernel version to another.
>>>
>>> Yes, I know Linus insists that kernel changes should not cause
>>> regressions in userspace, but the line has to be drawn somewhere.
>>> Otherwise the kernel could never change at all.
>>>
>>
>> I have had to change the usbip sysfs interface api in the past to
>> address security bugs related to information leaks. I am not saying
>> no. I am asking if there is a good reason to do this. So far I haven't
>> heard one.
> 
> I agree with Hongren that values defined in include/linux/ should not be
> part of a user API.  There are two choices:
> 

I agree with this in general. I don't think this is an explicit decision
to make them part of API. It is a consequence of simply copying the
transfer_flags. I am with you both on not being able to recognize the
impact until as this is rather obscure usage hidden away in the packets.
These defines aren't directly referenced.

> 	Move the definitions into include/uapi/linux/, or
> 

Wouldn't this be easier way to handle the change? With this option
the uapi will be well documented.

> 	Add code to translate the values between the numbers used in
> 	userspace and the numbers used in the kernel.  (This is what
> 	was done for urb->transfer_flags in devio.c:proc_do_submiturb()
> 	near line 1862.)
> 

I looked at the code and looks simple enough. I am okay going this route
if we see issues with the option 1.

thanks,
-- Shuah