Re: corsair-cpro and hidraw

[Marius Zachmann <mail@mariuszachmann.de>]

On 18.06.21 at 14:13:00 CEST, Guenter Roeck wrote
> On Fri, Jun 18, 2021 at 09:06:02AM +0200, Marius Zachmann wrote:
> > On 18.06.21 at 08:47:37 CEST, Wilken Gottwalt wrote
> > > On Fri, 18 Jun 2021 08:18:23 +0200
> > > Marius Zachmann <mail@mariuszachmann.de> wrote:
> > > 
> > > > On 18.06.21 at 07:45:00 CEST, Wilken Gottwalt wrote
> > > > > On Fri, 18 Jun 2021 05:56:29 +0600
> > > > > Aleksandr Mezin <mezin.alexander@gmail.com> wrote:
> > > > > 
> > > > > > I've looked through corsair-psu sources, and I think filtering in
> > > > > > raw_event won't be enough.
> > > > > > 
> > > > > > For example, in corsairpsu_request, there are 2 commands, and they
> > > > > > should be executed consecutively:
> > > > > > 1) corsairpsu_usb_cmd(priv, 2, PSU_CMD_SELECT_RAIL, rail, NULL);
> > > > > > 2) corsairpsu_usb_cmd(priv, 3, cmd, 0, data);
> > > > > > 
> > > > > > If the userspace will squeeze another PSU_CMD_SELECT_RAIL between (1)
> > > > > > and (2), the driver will get data for a wrong rail (and with the
> > > > > > current code won't even notice it).
> > > > > > 
> > > > > > So unless there is a way to "lock" hidraw (and it seems that there
> > > > > > isn't - looking at the code, hidraw calls the low-level hid driver
> > > > > > directly, as far as I understand), it won't work correctly.
> > > > > > 
> > > > > > And if a driver can't work correctly with hidraw enabled - maybe it
> > > > > > shouldn't enable hidraw? So that the user can 1) notice that something
> > > > > > is wrong 2) blacklist or unbind the driver (or userspace tools that
> > > > > > use hidraw can unbind automatically). Otherwise everything seems to be
> > > > > > silently broken.
> > > > > > 
> > > > > > On the other hand, maybe races between the kernel driver and userspace
> > > > > > tools are unlikely, because the driver doesn't talk to the device
> > > > > > continuously - only when sysfs reads happen.
> > > > > 
> > > > > I never noticed any issues of that kind. I actually did quite a lot of
> > > > > userspace testing. A result of this a userspace tool you can find here:
> > > > > https://github.com/wgottwalt/corsair-psu/tree/main/tools/rmi-hxi-query
> > > > > 
> > > > > Though, if you find a way to trigger such a race condition I have no
> > > > > problem to remove the hidraw part.
> > > > > 
> > > > > greetings
> > > > > Will
> > > > 
> > > > It is possible. Making a userspace tool with just a loop of read/writes
> > > > will get you wrong readings in the driver sometimes.
> > > 
> > > Hmm, did you read the comments in the driver? I warn about writing nonsense
> > > values to the micro-controller because you can make it stall. If I let you
> > > access the device by hidraw I assume you know what you are doing. You
> > > actually can damage your PSU by this, something I also warn about. I even
> > > mention that I may remove the hidraw feature in future versions.
> > 
> > Sorry for the confusion. I did not test with corsair-psu. (Do not have
> > the hardware) I tested with corsair-cpro. Reading temps with userspace and
> > reading fan speed with the driver simultaneously.
> > But you are right. This is also not, what a userspace tool should do this
> > fast and if it doesn't, races are really unlikely.
> > 
> 
> Same there: Make userspace and kernel mutually exclusive if parallel access
> is shown to be problematic. "Mutually exclusive" means disable userspace
> access completely while the driver is loaded, not some cross-subsystem
> mutex.
> 
> Guenter
> 

For now I did not get a bug report nor did anyone seem to have a real problem.
It is mostly a theoretical issue.
I am not unhappy about how it is.
Maybe until a real problem occurs, we should just do nothing?

Greetings
Marius