RE: [EXT] Re: [PATCH] usb: roles: try to get/put all relevant modules

Xu Yang
2024-01-15T03:02:09Z

Hi Greg,

>
> On Fri, Jan 12, 2024 at 09:28:04AM +0000, Xu Yang wrote:
> > Hi Greg,
> >
> > >
> > > On Fri, Jan 12, 2024 at 09:24:11AM +0100, Greg KH wrote:
> > > > On Fri, Jan 12, 2024 at 04:01:08PM +0800, Xu Yang wrote:
> > > > > +void usb_role_switch_get_modules(struct device *dev)
> > > > > +{
> > > > > + while (dev) {
> > > > > + if (dev->driver)
> > > > > + WARN_ON(!try_module_get(dev->driver->owner));
> > > >
> > > > You just crashed all systems that have panic-on-warn enabled, which is
> > > > by far (i.e. in the billions) the huge majority of Linux systems in the
> > > > world.
> > > >
> > > > If this is something that can fail, then properly handle the issue,
> > > > don't just give up and say "let's fill the kernel log with a mess and
> > > > reboot the box!".
> > >
> > > Ah, I see now you are just moving the code, but please, let's fix this
> > > properly, don't persist in the wrong code here.
> >
> > This is a true module dependency issue and it only occurs when
> > load/unload modules. The dependency of usb controller glue driver,
> > usb controller driver and the user driver (such as tcpm) of usb role
> > switch is not correctly established.
>
> Then the driver model is not being used properly, as no module
> references should be needed here.

Taking below diagram as example:

ci_hdrc.0 register usb get tcpm_port
(driver: ci_hdrc) ---------> role <---- (driver: tcpm)
^ ^ switch | ^
| | | |
+1| | +1 | |+1
| +-------------------------------------+ |
| |
4c200000.usb 1-0050
(driver: ci_hdrc_imx) (driver: tcpci)

1. Driver ci_hdrc_imx and tcpci are built as module at least.
2. When module ci_hdrc_imx is loaded, it will register ci_hdrc.0 device
and try to get ci_hdrc module's reference. ci_hdrc will register
usb-role-switch device.
3. When module tcpci is loaded, it will register tcpm port device and try
to get tcpm module's reference. The tcpm module will get usb-role-switch
which is registered by ci_hdrc. In current design, tcpm will also try to
get ci_hdrc module's reference after get usb-role-switch.

>
> > This patch is used to adjust dependency of them, without it, two issues
> > may happen:
> > 1. "NULL pointer dereference" kernel dump will be shown
>
> For when?

4. Due to no modules depend on ci_hdrc_imx, ci_hdrc_imx can be manually
unloaded. Then device ci_hdrc.0 will be removed by ci_hdrc_imx and
device usb-role-switch is also unregistered.
5. Then, if I try to unload module tcpci, "NULL pointer dereference"
will be shown due to below code:

module_put(sw->dev.parent->driver->owner);

parent->driver is NULL at this time.

>
> > 2. The reference count of usb controller module may never decrease to 0
>
> The reference shouldn't have been increased as you want to be able to
> unload the driver if a device is still present in the system.
>
> So I think there's a larger issue here, module references shouldn't be
> incremented just if a driver binds to a device, right? Only if other
> code is using the module, which is different.

6. If ci_hdrc is also built as module, the module reference will keep at
value 1 at least due to it failed to subtract 1 at step 5.

This issue is observed on chipidea and dwc3, the usb glue driver shouldn't
be unloaded befer tcpci module.

Thanks,
Xu Yang

>
> thanks,
>
> greg k-h