Bug#988776: Bug#983357: Netinst crashes xen domU when loading kernel

[Ben Hutchings]

On Tue, 2021-08-24 at 10:56 -0400, Chuck Zmudzinski wrote:
> On 5/24/2021 3:30 AM, Michael Biebl wrote:
> > Hi Phillip
> > 
> > Am 24.05.2021 um 06:19 schrieb Cyril Brulebois:
> > > > trigger to cold plug all devices.  Both scripts are set -e.  The Xen
> > > > Virtual Keyboard driver and at least one other driver have always 
> > > > failed
> > > > to trigger due to having absurdly long modalias, but the error used to
> > > > be ignored.  The kernel now returns the error to udevadm
> > 
> > So this is a change in behaviour in the kernel?
> > What happens if you boot the installed system? Does udevadm trigger 
> > fail there as well?
> > 
> > I feel a bit uneasy changing the udev start script this late in the 
> > release cycle (especially when it appears like covering up an issue 
> > someplace else).
> > 
> > I'll let Marco make the judgement on this though, as he has the most 
> > experience with those udev udeb start scripts as the original author.
> > 
> > Michael
> > 
> 
> After reviewing Philip's message at
> 
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=983357#43
> 
> which seems to point to the root cause of this bug, I can add:
> 
> On my Xen HVM DomU I see the absurdly long modalias for the Xen
> Virtual keyboard that seems to be causing this crash in sysfs at
> 
> /sys/devices/virtual/input/input2/modalias
> 
> But at /sys/devices/vkbd-0/modalias, I see just 'xen:vkbd', which would
> probably not result in an error in the udev script if this was also
> written as the modalias at /sys/devices/virtual/input/input2/modalias
>
> So the Xen virtual keyboard appears more than once in sysfs, and
> modalias is not the same in the different places. This seems
> to be a problem.

They are two different devices, and they should have different
modaliases.

Linux has code for discovering devices on each kind of bus, including
virtual buses, and that code creates "bus devices" such as vkbd-0.  At
this point the kernel doesn't know what the device is capable of.  The
modalias for a bus device carries some identifying information that can
be used to select a driver module for it.

The driver does know what the device is capable of, and how to use it.
It will normally create one or more "class devices" that support a
particular set of operations; in this case input device operations. 
Class devices typically don't have modaliases, since they don't need
another layer of drivers on top.  However, for input devices the
modalias carries information about the device's capabilities.  These
may trigger loading of the evdev or joydev module.

> I understand the correct way to fix this bug is by modifying the
> Xxen virtual keyboard (and any other devices that might cause
> this crash) and not the start-udev script on the netinst
> installation media, which is so far the only available workaround.
> Hopefully Xen will accept a fix if we can come up with a fix.
[...]

I think a proper fix would be one of:

a. If the Xen virtual keyboard driver is advertising capabilities it
   doesn't have, stop it doing that.
b. Change the implementation of modalias attributes to allow longer
   values.

It's not clear to me whether the Xen driver is advertising correctly or
not.  If it is, then the solution should be b, but that may be too
disruptive a change to the kernel.  So a reasonable workaround might
be:

c. Change the input subsystem to limit the length of the
   capabilities part of the modalias.


Ben.

-- 
Ben Hutchings
73.46% of all statistics are made up.
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