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

[Chuck Zmudzinski]

On 8/26/2021 8:01 AM, Chuck Zmudzinski wrote:
> On 8/24/2021 7:12 PM, Ben Hutchings wrote:
>>
>> The current limit on the environment of a uevent appears to be 2 KB
>> (UEVENT_BUFFER_SIZE defined in <linux/kobject.h>).� That seems like it
>> *might* be easier to change, so long as user-space doesn't have a
>> similar limit.
>>
>> I looked into systemd/udev, and it seems to use an 8 KB buffer for
>> receiving uevents:
>>
>> https://sources.debian.org/src/systemd/247.9-1/src/libsystemd/sd-device/device-monitor.c/?hl=390#L390 
>>
>>
>> But as a first step I think increasing the kernel buffer size to 4 KB
>> would be enough.� Perhaps someone could test whether this patch to the
>> domU kernel makes udev happier:
>>
>> --- a/include/linux/kobject.h
>> +++ b/include/linux/kobject.h
>> @@ -30,7 +30,7 @@
>> � � #define UEVENT_HELPER_PATH_LEN������� 256
>> � #define UEVENT_NUM_ENVP����������� 64��� /* number of env pointers */
>> -#define UEVENT_BUFFER_SIZE������� 2048��� /* buffer for the 
>> variables */
>> +#define UEVENT_BUFFER_SIZE������� 4096��� /* buffer for the 
>> variables */
>> � � #ifdef CONFIG_UEVENT_HELPER
>> � /* path to the userspace helper executed on an event */
>> --- END ---
>>
>> ?
>>
>> Ben.
>>
>
> I tested this patch on my Xen HVM bullseye system and
> it appears 4k is enough for the UEVENT_BUFFER_SIZE
> to accommodate the Xen Virtual Keyboard's large
> modalias. I needed to follow the instructions in
> the Kernel team's handbook for changing the ABI
> name of the kernel for the build to succeed with
> the patch. I just bumped it from 8 to 8.1.
>
> Results:
>
> 1. No coldplug failure reported at boot time.
>
> 2. With the patch the system can write uevent
> data to sysfs for the Xen Virtual Keyboard device.
>
> With the current 5.10.0-8 kernel:
>
> chuckz at debian:~$ cat /sys/devices/virtual/input/input2/uevent
> chuckz at debian:~$
>
> With the patched kernel with a change to the ABI version from 8 to 8.1:
>
> chuckz at debian:~$ uname -r
> 5.10.0-8.1-amd64
> chuckz at debian:~$ cat /sys/devices/virtual/input/input2/uevent
> PRODUCT=1/5853/ffff/0
> NAME="Xen Virtual Keyboard"
> PHYS="xenbus/device/vkbd/0"
> PROP=0
> EV=3
> KEY=7fffffffffffffff ffffffffffffffff ffffffffffffffff...
> MODALIAS=input:b0001v5853pFFFFe0000-e0,1,k71,72... really long MODALIAS
> --------------------------------------------------------------------------- 
>
>
> So I think a test of the installation media in a Xen HVM with the
> 4k buffer in the kernel is the next step.
>
> I would also like to test a live CD in a Xen HVM with this patch.
> It was also reported to fail to boot in a Xen HVM on the
> debian-user list.
>
> BTW, my complements to the Debian Kernel Team for the
> excellent handbook on building kernels for Debian. It is
> easy to understand and made it very easy for me to
> build and test the patch even though I have not built
> a Linux kernel in many years, and I never built a Debian
> kernel before.
>
> All the best,
>
> Chuck
>

Results of more tests with the patched kernel:

1. Boot on dom0 - works normally, can create VMs, run Liinux container, etc.
2. Boot in Xen PV - works normally
3. Boot on bare hardware - works normally

I do not see any issues with the patched kernel on my system.

Cheers,

Chuck