[Xen-devel] Race condition in console_available callback? (libvirt + libxl + xenconsoled)

[Pawel Marczewski <pawel@xxxxxxxxxxxxxxxxxxxxxx>]

Hello,

I am trying to debug an issue in QubesOS where a domain created by
libvirt often does not have information stored about the console TTY path.

The relevant part of libvirt creates a domain using
libxl_domain_create_new() and registers a callback (aop_console_how)
that is supposed to fire when the console is available. The callback
then calls libxl_console_get_tty(), but that fails with:

2020-01-06 11:52:30.952+0000: libxl: libxl.c:1853:libxl_console_get_tty:
unable to read console tty path `/local/domain/4/console/tty': Resource
temporarily unavailable

Based on my reading of the libxl code, it's supposed to set the path in
xenstore and then call the console_available callback, but only if the
bootloader is configured. Otherwise, we call console_available at a
later point (in domcreate_attach_devices()) and the path in xenstore is
being set by xenconsoled independently.

However, there is no guarantee that xenconsoled will do that before we
call console_available. And indeed, looking at the traces from
xenstored, the read and write of `.../console/tty` are ordered randomly
depending on the machine.

Should libxl wait for the information appearing in '.../console/tty' at
this point? Perhaps similar as the code I see in xenconsoled client
(xen/tools/console/client.c)?

I would be happy to work on a patch but I'm unfamiliar with the project
so I want to check my assumptions.

(I am testing with Xen 4.8.5 because that's what the stable version of
QubesOS uses, but as far as I can tell, that part has not changed since).

Original QubesOS bug here:
https://github.com/QubesOS/qubes-issues/issues/5156


-- 
Paweł Marczewski
Invisible Things Lab

Attachment: signature.asc
Description: OpenPGP digital signature

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel