[uml-devel] (Solved) Hanging using 'iptables -L' in UML vm booted with con0=pts

["Fermín Galán Márquez" <fermin.galan@cttc.es>]

Hi,

After some more debugging I think I've solved the problem (at least :) 
An explanation follows.

First, although apparently it seemed that the modules causing the 
problem were the ones related with networking (as David Fernández said 
in his mail) that assumption was wrong. In fact, the modules causing the 
problem are the ones that print some message in the "kernel message 
buffer" (I don't know what is the right name, but I'm referring to the 
message log that appears when the dmesg command is used) when modprobe 
load them. For example, ip_tables prints something like "ip_tables: (C) 
2000-2006 Netfilter Core Team".

The solution to the problem is using con1 instead of con0, I mean:

./linux ubd0=/tmp/root_fs_debug con=null con1=pts uml_dir=/tmp umid=run

instead of

./linux ubd0=/tmp/root_fs_debug con=null con1=pts uml_dir=/tmp umid=run

In that case you can 'iptables -L' works without problems.

Why? When the module is loaded it prints its message not only in the 
internal kernel message buffer but also in con0 (this can be checked 
running simply "./linux ubd0=/tmp/root_fs_debug", without con= 
redirectors). When con0 is redirected to null (con0=null) there is no 
problem, but if con0 is redirected to a pts (con0=pts) I guess that, in 
the moment of printing the message, some problem occurs with the output 
(as Jeff suggest in its mail) thus causing the vm hang.

This explains also why putting the module in /etc/modules works. The 
loading of the list of modules in /etc/modules is performed before UML 
assign virtual console to pts devices (the sequence can be checked 
observing the booting log).

I think that my solution is more a workaround that a definitive 
solution. Why when con0 is assigned to a pts modules can not (because of 
it hangs the vm) print its message but when it is assigned to null it 
works? Is there a bug in the UML kernel that need to be fixed? Or maybe 
the bug is in modprobe? I leave the question open for the experts in the 
UML internals... :)

Regarding the tests suggested by Paolo:

> Please try logging in via SSH and reproducing the problem and the stacktrace, 
> and also removing con=null - also have you double checked con=null is ok 
> (maybe it was con=none, I'm not sure). I'm not sure screen is perfectly safe 
> to use (it should be).

Do you really need that I perform these test or considers the report 
above is enough? If it's really needed I can do them, but it would take 
me some time (and maybe now it isn't a good idea because of they won't 
provide additional useful information :)

Best regards,

--------------------
Fermín Galán Márquez
CTTC - Centre Tecnològic de Telecomunicacions de Catalunya
Parc Mediterrani de la Tecnologia, Av. del Canal Olímpic s/n, 08860
Castelldefels, Spain
Room 1.02
Tel : +34 93 645 29 12
Fax : +34 93 645 29 01
Email address: fermin dot galan at cttc dot es

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