Re: [PATCH]: Add Network Sysrq Support

On Tue, Jun 21, 2011 at 10:08:58AM -0700, Stephen Hemminger wrote:

On Tue, 21 Jun 2011 09:00:40 -0400
Prarit Bhargava [off-list ref] wrote:

quoted

Add Network Sysrq Support

In some circumstances, a system can hang/lockup in such a way that the system
is completely unresponsive to keyboard or console input but is still
responsive to ping.  The config option, CONFIG_SYSRQ_PING, builds
net/ipv4/sysrq-ping.ko which allows a root user to configure the system for
a remote sysrq.

To use this do:

mount -t debugfs none /sys/kernel/debug/
echo 1 > /proc/sys/kernel/sysrq
echo <hex digit val> > /sys/kernel/debug/network_sysrq_magic
echo 1 > /sys/kernel/debug/network_sysrq_enable

Then on another system on the network you can do:

ping -c 1 -p <up to 30 hex digit val><hex val of sysrq> <target_system_name>

ex) sysrq-m, m is ascii 0x6d

ping -c 1 p 1623a06f554d46d676d <target_system_name>

Note that the network sysrq automatically disables after the receipt of
the ping, ie) it is single-shot mode.  If you want to use this again, you
must complete the above four steps again.

Signed-off-by: Prarit Bhargava <redacted>

Isn't same functionality already available with netconsole?

Negative, netconsole is a unidirectional protocol.  It registers no rx_hook and
so it doesn't receive frames.  It could be added, and we could add this
functionality that way, but that would seem wierd (to me at least):

1) It would mean that netconsole was writeable, which means we would need a
utility on the remote server to do that writing to the proper udp port

2) Said utility in (1) would have some security concerns, unless we implemented
the exact same magic key sequencing that prarit has here.  Otherwise any
attacker could send an unsolicited request to the netconsole port on a system.
I like the idea of keeping this functionality separate from a feature that gets
such wide use in a production environment.

I've hacked this kind of feature together to solve this problem a few times now,
I think it would be good to have it as a well defined feature permanently.
Acked-by: Neil Horman <nhorman@tuxdriver.com> 

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