Getting the magic Sysrq key to work (on Ubuntu)

[kernelmail.jms@gmail.com (julie Sullivan)]

Hi all,

I've been having fun today trying to get the Magic Sysrq key to work on my
Ubuntu 10.04
system (most recent kernel .config was for 2.6.37-rc7). I noticed it wasn't
working by
accident when I tried to use it (which presumably means I haven't used it
since I was
using Gentoo - ouch, a while ago.)
This is probably not strictly a kernel issue - I apologise if you think it
doesn't
belong here, but I thought I'd share my adventure with the list in case
anyone else had
this problem and looks for help here.

I always build the Sysrq into the kernel so I went back to check that my
.config was
right - sure enough, CONFIG_MAGIC_SYSRQ was set to "y".

Documentation/sysrq.txt states:

*'When running a kernel with SysRq compiled in,
/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
the SysRq key. By default the file contains 1 which means that every
possible SysRq request is allowed (in older versions SysRq was disabled
by default, and you were required to specifically enable it at run-time
but this is not the case any more).'*

Hmm, not on my machine:

*julie at zeus:/proc/sys/kernel$ cat sysrq
0*

After a considerable amount of faffing about and searching I managed to find
the solution,
which is to find the sysctl.conf file (*/etc/sysctl.conf*) and add the
following line:

*kernel.sysrq = 1*

The system now boots with the Magic Sysrq key enabled by default.



Note that trying to sudo echo a value into /proc/sys/kernel/sysrq doesn't
work:

*julie at zeus:/proc/sys/kernel$ cat sysrq
0
julie at zeus:/proc/sys/kernel$ sudo echo "1" > sysrq
bash: sysrq: Permission denied
julie at zeus:/proc/sys/kernel$ ls -l sysrq
-rw-r--r-- 1 root root 0 2010-12-29 19:37 sysrq
*
Instead, I have to explicitly log in as root:

*julie at zeus:/proc/sys/kernel$ cat sysrq
0
julie at zeus:/proc/sys/kernel$ su
Password:
root at zeus:/proc/sys/kernel# echo "1" > sysrq
root at zeus:/proc/sys/kernel# exit
exit
julie at zeus:/proc/sys/kernel$ cat sysrq
1*

or sudo a shell:

*julie at zeus:/proc/sys/kernel$ sudo bash
[sudo] password for julie:
root at zeus:/proc/sys/kernel# echo "1" > sysrq
root at zeus:/proc/sys/kernel# exit
exit
julie at zeus:/proc/sys/kernel$ cat sysrq
1*

Which I presume might be counter-intuitive for some Ubuntu users, as AFAICR
they're not encouraged to create an explicit root account but are expected
to sudo
everything instead - in other words they're likely to simply tag 'sudo' in
front of
any CL operation that would normally be root-only and expect it to work.

.............................................

I don't know enough about things that are outside the kernel tree to know
whether
the /proc/sysrq configuration default is a distro thing or whether it's
resulted
from the recent kernel discussions about restricting access for security
reasons to
files/settings exposed by /proc. Does anyone know this or might anyone point
me to
the corresponding discussion?

Thanks
Julie
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