Hi all,<br><br>I've been having fun today trying to get the Magic Sysrq key to work on my Ubuntu 10.04<br>system (most recent kernel .config was for 2.6.37-rc7). I noticed it wasn't working by<br>accident when I tried to use it (which presumably means I haven't used it since I was<br>
using Gentoo - ouch, a while ago.)<br>This is probably not strictly a kernel issue - I apologise if you think it doesn't <br>belong here, but I thought I'd share my adventure with the list in case anyone else had <br>
this problem and looks for help here.<br><br>I always build the Sysrq into the kernel so I went back to check that my .config was<br>right - sure enough, CONFIG_MAGIC_SYSRQ was set to "y".<br> <br>Documentation/sysrq.txt states:<br>
<br><i>'When running a kernel with SysRq compiled in,<br>/proc/sys/kernel/sysrq controls the functions allowed to be invoked via<br>the SysRq key. By default the file contains 1 which means that every<br>possible SysRq request is allowed (in older versions SysRq was disabled<br>
by default, and you were required to specifically enable it at run-time<br>but this is not the case any more).'</i><br><br>Hmm, not on my machine:<br><br><b>julie@zeus:/proc/sys/kernel$ cat sysrq<br>0</b><br><br>After a considerable amount of faffing about and searching I managed to find the solution,<br>
which is to find the sysctl.conf file (<b>/etc/sysctl.conf</b>) and add the following line:<br><br><b>kernel.sysrq = 1</b><br><br>The system now boots with the Magic Sysrq key enabled by default.<br><br><br><br>Note that trying to sudo echo a value into /proc/sys/kernel/sysrq doesn't work:<br>
<br><b>julie@zeus:/proc/sys/kernel$ cat sysrq<br>0<br>julie@zeus:/proc/sys/kernel$ sudo echo "1" > sysrq<br>bash: sysrq: Permission denied<br>julie@zeus:/proc/sys/kernel$ ls -l sysrq<br>-rw-r--r-- 1 root root 0 2010-12-29 19:37 sysrq<br>
</b><br>Instead, I have to explicitly log in as root:<br><br><b>julie@zeus:/proc/sys/kernel$ cat sysrq<br>0<br>julie@zeus:/proc/sys/kernel$ su<br>Password:<br>root@zeus:/proc/sys/kernel# echo "1" > sysrq<br>root@zeus:/proc/sys/kernel# exit<br>
exit<br>julie@zeus:/proc/sys/kernel$ cat sysrq<br>1</b><br><br>or sudo a shell:<br><br><b>julie@zeus:/proc/sys/kernel$ sudo bash<br>[sudo] password for julie:<br>root@zeus:/proc/sys/kernel# echo "1" > sysrq<br>
root@zeus:/proc/sys/kernel# exit<br>exit<br>julie@zeus:/proc/sys/kernel$ cat sysrq<br>1</b><br><br>Which I presume might be counter-intuitive for some Ubuntu users, as AFAICR<br>they're not encouraged to create an explicit root account but are expected to sudo<br>
everything instead - in other words they're likely to simply tag 'sudo' in front of<br>any CL operation that would normally be root-only and expect it to work.<br><br>.............................................<br>
<br>I don't know enough about things that are outside the kernel tree to know whether<br>the /proc/sysrq configuration default is a distro thing or whether it's resulted<br>from the recent kernel discussions about restricting access for security reasons to<br>
files/settings exposed by /proc. Does anyone know this or might anyone point me to<br>the corresponding discussion?<br><br>Thanks<br>Julie<br><br>