UNIX for Advanced & Expert Users

I wonder if anyone could assist with some problems I'm having with Linux Capabilities and their use when using the commands "nice" and "schedtool".

I run a couple of PCs, one is an elderly AMD Sempron 2800+ (32-bit, 2GHz clock and 3GB memory) that is used as a family multimedia system running mythTV. The other is an AMD Phenom II X3 (64-bit, three core, 2.5GHz, 4GB memory). Both systems run Ubuntu 10.04 LTS 32-bit and 64-bit versions respectively.

Ever since upgrading both systems to 10.04 I have had persistent, initially severe stuttering audio when using Rhythmbox and, on the media PC only, lesser problems with stuttering TV playback with mythTV. When I reverted from pulseaudio to ALSA, the severe problems with rhythmbox were cured on the desktop PC and alleviated on the media PC.

I am now attempting deal with the remaining problems on the media PC and, using rhythmbox as a guinea pig, I would like to try running rhythmbox either with a lower nice setting or scheduled as a real time process (SCHED_RR).

I wrote a (very short) script containing "nice --adjustment=-10 rhythmbox", gave the script execute and setuid root priviledges and called it. Gnome objected to this telling me that, as a matter of policy, gnome will not run any process as root.

Further research suggested that I needed instead to give the script file the capability cap_sys_nice+eip. The script's nice call was rejected due "insufficient privilege".

I've therefore written the following trivial script, test-file-caps:

Code:

#! /bin/bash
# simple script to trial file capabilities.

echo
echo "nice replies: "
nice --adjustment=-10 rhythmbox
echo
echo "capsh replies: "
/sbin/capsh --print

and here is an example of it's use. You will see that despite the file having the cap_sys_nice capability, that capability is not present when it executes.

Code:

mike@orion:~/scripts$ ls -l ./test-file-caps
-rwxr--r-- 1 mike mike 167 2012-02-08 16:12 ./test-file-caps
mike@orion:~/scripts$ getcap ./test-file-caps
./test-file-caps = cap_sys_nice+eip
mike@orion:~/scripts$ ./test-file-caps

nice replies: 
nice: cannot set niceness: Permission denied

capsh replies: 
Current: =
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,
cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,
cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,
cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,
cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,
cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,
cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,
cap_mac_admin
Securebits: 00/0x0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=1000
mike@orion:~/scripts$

Can anyone please give me any suggestions or pointers to information that I'm lacking.