strace cannot attach to process despite being root and setting scope

[Mario Vietri]

I am trying to run `strace` against kworker, but I cannot attach strace despite havingset ptrace_scope=0 and being root.

Here is the proof:

root@rasal:/home/me# whoami
root
root@rasal:/home/me# cat /proc/sys/kernel/yama/ptrace_scope
0
root@rasal:/home/me# ps ax | grep kworker
    5 ? S< 0:00 [kworker/0:0H]
   31 ? S< 0:00 [kworker/1:0H]
   35 ? S 0:00 [kworker/2:0]
   36 ? S< 0:00 [kworker/2:0H]
   41 ? S< 0:00 [kworker/3:0H]
   58 ? S 0:00 [kworker/1:1]
   76 ? S 0:00 [kworker/3:1]
   78 ? S 0:00 [kworker/1:2]
   79 ? S 0:00 [kworker/2:1]
  149 ? S 0:00 [kworker/0:1]
  162 ? S 0:00 [kworker/u16:3]
  163 ? S 0:00 [kworker/u16:4]
  164 ? S 0:00 [kworker/u16:5]
  448 ? S< 0:00 [kworker/3:1H]
  449 ? S< 0:00 [kworker/1:1H]
  451 ? S< 0:00 [kworker/2:1H]
  452 ? S< 0:00 [kworker/0:1H]
  560 ? S 0:00 [kworker/3:2]
  604 ? S 0:00 [kworker/0:2]
 1006 ? S< 0:00 [kworker/u17:0]
 1017 ? S< 0:00 [kworker/u17:1]
 4222 ? S 0:00 [kworker/3:0]
 4249 pts/4 S+ 0:00 grep --color=auto kworker
root@rasal:/home/me# strace -p 4222
strace: attach: ptrace(PTRACE_ATTACH, ...): Operation not permitted
Could not attach to process. If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user. For more details, see /etc/sysctl.d/10-ptrace.conf
root@rasal:/home/me# uname -a
Linux rasal 3.16.0-49-generic #65~14.04.1-Ubuntu SMP Wed Sep 9 10:03:23 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Any info on this?

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.16.0-49-generic 3.16.0-49.65~14.04.1
ProcVersionSignature: Ubuntu 3.16.0-49.65~14.04.1-generic 3.16.7-ckt15
Uname: Linux 3.16.0-49-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.13
Architecture: amd64
CurrentDesktop: KDE
Date: Tue Sep 15 00:01:12 2015
InstallationDate: Installed on 2014-05-05 (497 days ago)
InstallationMedia: Kubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140416.1)
SourcePackage: linux-lts-utopic
UpgradeStatus: Upgraded to trusty on 2014-11-21 (297 days ago)