/proc/self/cgroup does not contain relative paths in docker container

[tmds]

Description

/proc/self/cgroup does not contain relative paths in docker container.

man cgroups describes this file should contain relative paths:

This field contains the pathname of the control group in
the hierarchy to which the process belongs. This pathname
is relative to the mount point of the hierarchy.

And so does man cgroup_namespaces:

When viewing /proc/[pid]/cgroup, the pathname shown in the third
field of each record will be relative to the reading process's root
directory for the corresponding cgroup hierarchy.

Steps to reproduce the issue:

For example, in a docker container

/proc/self/mountinfo:
396 394 0:21 /kubepods/besteffort/pod9a18ffb8-8513-11e7-b26e-7e29fbe2a5a3/d28e0087cf8f3f0429f755d60b0de415b20fcf76736ded7bab6e30e7b739ee36 /sys/fs/cgroup/cpu ro,nosuid,nodev,noexec,relatime - cgroup cgroup rw,cpu
/proc/self/cgroup:
2:cpu:/kubepods/besteffort/pod9a18ffb8-8513-11e7-b26e-7e29fbe2a5a3/d28e0087cf8f3f0429f755d60b0de415b20fcf76736ded7bab6e30e7b739ee36

The path in the cgroup file is not a relative path but it matches the mount root. The limits are not found at that path under the mount point, but directly under the mount point:

$ ls /sys/fs/cgroup/cpu                                                                                                                                                                                                             
cgroup.clone_children  cgroup.procs  cpu.cfs_period_us  cpu.cfs_quota_us  cpu.rt_period_us  cpu.rt_runtime_us  cpu.shares  cpu.stat  notify_on_release  tasks

Expected behavior:

/proc/self/cgroup to behave as described in the man pages.
Or document the difference in the docker docs.