accessing namespaces of a docker/podman container (nsenter)

There is a nice utility `nsenter` that allows you to switch to the namespace of another process. It took me considerable time to search it out today so thought to write a short blog about it.

Now I have a Podman container (for docker just use `docker` command instead of `podman` below). I started that container by:

$ sudo podman run -t -a STDIN -a STDOUT -a STDERR --rm=true --entrypoint /bin/bash quay.io/example/image:version

And I've been running some testing on it but it turned out I want to increase limits without destroying my preparations if I exit the process. So first thing is to figure out pid namespace of my container:

$ sudo podman ps --ns
CONTAINER ID  NAMES                PID   CGROUPNS    IPC         MNT         NET         PIDNS       USERNS      UTS
a147a3a5b35f  fervent_stonebraker  1408  4026531835  4026532431  4026532429  4026532360  4026532432  4026531837  4026532430

I see different namespaces but `nsenter` requires a file name to switch to a PID namespace. SO I will use the PID information in above output.

$ sudo nsenter --pid=/proc/1408/ns/pid

The above starts a shell for me in the PID namespace of my container. Now I want to change limits. Interesting to note here is that I change pid 1 as it is the PID of my bash shell in the container:

$ sudo prlimit --rss=-1 --memlock=33554432 --pid 1

Finally verify limits in my container shell:

bash-4.2$ ulimit -a
core file size          (blocks, -c) unlimited
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 23534
max locked memory       (kbytes, -l) 32768
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1048576
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 16384
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1048576
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

One interesting thing is `ps` inside namespace. If I run these two

$ ps -ef
$ sudo nsenter --pid=/proc/1408/ns/pid ps -ef

They will show exactly the same output. It is because I still have same `/proc` mounted even though my PID namespace is changed. And it is what `ps` looks at.

With `nsenter` you can switch any namespace, not only PID. I hope this is a useful short demonstration how to do fun things with linux namespaces.

Some links:

• https://lwn.net/Articles/531114/ - namespaces overview series