github.com/containers/psgo

psgo

A ps(1) AIX-format compatible golang library extended with various descriptors useful for displaying container-related data.

The idea behind the library is to provide an easy to use way of extracting process-related data, just as ps(1) does. The problem when using ps(1) is that the ps format strings split columns with whitespaces, making the output nearly impossible to parse. It also adds some jitter as we have to fork and execute ps either in the container or filter the output afterwards, further limiting applicability.

This library aims to make things a bit more comfortable, especially for container runtimes, as the API allows to join the mount namespace of a given process and will parse /proc and /dev/ from there. The API consists of the following functions:

• psgo.ProcessInfo(descriptors []string) ([][]string, error)

  • ProcessInfo returns the process information of all processes in the current mount namespace. The input descriptors must be a slice of supported AIX format descriptors in the normal form or in the code form, if supported. If the input descriptor slice is empty, the psgo.DefaultDescriptors are used. The return value contains the string slice of process data, one per process.

• psgo.ProcessInfoByPids(pids []string, descriptors []string) ([][]string, error)

  • ProcessInfoByPids is similar to psgo.ProcessInfo, but limits the return value to a list of specified pids. The pids input must be a slice of PIDs for which process information should be returned. If the input descriptor slice is empty, only the format descriptor headers are returned.

• psgo.JoinNamespaceAndProcessInfo(pid string, descriptors []string) ([][]string, error)

  • JoinNamespaceAndProcessInfo has the same semantics as ProcessInfo but joins the mount namespace of the specified pid before extracting data from /proc. This way, we can extract the /proc data from a container without executing any command inside the container.

• psgo.JoinNamespaceAndProcessInfoByPids(pids []string, descriptors []string) ([][]string, error)

  • JoinNamespaceAndProcessInfoByPids is similar to psgo.JoinNamespaceAndProcessInfo but takes a slice of pids as an argument. To avoid duplicate entries (e.g., when two or more containers share the same PID namespace), a given PID namespace will be joined only once.

• psgo.ListDescriptors() []string

  • ListDescriptors returns a sorted string slice of all supported AIX format descriptors in the normal form (e.g., "args,comm,user"). It can be useful in the context of bash-completion, help messages, etc.

Listing processes

We can use the psgo sample tool from this project to test the core components of this library. First, let's build psgo via make build. The binary is now located under ./bin/psgo. By default psgo displays data about all running processes in the current mount namespace, similar to the output of ps -ef.

$ ./bin/psgo | head -n5
USER         PID     PPID    %CPU     ELAPSED              TTY      TIME        COMMAND
root         1       0       0.064    6h3m27.677997443s    ?        13.98s      systemd
root         2       0       0.000    6h3m27.678380128s    ?        20ms        [kthreadd]
root         4       2       0.000    6h3m27.678701852s    ?        0s          [kworker/0:0H]
root         6       2       0.000    6h3m27.678999508s    ?        0s          [mm_percpu_wq]

Listing processes

You can use the --pids flag to restrict psgo output to a subset of processes. This option accepts a list of comma separate process IDs and will return exactly the same kind of information per process as the default output.

$ ./bin/psgo --pids 1,$(pgrep bash | tr "\n" ",")
USER   PID     PPID    %CPU    ELAPSED                TTY     TIME   COMMAND
root   1       0       0.009   128h52m44.193475932s   ?       40s    systemd
root   20830   20827   0.000   105h2m44.19579679s     pts/5   0s     bash
root   25843   25840   0.000   102h56m4.196072027s    pts/6   0s     bash

Listing processes within a container

Let's have a look at how we can use this library in the context of containers. As a simple show case, we'll start a Docker container, extract the process ID via docker-inspect and run the psgo binary to extract the data of running processes within that container.

$ docker run -d alpine sleep 100
473c9a05d4223b88ef7f5a9ac11e3d21e9914e012338425cc1cef853fc6c32a2

$ docker inspect --format '{{.State.Pid}}' 473c9
5572

$ sudo ./bin/psgo -pids 5572 -join
USER   PID   PPID   %CPU    ELAPSED         TTY   TIME   COMMAND
root   1     0      0.000   17.249905587s   ?     0s     sleep

Format descriptors

The ps library is compatible with all AIX format descriptors of the ps command-line utility (see man 1 ps for details) but it also supports some additional descriptors that can be useful when seeking specific process-related information.

• capamb
  • Set of ambient capabilities. See capabilities(7) for more information.
• capbnd
  • Set of bounding capabilities. See capabilities(7) for more information.
• capeff
  • Set of effective capabilities. See capabilities(7) for more information.
• capinh
  • Set of inheritable capabilities. See capabilities(7) for more information.
• capprm
  • Set of permitted capabilities. See capabilities(7) for more information.
• groups
  • Supplmentary groups inside the container.
• hgroup
  • The corresponding effective group of a container process on the host.
• hgroups
  • Supplmentary groups on the host.
• hpid
  • The corresponding host PID of a container process.
• huser
  • The corresponding effective user of a container process on the host.
• huid
  • The corresponding host UID of a container process.
• label
  • Current security attributes of the process.
• seccomp
  • Seccomp mode of the process (i.e., disabled, strict or filter). See seccomp(2) for more information.
• state
  • Process state codes (e.g, R for running, S for sleeping). See proc(5) for more information.
• stime
  • Process start time (e.g, "2019-12-09 10:50:36 +0100 CET).

We can try out different format descriptors with the psgo binary:

$ ./bin/psgo -format "pid, user, group, seccomp" | head -n5
PID     USER         GROUP        SECCOMP
1       root         root         disabled
2       root         root         disabled
4       root         root         disabled
6       root         root         disabled