Simple Go-based setuid+setgid+setgroups+exec
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README.md

gosu

Build Status

This is a simple tool grown out of the simple fact that su and sudo have very strange and often annoying TTY and signal-forwarding behavior. They're also somewhat complex to setup and use (especially in the case of sudo), which allows for a great deal of expressivity, but falls flat if all you need is "run this specific application as this specific user and get out of the pipeline".

The core of how gosu works is stolen directly from how Docker/libcontainer itself starts an application inside a container (and in fact, is using the /etc/passwd processing code directly from libcontainer's codebase).

$ gosu
Usage: ./gosu user-spec command [args]
   ie: ./gosu tianon bash
       ./gosu nobody:root bash -c 'whoami && id'
       ./gosu 1000:1 id

./gosu version: 1.1 (go1.3.1 on linux/amd64; gc)

Once the user/group is processed, we switch to that user, then we exec the specified process and gosu itself is no longer resident or involved in the process lifecycle at all. This avoids all the issues of signal passing and TTY, and punts them to the process invoking gosu and the process being invoked by gosu, where they belong.

Warning

The core use case for gosu is to step down from root to a non-privileged user during container startup (specifically in the ENTRYPOINT, usually).

Uses of gosu beyond that could very well suffer from vulnerabilities such as CVE-2016-2779 (from which the Docker use case naturally shields us); see tianon/gosu#37 for some discussion around this point.

Installation

High-level steps:

  1. download gosu-$(dpkg --print-architecture | awk -F- '{ print $NF }') as gosu
  2. download gosu-$(dpkg --print-architecture | awk -F- '{ print $NF }').asc as gosu.asc
  3. fetch my public key (to verify your download): gpg --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4
  4. gpg --batch --verify gosu.asc gosu
  5. chmod +x gosu

For explicit Dockerfile instructions, see INSTALL.md.

Why?

$ docker run -it --rm ubuntu:trusty su -c 'exec ps aux'
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  46636  2688 ?        Ss+  02:22   0:00 su -c exec ps a
root         6  0.0  0.0  15576  2220 ?        Rs   02:22   0:00 ps aux
$ docker run -it --rm ubuntu:trusty sudo ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  3.0  0.0  46020  3144 ?        Ss+  02:22   0:00 sudo ps aux
root         7  0.0  0.0  15576  2172 ?        R+   02:22   0:00 ps aux
$ docker run -it --rm -v $PWD/gosu-amd64:/usr/local/bin/gosu:ro ubuntu:trusty gosu root ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0   7140   768 ?        Rs+  02:22   0:00 ps aux

Additionally, due to the fact that gosu is using Docker's own code for processing these user:group, it has exact 1:1 parity with Docker's own --user flag.

If you're curious about the edge cases that gosu handles, see Dockerfile.test for the "test suite" (and the associated test.sh script that wraps this up for testing arbitrary binaries).

Alternatives

su-exec

As mentioned in INSTALL.md, su-exec is a very minimal re-write of gosu in C, making for a much smaller binary, and is available in the main Alpine package repository.

chroot

With the --userspec flag, chroot can provide similar benefits/behavior:

$ docker run -it --rm ubuntu:trusty chroot --userspec=nobody / ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
nobody       1  5.0  0.0   7136   756 ?        Rs+  17:04   0:00 ps aux

Others

I'm not terribly familiar with them, but a few other alternatives I'm aware of include:

  • chpst (part of runit)