Can Not Add New User in Docker Container with Mounted /Etc/Passwd and /Etc/Shadow

Can not add new user in docker container with mounted /etc/passwd and /etc/shadow

It's failing because passwd manipulates a temporary file, and then attempts to rename it to /etc/shadow. This fails because /etc/shadow is a mountpoint -- which cannot be replaced -- which results in this error (captured using strace):

102   rename("/etc/nshadow", "/etc/shadow") = -1 EBUSY (Device or resource busy)

You can reproduce this trivially from the command line:

# cd /etc
# touch foo
# mv foo shadow
mv: cannot move 'foo' to 'shadow': Device or resource busy

You could work around this by mounting a directory containing my_shadow and my_passwd somewhere else, and then symlinking /etc/passwd and /etc/shadow in the container appropriately:

$ docker run -it --rm -v $PWD/my_etc:/my_etc centos
[root@afbc739f588c /]# ln -sf /my_etc/my_passwd /etc/passwd
[root@afbc739f588c /]# ln -sf /my_etc/my_shadow /etc/shadow
[root@afbc739f588c /]# ls -l /etc/{shadow,passwd}
lrwxrwxrwx. 1 root root 17 Oct  8 17:48 /etc/passwd -> /my_etc/my_passwd
lrwxrwxrwx. 1 root root 17 Oct  8 17:48 /etc/shadow -> /my_etc/my_shadow
[root@afbc739f588c /]# passwd root
Changing password for user root.
New password: 
Retype new password: 
passwd: all authentication tokens updated successfully.
[root@afbc739f588c /]# 

How do I add the local users to my docker container?

You would have to mount all relevant linux files using -v like /etc/passwd, /etc/shadow, /ect/group, and /etc/sudoers. Though I can't recommend this due to the security risks, if anyone gets root access in the container they can add users on the host or change passwords since he mount works both ways.

The list of files is not exhaustive, for example, you have to also make sure the shell exacutables exist within the container. When testing this I had to make a symbolic link from /usr/bin/zsh to /bin/bash for example since my user has the zsh shell configured which was not present in the docker image.

If you want to use these users to interact with mounted files, you also have to make sure that user namespace remapping is disabled, or specify that you want to use the same user namespace as the host with the --userns=host flag. Again, not recommended since it is a security feature, so use with care.

Note: Once you have done all this you can use su - {username} to switch to all your existing users. The -u options doesn't work since docker checks the /etc/passwd file before mounting and will give an error.

Create a user on the docker host from inside a container

When you mount an individual file, you end up mounting the inode of that file with the bind mount. And when you write to the file, many tools create a new file, with a new inode, and replace the existing file with that. This avoids partial reads, and other file corruption risks if you were to modify the file in place.

What you are attempting to do is likely a very bad idea, it's the very definition of a container escape, allowing the container to setup credentials on the host. If you really need host access, I'd mount the folder in a different location because containers have other files that are automatically mounted in /etc. So you could say /etc:/host/etc and access the files in the container under /host/etc. Just realize that's even a larger security hole.

Note, if the entire goal is to avoid permission issues between the host and the container, there are much better ways to do this, but that would be an X-Y problem.

Allow Docker Container & Host User To Write on Bind Mounted Host Directory

Problem: if I set "ubuntu" as owner, container can't write (using php to write), if I set "nobody" as owner, VSCode SSH can't write. I am finding a way to allow both to write without changing directory owner user again and again, or similar ease.

First, I'd recommend the container image should create a new username for the files inside the container, rather than reusing nobody since that user may also be used for other OS tasks that shouldn't have any special access.

Next, as Triet suggests, an entrypoint that adjusts the container's user/group to match the volume is preferred. My own version of these scripts can be found in this base image that includes a fix-perms script that makes the user id and group id of the container user match the id's of a mounted volume. In particular, the following lines of that script where $opt_u is the container username, $opt_g is the container group name, and $1 is the volume mount location:

# update the uid
if [ -n "$opt_u" ]; then
  OLD_UID=$(getent passwd "${opt_u}" | cut -f3 -d:)
  NEW_UID=$(stat -c "%u" "$1")
  if [ "$OLD_UID" != "$NEW_UID" ]; then
    echo "Changing UID of $opt_u from $OLD_UID to $NEW_UID"
    usermod -u "$NEW_UID" -o "$opt_u"
    if [ -n "$opt_r" ]; then
      find / -xdev -user "$OLD_UID" -exec chown -h "$opt_u" {} \;
    fi
  fi
fi

# update the gid
if [ -n "$opt_g" ]; then
  OLD_GID=$(getent group "${opt_g}" | cut -f3 -d:)
  NEW_GID=$(stat -c "%g" "$1")
  if [ "$OLD_GID" != "$NEW_GID" ]; then
    echo "Changing GID of $opt_g from $OLD_GID to $NEW_GID"
    groupmod -g "$NEW_GID" -o "$opt_g"
    if [ -n "$opt_r" ]; then
      find / -xdev -group "$OLD_GID" -exec chgrp -h "$opt_g" {} \;
    fi
  fi
fi

Then I start the container as root, and the container runs the fix-perms script from the entrypoint, followed by a command similar to:

exec gosu ${container_user} ${orig_command}

This replaces the entrypoint that's running as root with the application running as the specified user. I've got more examples of this in:

• DockerCon presentation
• Similar SO questions

What I tried: In Container, I added user "nobody" to group "ubuntu".
On host, directory (used as mount) was set "sudo chown -R
ubuntu:ubuntu directory", user "ubuntu" was already added to group
"ubuntu". VSCode did edit, container was unable to edit.

I'd avoid this and create a new user. Nobody is designed to be as unprivileged as possible, so there could be unintended consequences with giving it more access.

Edit: the container already created without Dockerfile also ran and
maybe edited with important changes, so maybe I can't use Dockerfile
or entrypoint.sh way to solve problem. Can It be achieved through
running commands inside container or without creating container again?
This container can be stopped.

This is a pretty big code smell in containers. They should be designed to be ephemeral. If you can't easily replace them, you're missing the ability to upgrade to a newer image, and creating a lot of state drift that you'll eventually need to cleanup. Your changes that should be preserved need to be in a volume. If there are other changes that would be lost when the container is deleted, they will be visible in docker diff and I'd recommend fixing this now rather than increasing the size of the technical debt.

Edit: I am wondering, in Triet Doan's answer, an option is to modify
UID and GID of already created user in the container, will doing this
for the user and group "nobody" can cause any problems inside
container, I am wondering because probably many commands for settings
already executed inside container, files are already edited by php on
mounted directory & container is running for days

I would build a newer image that doesn't depend on this username. Within the container, if there's data you need to preserve, it should be in a volume.

Edit: I found that alpine has no usermod & groupmod.

I use the following in the entrypoint script to install it on the fly, but the shadow package should be included in the image you build rather than doing this on the fly for every new container:

if ! type usermod >/dev/null 2>&1 || \
   ! type groupmod >/dev/null 2>&1; then
  if type apk /dev/null 2>&1; then
    echo "Warning: installing shadow, this should be included in your image"
    apk add --no-cache shadow
  else
    echo "Commands usermod and groupmod are required."
    exit 1
  fi
fi

Can't add a user with a high UID in docker Alpine

Here is a working but dirty workaround, by manually creating the user, using $UID_TO_SET as the bash variable containing the high UID to set:

# Create user
echo "user:x:$UID_TO_SET:$UID_TO_SET::/home/user:" >> /etc/passwd
## thanks for http://stackoverflow.com/a/1094354/535203 to compute the creation date
echo "user:!:$(($(date +%s) / 60 / 60 / 24)):0:99999:7:::" >> /etc/shadow
echo "user:x:$UID_TO_SET:" >> /etc/group
mkdir /home/user && chown user: /home/user

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