cboettig cboettig - 1 month ago 11
Linux Question

Understanding user file ownership in docker: how to avoid changing permissions of linked volumes

Consider the following trivial Dockerfile:

FROM debian:testing
RUN adduser --disabled-password --gecos '' docker
RUN adduser --disabled-password --gecos '' bob


in a working directory with nothing else. Build the docker image:

docker build -t test .


and then run a bash script on the container, linking the working directory into a new subdir on bob's home directory:

docker run --rm -it -v $(pwd):/home/bob/subdir test


Who owns the contents of
subdir
on the container? On the container, run:

cd /home/bob/subdir
ls -l


ad we see:

-rw-rw-r-- 1 docker docker 120 Oct 22 03:47 Dockerfile


Holy smokes!
docker
owns the contents! Back on the host machine outside the container, we see that our original user still owns the
Dockerfile
. Let's try and fix the ownership of
bob
's home directory. On the container, run:

chown -R bob:bob /home/bob
ls -l


and we see:

-rw-rw-r-- 1 bob bob 120 Oct 22 03:47 Dockerfile


But wait! outside the container, we now run
ls -l


-rw-rw-r-- 1 1001 1001 120 Oct 21 20:47 Dockerfile


we no longer own our own file. Terrible news!




If we had only added one user in the above example, everything would have gone more smoothly. For some reason, Docker seems to be making any home directory owned by the first non-root user it encounters (even if that user is declared on an earlier image). Likewise, this first user is the one that corresponds to the same ownership permissions as my home user.

Question 1 Is that correct? Can someone point me to documentation of this, I'm just conjecturing based on the above experiment.

Question 2: Perhaps this is just because they both have the same numerical value on the kernel, and if I tested on a system where my home user was not id
1000
then permissions would get changed in every case?

Question 3: The real question is, of course, 'what do I do about this?' If
bob
is logged in as
bob
on the given host machine, he should be able to run the container as
bob
and not have file permissions altered under his host account. As it stands, he actually needs to run the container as user
docker
to avoid having his account altered.

I hear you asking Why do I have such a weird Dockerfile anyway?. I wonder too sometimes. I am writing a container for a webapp (RStudio-server) that permits different users to log in, which simply uses the user names and credentials from the linux machine as the valid user names. This brings me the perhaps unusual motivation of wanting to create multiple users. I can get around this by creating the user only at runtime and everthing is fine. However, I use a base image that has added a single
docker
user so that it can be used interactively without running as root (as per best practice). This ruins everything since that user becomes the first user and ends up owning everything, so attempts to log on as other users fail (the app cannot start because it lacks write permissions). Having the startup script run
chown
first solves this issue, but at the cost of linked volumes changing permissions (obviously only a problem if we are linking volumes).

Answer

Is that correct? Can someone point me to documentation of this, I'm just conjecturing based on the above experiment.

Perhaps this is just because they both have the same numerical value on the kernel, and if I tested on a system where my home user was not id 1000 then permissions would get changed in every case?

Have a read of info coreutils 'chown invocation', that might give you a better idea of how file permissions / ownership works.

Basically, though, each file on your machine has a set of bits tacked on to it that defines its permissions and ownership. When you chown a file, you're just setting these bits.

When you chown a file to a particular user/group using the username or group name, chown will look in /etc/passwd for the username and /etc/group for the group to attempt to map the name to an ID. If the username / group name doesn't exist in those files, chown will fail.

root@dc3070f25a13:/test# touch test
root@dc3070f25a13:/test# ll
total 8
drwxr-xr-x  2 root root 4096 Oct 22 18:15 ./
drwxr-xr-x 22 root root 4096 Oct 22 18:15 ../
-rw-r--r--  1 root root    0 Oct 22 18:15 test
root@dc3070f25a13:/test# chown test:test test
chown: invalid user: 'test:test'

However, you can chown a file using IDs to whatever you want (within some upper positive integer bounds, of course), whether there is a user / group that exists with those IDs on your machine or not.

root@dc3070f25a13:/test# chown 5000:5000 test
root@dc3070f25a13:/test# ll
total 8
drwxr-xr-x  2 root root 4096 Oct 22 18:15 ./
drwxr-xr-x 22 root root 4096 Oct 22 18:15 ../
-rw-r--r--  1 5000 5000    0 Oct 22 18:15 test

The UID and GID bits are set on the file itself, so when you mount those files inside your docker container, the file has the same owner / group UID as it does on the host, but is now mapped to /etc/passwd in the container, which is probably going to be a different user unless it's owned by root (UID 0).

The real question is, of course, 'what do I do about this?' If bob is logged in as bob on the given host machine, he should be able to run the container as bob and not have file permissions altered under his host account. As it stands, he actually needs to run the container as user docker to avoid having his account altered.

It seems like, with your current set-up, you'll need to make sure your UIDs > usernames in /etc/passwd on your host match up to your UIDs > usernames in your containers /etc/passwd if you want to interact with your mounted user directory as the same user that's logged in on the host.

You can create a user with a specific user id with useradd -u xxxx. Buuuut, that does seem like a messy solution...

You might have to come up with a solution that doesn't mount a host users home directory.