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I have a docker-compose file that I'm trying to secure by making the root volumes of the containers it creates read-only.

Relevant parts of docker-compose.yml:

version: '2' services: mysql: image: mariadb:10.1 read_only: true tmpfs: - /var/run/mysqld:uid=999,gid=999 - /tmp volumes: - mysql:/var/lib/mysql restart: always volumes: mysql:

Trouble is, the tmpfs isn't being created. If I run an instance of the container using docker-compose run --rm mysql /bin/bash, the /var/run/mysqld directory is still read-only despite the tmpfs entry, and any attempt to touch /var/run/mysqld/foo fails. Since this is where MySQL puts its socket and pid file, this causes the whole thing to fail. I'm not sure why the tmpfs entry isn't working in this case. 

mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] mysqld (mysqld 10.1.21-MariaDB-1~jessie) starting as process 1 ... mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Using mutexes to ref count buffer pool pages mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: The InnoDB memory heap is disabled mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Mutexes and rw_locks use GCC atomic builtins mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: GCC builtin __atomic_thread_fence() is used for memory barrier mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Compressed tables use zlib 1.2.8 mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Using Linux native AIO mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Using SSE crc32 instructions mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Initializing buffer pool, size = 256.0M mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Completed initialization of buffer pool mysql_1 | 2017-01-27 20:53:45 140515784030144 [Note] InnoDB: Highest supported file format is Barracuda. mysql_1 | 2017-01-27 20:53:48 140515784030144 [Note] InnoDB: 128 rollback segment(s) are active. mysql_1 | 2017-01-27 20:53:48 140515784030144 [Note] InnoDB: Waiting for purge to start mysql_1 | 2017-01-27 20:53:48 140515784030144 [Note] InnoDB: Percona XtraDB (http://www.percona.com) 5.6.34-79.1 started; log sequence number 239403989 mysql_1 | 2017-01-27 20:53:48 140515005662976 [Note] InnoDB: Dumping buffer pool(s) not yet started mysql_1 | 2017-01-27 20:53:48 140515784030144 [Note] Plugin 'FEEDBACK' is disabled. mysql_1 | 2017-01-27 20:53:49 140515784030144 [Note] Server socket created on IP: '::'. mysql_1 | 2017-01-27 20:53:49 140515784030144 [ERROR] Can't start server : Bind on unix socket: Read-only file system mysql_1 | 2017-01-27 20:53:49 140515784030144 [ERROR] Do you already have another mysqld server running on socket: /var/run/mysqld/mysqld.sock ? mysql_1 | 2017-01-27 20:53:49 140515784030144 [ERROR] Aborting

I can verify the permissions on the directory are correct (and that the UID of the mysql user is 999):

$ ls -la /var/run/mysqld total 8 drwxrwxrwx 2 mysql mysql 4096 Jan 17 22:14 . drwxr-xr-x 4 root root 4096 Jan 18 22:55 ..

But I still cannot:

$ touch /var/run/mysqld/foo touch: cannot touch '/var/run/mysqld/foo': Read-only file system

Even if I run as root.

Any ideas what I'm doing wrong?

As an aside, the /tmp filesystem works fine.

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3
  • 1
    Are you using docker swarm? docs.docker.com/compose/compose-file/#tmpfs says "Note: This option is ignored when deploying a stack in swarm mode with a (version 3) Compose file." Commented Mar 7, 2017 at 22:55
  • Nope. Straight up docker-compose. Commented Mar 10, 2017 at 18:40
  • Me too. (filler) Commented Jul 17, 2017 at 11:38

2 Answers 2

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59

I have been doing some testing in this regards, it looks like the /var/run directory is special in docker.

Here is some sample config and output:

 ubuntu: image: ubuntu command: "bash -c 'mount'" tmpfs: - /var/run - /var/cache

Running docker-compose up ubuntu shows what is mounted. Can see /var/cache is mounted but /var/run isn't.

... ubuntu_1 | tmpfs on /var/cache type tmpfs (rw,nosuid,nodev,noexec,relatime) ...

If you use docker-compose run ubuntu bash you can see it's also mounted there but not /var/run.

The reason is that /var/run is normally a symlink to /run and hence you creating /var/run/mysql as a tmpfs doesn't work.

It will work if you change it to /run/mysql, but /run is normally mounted as tmpfs anyway so you might as well just make /run a tmpfs. Like so:

 ubuntu: image: ubuntu command: "bash -c 'mount'" tmpfs: - /run - /var/cache

Note: I'd like to amend my answer and show the way to do it using volumes:

services: ubuntu: image: ubuntu command: "bash -c 'mount'" volumes: - cache_vol:/var/cache - run_vol:/run volumes: run_vol: driver_opts: type: tmpfs device: tmpfs cache_vol: driver_opts: type: tmpfs device: tmpfs

This also allows you to share the tmpfs mounts if needed.

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6 Comments

According to documentation tmpfs mounts cannot be shared among containers: docs.docker.com/engine/admin/volumes/tmpfs/…
Yes the tmpfs mount cannot, but above is a volume that is using the tmpfs type. Which is not a tmpfs mount but a volume mount using the tmpfs device. Which in theory can be shared like any other volume.
Has Docker's behaviour changed recently in this regard, as I used be able to tmpfs mount /var/run/ and this has only stopped working today during a reboot on a frequently updated CoreOS machine.
Bardi it's probably because /var/run is a symlink
@dalore Tmpfs's lifespan is normally 1:1 with the container's lifespan. If multiple containers share a volume using the tmpfs type and device, then what becomes of the lifecycle of the data in the volume?
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In some images like alpine, the directory /var/run is just a link to /run. You can verify using:

$ docker run --rm -ti mariadb:10.1 ls -lh /var/run lrwxrwxrwx 1 root root 4 Aug 7 13:02 /var/run -> /run

This means that /var/run/mysqld is actually /run/mysqld.

Your updated docker-compose.yml is:

version: '2' services: mysql: image: mariadb:10.1 read_only: true tmpfs: - /run/mysqld:uid=999,gid=999 - /tmp volumes: - mysql:/var/lib/mysql restart: always volumes: mysql:

In that case, just make your tmpfs point to /run.

it looks like the /var/run directory is special in docker.

No, it's just because it's a link.

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1 Comment

Thanks, :uid=999,gid=999 fixed my issue with non-writable directory.

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