Unable to view filesystems other than / using Docker on Linux?

I was running a Docker container on my home machine today (Linux, x86_64). Usually Docker is trouble-free and reliable, but today I ended up navigating a deep labyrinth. Since I couldn't find any good documentation on this, and to avoid this problem in the future,  I wanted to document it.

The problem was that Docker refused to mount an external filesystem on the host as a directory on the image. I'll condense the problem to make it easy to understand. Here is the symptom:

Let's say /mnt has a filesystem that is mounted (an external disk, or a thumb drive). / is the root partition, and the only other physical partition. There are a few files on the thumb drive in /mnt/

host$ ls /mnt

fileA fileB directoryA/

The following command works file, and mounts /home/user/work-dir as /external

host$ docker run -it -v ~/work-dir:/external busybox

/external in the docker image correctly shows the contents of /home/user/work-dir, as expected.

But, if I point it to the external mount, the directory /external is empty in the docker container

host$ docker run -it -v /mnt:/external busybox

You'd expect /external in the busybox image to contain fileA, fileB and directoryA. But they don't. It gets weirder.

 If you export all of / as /external, then busybox can see everything in / except /mnt/

host$ docker run -it -v /:/external busybox

# ls /external/mnt

The output is empty, so /external/mnt/ exists, and is empty. /external/bin/ exists, and maps to /bin on the host machine, correctly, as expected.

I struggled with this for a long time. strace on docker clearly shows that it thinks /mnt doesn't exist, or is read only, even though it is definitely visible and writable in the host.

host$ date >> /mnt/out; cat /mnt/out

Sun 09 Aug 2020 04:26:16 PM PDT

So the directory structure is visible, writable, but never in a docker image. Doesn't help if you try all the commands as root. What gives?

The problem was that my docker installation was from snap. Snaps have limited visibility of the remaining system. This might be fine for a snap containing a calculator app, but docker needs a lot to function.

$ sudo snap remove docker; apt install docker-ce{,-cli}

When you do that, remember to log out because otherwise bash might have hashed the location of 'docker' to /snap/bin/docker, and it takes a logout to clear bash's state and realize that docker is now at /usr/bin/docker.

Snaps are a great idea, and I look forward to a time when they are functionally equivalent to packages installed through apt. For now, my experience with docker on snap makes me reluctant to use them. They're an additional layer of complexity to understand and debug, in a system that is already plenty complicated.

Playing flac files on Linux

It used to be that you just ran flac files through flac123, like so:
$ flac123 file.flac

That doesn't work any more, and the flac commandline tool only contains a single binary called 'flac' that decodes, encodes, etc.

Playing with 'mplayer' still works but it produces a lot of errors, and the audio level is low. The trick seems to be to call flac with the decode (-d) option, output to stdout (-c) and then pipe it to alsa player over stdin

$ flac -c -d /path/to/files/*flac | aplay

Social trackers on banks: Etrade

Etrade has a single advertiser: Wall Street on Demand.

This is much better than other financial websites, though I'm not sure what Wall Street on Demand does.

Social tracker on banks: Schwab

Charles Schwab only has trackers from Tealium, Confirmit and SOASTA mPulse.

The site works well without it, and I feel a lot safer knowing that these companies are not snooping on my online financial activity.

It is never a good sign when you have to search online for these company names. They know a lot more about you than you know about them.

Social trackers on banks: Vanguard

Vanguard is much better on their web interface. You only have trackers from Adobe Audience Manager, and Doubleclick.

There are site analytics from two companies. It is best not to send this data to third-parties like Adobe or AppDynamics, since I don't know what these companies are tracking. They almost certainly have my IP address, time of day, browser and OS version, do they have my bank account number and other details as well?

Social trackers on banks: Ally bank

There are 12 trackers on ally.com that have nothing to do with my relation with ally.

These are advertisers, trackers by Snapchat, Adobe, Bing, Pinterest, Qualtrics and Facebook.

The only legitimate tracker that deserves to be there is LivePerson, for customer service. Even that should only start when I have an actual customer support interaction rather than being aware of my page interactions all the time.

What data are these trackers collecting, and why are they required on a bank website?

(Data obtained from Ghostery, the privacy extension for Google Chrome)