1

I'm wondering if devtmpfs is special with respect to namespaces.

Here's my system info (using Vagrant for testing)

vagrant@ubuntu-xenial:~/test$ uname -a
Linux ubuntu-xenial 4.4.0-135-generic #161-Ubuntu SMP Mon Aug 27 10:45:01 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

A basic demonstration explains best:

vagrant@ubuntu-xenial:~/test$ unshare --ipc --uts --user --mount --fork --pid --net --map-root-user
root@ubuntu-xenial:~/test# mkdir proc
root@ubuntu-xenial:~/test# mkdir sys
root@ubuntu-xenial:~/test# mount -t proc proc ./proc/
root@ubuntu-xenial:~/test# mount -t sysfs sysfs ./sys/
root@ubuntu-xenial:~/test# mkdir dev
root@ubuntu-xenial:~/test# mount -t devtmpfs devtmpfs ./dev/
mount: permission denied
root@ubuntu-xenial:~/test# exit
logout

I have the rights to mount proc and sys, but not a devtmpfs?

A tmpfs works, though (also run with the same unshare command)

root@ubuntu-xenial:~/test# mount -t tmpfs tmpfs ./dev/

EDIT2: Moved previous edit to an answer based on sourcejedi's feedback.

2

I believe it is related to the file system flag FS_USERNS_MOUNT (https://lwn.net/Articles/652468/).

A quick grep of the kernel seems to indicate it's not set for devtmpfs: livegrep.com results

If someone has a better answer I'd be happy to accept it instead!

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