ffmpeg chroot jail from Gentoo linux (ffmpeg 4.1 with gcc 8.2)

Here we build a Gentoo ssh chroot jail. You may need this because

  • you want the bleeding edge versions of the video libraries like x264, x265, ffmpeg the latest version and so on.
  • latest version of GNU C library and the compiler GNU GCC (version 8.2) at the moment
  • use all the optimizations for compiling the ffmpeg and the libraries for you current processor. Use “-march=native”
  • you want just to be able to update your system and not to break your carefully compiled latest version ffmpeg
  • experiment with GCC compile flags
  • update and test in another directory by just copying the old chroot jail directory
  • no need of any additional software on the host (like virtualization lxc, docker, qemu, etc)
  • or you might want a 32bit version (why? really? but it is possible…)

Of course, you can use for the base any Linux system, but it is easy with Gentoo – the latest versions of almost all important libraries and a packet system, which builds everything for you after a little bit of first tuning…

So here is how to do it:
Keep on reading!

tmpfs mount on /dev/shm in LXC container or chroot environment

When using LXC containers booting the lxc container would not populate it as the normal boot process. Or when you create a chroot jail /dev is not mounted or just some devices are created.
There is an option to populate (when using LXC containers) it with minimal required devices:

lxc.autodev = 1

which will create a tmpfs mount under /dev and create some basic devices, it will ensure /dev/shm to be mounted on with tmpfs!
If you omit this option, the /dev directory won’t be populated and will stay with the devices you made or copied when you made the LXC container (or the chroot jail) and /dev/shm will not be mounted using tmps, which could create numerous bad issues.
If you get errors like

 * configure has detected that the sem_open function is broken.
 * Please ensure that /dev/shm is mounted as a tmpfs with mode 1777.

You could mount the /dev/shm of the LXC container or the chroot jail (usually you can tune the size half of the server’s RAM) with

mkdir -p /dev/shm
mount -t tmpfs -o nodev,nosuid,noexec,mode=1777,size=6144m tmpfs /dev/shm

Or reboot your LXC container with a new configuration (probably in the “/var/lxc/[lxc_name]/config”) adding the following line:

lxc.mount.entry = none dev/shm tmpfs nodev,nosuid,noexec,mode=1777,create=dir 0 0

Thus you ensure the /dev/shm to be mounted on tmpfs and all semaphore functions to work properly.

* Real output of Gentoo failed compilation of python package:
 * configure has detected that the sem_open function is broken.
 * Please ensure that /dev/shm is mounted as a tmpfs with mode 1777.
 * ERROR: dev-lang/python-3.3.4-r1::gentoo failed (configure phase):
 *   Broken sem_open function (bug 496328)
 * Call stack:
 *     ebuild.sh, line 124:  Called src_configure
 *   environment, line 3542:  Called die
 * The specific snippet of code:
 *           die "Broken sem_open function (bug 496328)";
 * If you need support, post the output of `emerge --info '=dev-lang/python-3.3.4-r1::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=dev-lang/python-3.3.4-r1::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/dev-lang/python-3.3.4-r1/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/dev-lang/python-3.3.4-r1/temp/environment'.
 * Working directory: '/var/tmp/portage/dev-lang/python-3.3.4-r1/work/x86_64-pc-linux-gnu'
 * S: '/var/tmp/portage/dev-lang/python-3.3.4-r1/work/Python-3.3.4'

>>> Failed to emerge dev-lang/python-3.3.4-r1, Log file: