Chapter 12. Linux Binary Compatibility

Table of Contents

12.1. Synopsis

FreeBSD provides optional binary compatibility with Linux®, commonly referred to as Linuxulator, allowing users to install and run unmodified Linux binaries. It is available for the x86 (both 32 and 64 bit) and AArch64 architectures. Some Linux-specific operating system features are not yet supported; this mostly happens with functionality specific to hardware or related to system management, such as cgroups or namespaces.

Before reading this chapter, you should:

After reading this chapter, you will know:

  • How to enable Linux binary compatibility on a FreeBSD system.

  • How to install additional Linux shared libraries.

  • How to install Linux applications on a FreeBSD system.

  • The implementation details of Linux compatibility in FreeBSD.

12.2. Configuring Linux Binary Compatibility

By default, linux(4) binary compatibility is not enabled.

To enable the Linux ABI at boot time, execute the following command:

# sysrc linux_enable="YES"

Once enabled, it can be started without rebooting executing the following command:

# service linux start

This is enough for statically linked Linux binaries to work.

The Linux service will load necessary kernel modules and mount filesystems expected by Linux applications under /compat/linux. They can be started in the same way native FreeBSD binaries can; they behave almost exactly like native processes and can be traced and debugged the usual way.

The current content of /compat/linux can be checked executing the following command:

# ls -l /compat/linux/

The output should be similar to the following:

total 1
dr-xr-xr-x  13 root  wheel  512 Apr 11 19:12 dev
dr-xr-xr-x   1 root  wheel    0 Apr 11 21:03 proc
dr-xr-xr-x   1 root  wheel    0 Apr 11 21:03 sys

12.3. Linux userlands

Linux software requires more than just an ABI to work. In order to run Linux software a Linux userland must be installed first.

If all that is wanted is to run some software already included in the Ports tree, it can be installed via package manager and pkg(8) will automatically setup the required Linux userland.

For example, to install Sublime Text 4, along with all the Linux libraries it depends on, run this command:

# pkg install linux-sublime-text4

12.3.1. CentOS Base System from FreeBSD Packages

To install the CentOS userland execute the following command:

# pkg install linux_base-c7

emulators/linux_base-c7 will place the base system derived from CentOS 7 into /compat/linux.

After installing the package, the contents of /compat/linux can be verified by running the following command to check that the CentOS userland has been installed:

# ls -l /compat/linux/

The output should be similar to the following:

total 30
lrwxr-xr-x   1 root  wheel    7 Apr 11  2018 bin -> usr/bin
drwxr-xr-x  13 root  wheel  512 Apr 11 21:10 dev
drwxr-xr-x  25 root  wheel   64 Apr 11 21:10 etc
lrwxr-xr-x   1 root  wheel    7 Apr 11  2018 lib -> usr/lib
lrwxr-xr-x   1 root  wheel    9 Apr 11  2018 lib64 -> usr/lib64
drwxr-xr-x   2 root  wheel    2 Apr 11 21:10 opt
dr-xr-xr-x   1 root  wheel    0 Apr 11 21:25 proc
lrwxr-xr-x   1 root  wheel    8 Feb 18 02:10 run -> /var/run
lrwxr-xr-x   1 root  wheel    8 Apr 11  2018 sbin -> usr/sbin
drwxr-xr-x   2 root  wheel    2 Apr 11 21:10 srv
dr-xr-xr-x   1 root  wheel    0 Apr 11 21:25 sys
drwxr-xr-x   8 root  wheel    9 Apr 11 21:10 usr
drwxr-xr-x  16 root  wheel   17 Apr 11 21:10 var

12.3.2. Debian / Ubuntu Base System with debootstrap

An alternative way of providing Linux shared libraries is by using sysutils/debootstrap. This has the advantage of providing a full Debian or Ubuntu distribution.

To install debootstrap execute the following command:

# pkg install debootstrap

debootstrap(8) needs linux(4) ABI enabled. Once enabled, execute the following command to install Ubuntu or Debian in /compat/ubuntu:

# debootstrap focal /compat/ubuntu

While it is technically possible to install into /compat/linux instead, it’s discouraged due to possible clashes with CentOS-based packages. Instead, derive the directory name from the distribution or version name, e.g., /compat/ubuntu.

The output should be similar to the following:

I: Retrieving InRelease
I: Checking Release signature
I: Valid Release signature (key id F6ECB3762474EDA9D21B7022871920D1991BC93C)
I: Retrieving Packages
I: Validating Packages
I: Resolving dependencies of required packages...
I: Resolving dependencies of base packages...
I: Checking component main on http://archive.ubuntu.com/ubuntu...
[...]
I: Configuring console-setup...
I: Configuring kbd...
I: Configuring ubuntu-minimal...
I: Configuring libc-bin...
I: Configuring ca-certificates...
I: Base system installed successfully.

Then set up mounts in /etc/fstab.

If the contents of the home directory should be shared and to be able to run X11 applications, /home and /tmp should be mounted in the linux compat area using nullfs(5) for loopback.

The following example can be added to /etc/fstab:

# Device        Mountpoint              FStype          Options                      Dump    Pass#
devfs           /compat/ubuntu/dev      devfs           rw,late                      0       0
tmpfs           /compat/ubuntu/dev/shm  tmpfs           rw,late,size=1g,mode=1777    0       0
fdescfs         /compat/ubuntu/dev/fd   fdescfs         rw,late,linrdlnk             0       0
linprocfs       /compat/ubuntu/proc     linprocfs       rw,late                      0       0
linsysfs        /compat/ubuntu/sys      linsysfs        rw,late                      0       0
/tmp            /compat/ubuntu/tmp      nullfs          rw,late                      0       0
/home           /compat/ubuntu/home     nullfs          rw,late                      0       0

Then execute mount(8):

# mount -al

To access the system using chroot(8) execute the following command:

# chroot /compat/ubuntu /bin/bash

Then uname(1) can be executed to check the Linux environment:

# uname -s -r -m

The output should be similar to the following:

Linux 3.17.0 x86_64

Once inside the chroot, the system behaves as in a normal Ubuntu installation While systemd doesn’t work, the service(8) command works as usual.

To add the package repositories missing from defaults edit the file /compat/ubuntu/etc/apt/sources.list.

For amd64 the following example can be used:

deb http://archive.ubuntu.com/ubuntu focal main universe restricted multiverse
deb http://security.ubuntu.com/ubuntu/ focal-security universe multiverse restricted main
deb http://archive.ubuntu.com/ubuntu focal-backports universe multiverse restricted main
deb http://archive.ubuntu.com/ubuntu focal-updates universe multiverse restricted main

For arm64 this other example can be used:

deb http://ports.ubuntu.com/ubuntu-ports bionic main universe restricted multiverse

12.4. Advanced Topics

A list of all Linux-related sysctl(8) knobs can be found in linux(4).

Some applications require specific filesystems to be mounted.

This is normally handled by the /etc/rc.d/linux script but can be disabled at boot executing the following command:

sysrc linux_mounts_enable="NO"

Filesystems mounted by the rc script will not work for Linux processes inside chroots or jails; if needed, configure them in /etc/fstab:

devfs      /compat/linux/dev      devfs      rw,late                    0  0
tmpfs      /compat/linux/dev/shm  tmpfs      rw,late,size=1g,mode=1777  0  0
fdescfs    /compat/linux/dev/fd   fdescfs    rw,late,linrdlnk           0  0
linprocfs  /compat/linux/proc     linprocfs  rw,late                    0  0
linsysfs   /compat/linux/sys      linsysfs   rw,late                    0  0

Since the Linux binary compatibility layer has gained support for running both 32- and 64-bit Linux binaries, it is no longer possible to link the emulation functionality statically into a custom kernel.

12.4.1. Installing Additional Libraries Manually

For base system subdirectories created with debootstrap(8), use the instructions above instead.

If a Linux application complains about missing shared libraries after configuring Linux binary compatibility, determine which shared libraries the Linux binary needs and install them manually.

From a Linux system using the same CPU architecture, ldd can be used to determine which shared libraries the application needs.

For example, to check which shared libraries linuxdoom needs, run this command from a Linux system that has Doom installed:

% ldd linuxdoom

The output should be similar to the following:

libXt.so.3 (DLL Jump 3.1) => /usr/X11/lib/libXt.so.3.1.0
libX11.so.3 (DLL Jump 3.1) => /usr/X11/lib/libX11.so.3.1.0
libc.so.4 (DLL Jump 4.5pl26) => /lib/libc.so.4.6.29

Then, copy all the files in the last column of the output from the Linux system into /compat/linux on the FreeBSD system. Once copied, create symbolic links to the names in the first column.

This example will result in the following files on the FreeBSD system:

/compat/linux/usr/X11/lib/libXt.so.3.1.0
/compat/linux/usr/X11/lib/libXt.so.3 -> libXt.so.3.1.0
/compat/linux/usr/X11/lib/libX11.so.3.1.0
/compat/linux/usr/X11/lib/libX11.so.3 -> libX11.so.3.1.0
/compat/linux/lib/libc.so.4.6.29
/compat/linux/lib/libc.so.4 -> libc.so.4.6.29

If a Linux shared library already exists with a matching major revision number to the first column of the ldd output, it does not need to be copied to the file named in the last column, as the existing library should work. It is advisable to copy the shared library if it is a newer version, though. The old one can be removed, as long as the symbolic link points to the new one.

For example, these libraries already exist on the FreeBSD system:

/compat/linux/lib/libc.so.4.6.27
/compat/linux/lib/libc.so.4 -> libc.so.4.6.27

and ldd indicates that a binary requires a later version:

libc.so.4 (DLL Jump 4.5pl26) -> libc.so.4.6.29

Since the existing library is only one or two versions out of date in the last digit, the program should still work with the slightly older version. However, it is safe to replace the existing libc.so with the newer version:

/compat/linux/lib/libc.so.4.6.29
/compat/linux/lib/libc.so.4 -> libc.so.4.6.29

Generally, one will need to look for the shared libraries that Linux binaries depend on only the first few times that a Linux program is installed on FreeBSD. After a while, there will be a sufficient set of Linux shared libraries on the system to be able to run newly installed Linux binaries without any extra work.

12.4.2. Branding Linux ELF Binaries

The FreeBSD kernel uses several methods to determine if the binary to be executed is a Linux one: it checks the brand in the ELF file header, looks for known ELF interpreter paths and checks ELF notes; finally, by default, unbranded ELF executables are assumed to be Linux anyway.

Should all those methods fail, an attempt to execute the binary might result in error message:

% ./my-linux-elf-binary

The output should be similar to the following:

ELF binary type not known
Abort

To help the FreeBSD kernel distinguish between a FreeBSD ELF binary and a Linux binary, use brandelf(1):

% brandelf -t Linux my-linux-elf-binary

12.4.3. Installing a Linux RPM Based Application

To install a Linux RPM-based application, first install the archivers/rpm4 package or port. Once installed, root can use this command to install a .rpm:

# cd /compat/linux
# rpm2cpio < /path/to/linux.archive.rpm | cpio -id

If necessary, brandelf the installed ELF binaries. Note that this will prevent a clean uninstall.

12.4.4. Configuring the Hostname Resolver

If DNS does not work or this error appears:

resolv+: "bind" is an invalid keyword resolv+:
"hosts" is an invalid keyword

configure /compat/linux/etc/host.conf as follows:

order hosts, bind
multi on

This specifies that /etc/hosts is searched first and DNS is searched second. When /compat/linux/etc/host.conf does not exist, Linux applications use /etc/host.conf in the host system but they complain since that file does not exist in FreeBSD. Remove bind if a name server is not configured using /etc/resolv.conf.

12.4.5. Miscellaneous

More information on how binary compatibility works with Linux® can be found in the article Linux emulation in FreeBSD.

Last modified on: July 6, 2023 by Sergio Carlavilla Delgado

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