LINUX(4)

NAME
linux - Linux ABI support

SYNOPSIS
To enable the Linux ABI at boot time, place the following line in rc.conf(5):

    linux_enable="YES"

DESCRIPTION
The linux kernel module provides limited Linux ABI (application binary interface) compatibility, making it possible to run many unmodified Linux applications without the need for virtualization or emulation. Some of the facilities provided are:

•   Linux to native system call translation

•   Linux-specific system calls

•   Special signal handling for Linux processes

•   Path translation mechanism

•   Linux-specific virtual file systems

The path translation mechanism makes Linux processes look up file paths under emul_path (defaulting to /compat/linux) before /. For example, when a Linux process attempts to open /etc/passwd, it will first access /compat/linux/etc/passwd, falling back to /etc/passwd if the compat path does not exist. This is used to make sure Linux processes load Linux shared libraries instead of their similarly-named FreeBSD counterparts, and also to provide alternative versions of certain other files and virtual file systems.

To install Linux shared libraries and system files into /compat/linux, either use the emulators/linux_base-c7 port or package, or debootstrap(8) installed from sysutils/debootstrap.

To avoid mounting Linux-specific filesystems at startup, add the following line to the rc.conf(5) file:

    linux_mounts_enable="NO"

SYSCTL VARIABLES
The following variables are available as both sysctl(8) variables and loader(8) tunables:

compat.linux.debug
    Enable debugging messages. Set to 0 to silence them. Defaults to 3. A setting of 1 prints debug messages, tells about unimplemented stuff (only once). Set to 2 is like 1, but also prints messages about implemented but not tested stuff (only once). Setting it to 3 or higher is like 2, but no rate limiting of messages.

compat.linux.default_openfiles
    Default soft openfiles resource limit for Linux applications. Set to -1 to disable the limit. Defaults to 1024.

compat.linux.emul_path
    Path to the Linux run-time environment. Defaults to /compat/linux.

compat.linux.osname
    Linux kernel operating system name. Defaults to "Linux".

compat.linux.osrelease
    Linux kernel operating system release. Changing this to something else is discouraged on non-development systems, because it may change the way Linux programs work. Some versions of GNU libc are known to use different syscalls depending on the value of this sysctl.

compat.linux.oss_version
    Linux Open Sound System version. Defaults to 198144.

compat.linux.preserve_vstatus
    When set to 1, it prevents Linux applications from resetting the termios(4) VSTATUS setting. From a user perspective, this makes SIGINFO work for Linux executables. Defaults to 1.

compat.linux.setid_allowed
    Enable handling of set-user-ID and set-group-ID mode bits for the new process image file when image is to be executed under Linux ABI. When set to 0, new Linux images always use credentials of the program that issued the execve(2) call, regardless of the image file mode. This might be reasonable or even required, because FreeBSD does not emulate the Linux environment completely, and missed features may result in security vulnerabilities. Defaults to 1.

compat.linux32.emulate_i386
    In the x86_64 (amd64) world enable the real i386 Linuxulator behavior. For example, when set to 0, Linux uname -m will return "x86_64" even if uname itself is a i386 Linux executable. When set to 1, Linux i386 uname -m will return "i686". Defaults to 0.

FILES
/compat/linux
    Linux run-time environment
/compat/linux/dev
    device file system, see devfs(4)
/compat/linux/dev/fd
    file descriptor file system mounted with the linrdlnk option, see fdescfs(4)
/compat/linux/dev/mqueue
    symbolic link to a mqueuefs mount, see mqueuefs(4)
/compat/linux/dev/shm
    in-memory file system, see tmpfs(4)
/compat/linux/proc
    Linux process file system, see linprocfs(4)
/compat/linux/sys
    Linux kernel objects file system, see linsysfs(4)

SEE ALSO
brandelf(1), dtrace_syscall(4), fdescfs(4), linprocfs(4), linsysfs(4), mqueuefs(4), pty(4), tmpfs(4), elf(5)

HISTORY
Linux ABI support first appeared for i386 in FreeBSD 2.1. Support for amd64 binaries first appeared in FreeBSD 10.3. Support for arm64 binaries first appeared in FreeBSD 12.0.

BUGS
Support for some of the Linux-specific system calls and system call arguments is missing.