.\" Copyright (c) 2000 Sheldon Hearn
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd December 16, 2019
.Dt LINUX 4
.Os
.Sh NAME
.Nm linux
.Nd Linux ABI support
.Sh SYNOPSIS
To compile support for this ABI into an i386 kernel
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "options COMPAT_LINUX"
.Ed
.Pp
for an amd64 kernel use:
.Bd -ragged -offset indent
.Cd "options COMPAT_LINUX32"
.Ed
.Pp
Alternatively, to load the ABI as a
module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
linux_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
module provides limited Linux ABI (application binary interface) compatibility,
making it possible to run many unmodified Linux applications and libraries
without the need for virtualization or emulation.
Some of the facilities provided are:
.Bl -bullet
.It
An image activator
for correctly branded
.Xr elf 5
executable images
.It
Special signal handling for activated images
.It
Linux to native system call translation
.It
Linux-specific system calls
.El
.Pp
Note that dynamically linked Linux executables
will require a suitable environment in
.Pa /compat/linux .
This includes native Linux shared libraries, and Linux-specific virtual
filesystems.
To set it up, install the
.Pa emulators/linux_base-c7
port or the linux_base-c7
package, and add the following line to the
.Xr rc.conf 5
file:
.Pp
.Dl linux_enable="YES"
.Pp
To avoid mounting Linux-specific filesystems at startup, also add the following
line:
.Pp
.Dl linux_mounts_enable="NO"
.Sh SYSCTL VARIABLES
The following variables are available as both
.Xr sysctl 8
variables and
.Xr loader 8
tunables:
.Bl -tag -width indent
.It Va compat.linux.emul_path
Path to the Linux run-time environment.
Defaults to
.Pa /compat/linux .
.It Va compat.linux.osname
Linux kernel operating system name.
.It Va 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.
Recent versions of GNU libc are known to use different syscalls depending
on the value of this sysctl.
.It Va compat.linux.oss_version
Linux Open Sound System version.
.It Va compat.linux.preserve_vstatus
When set to 1, it prevents Linux applications from resetting the
.Xr termios 4
VSTATUS setting.
From a user perspective, this makes
.Va SIGINFO
work for Linux executables.
Defaults to 0.
.Sh FILES
.Bl -tag -width /compat/linux/dev/shm -compact
.It Pa /compat/linux
minimal Linux run-time environment
.It Pa /compat/linux/dev/fd
file-descriptor file system, see
.Xr fdescfs 5
.It Pa /compat/linux/dev/shm
in-memory file system, see
.Xr tmpfs 5
.It Pa /compat/linux/proc
Linux process file system, see
.Xr linprocfs 5
.It Pa /compat/linux/sys
Linux kernel objects file system, see
.Xr linsysfs 5
.El
.Sh SEE ALSO
.Xr brandelf 1 ,
.Xr pty 4 ,
.Xr elf 5 ,
.Xr fdescfs 5 ,
.Xr linprocfs 5 ,
.Xr linsysfs 5 ,
.Xr tmpfs 5
.Sh HISTORY
Linux ABI support first appeared in
.Fx 2.1 .
.Sh BUGS
Support for some of the Linux-specific system calls and system call arguments
is missing.