.\" Copyright (c) 2000 Sheldon Hearn
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.\" $FreeBSD$
.\"
.Dd July 5, 2020
.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.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.
.It Va compat.linux.default_openfiles
Default soft openfiles resource limit for Linux applications.
Set to -1 to disable the limit.
Defaults to 1024.
.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.
.El
.Sh FILES
.Bl -tag -width /compat/linux/dev/shm -compact
.It Pa /compat/linux
minimal Linux run-time environment
.It Pa /compat/linux/dev
device file system, see
.Xr devfs 5
.It Pa /compat/linux/dev/fd
file descriptor file system mounted with the
.Cm linrdlnk
option, 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.