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	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)			.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT			.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY			.\" 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			.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	.\" SUCH DAMAGE.			.\" SUCH DAMAGE.
	.\"			.\"
	.\" $FreeBSD$			.\" $FreeBSD$
	.\"			.\"
	.Dd August 1, 2017			.Dd November 13, 2019
	.Dt LINUX 4			.Dt LINUX 4
	.Os			.Os
	.Sh NAME			.Sh NAME
	.Nm linux			.Nm linux
	.Nd Linux ABI support			.Nd Linux ABI support
	.Sh SYNOPSIS			.Sh SYNOPSIS
	To compile support for this ABI into an i386 kernel			To compile support for this ABI into an i386 kernel
	place the following line in your			place the following line in your
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	module at boot time, place the following line in			module at boot time, place the following line in
	.Xr loader.conf 5 :			.Xr loader.conf 5 :
	.Bd -literal -offset indent			.Bd -literal -offset indent
	linux_load="YES"			linux_load="YES"
	.Ed			.Ed
	.Sh DESCRIPTION			.Sh DESCRIPTION
	The			The
	.Nm			.Nm
	module provides limited			module provides limited Linux ABI (application binary interface) compatibility,
	Linux ABI (application binary interface) compatibility			making it possible to run many unmodified Linux applications and libraries
	for userland applications.			without the need for virtualization or emulation.
	The module provides the following significant facilities:			Some of the facilities provided are:
	.Bl -bullet			.Bl -bullet
	.It			.It
	An image activator			An image activator
	for correctly branded			for correctly branded
	.Xr elf 5			.Xr elf 5
	executable images			executable images
	.It			.It
	Special signal handling for activated images			Special signal handling for activated images
	.It			.It
	Linux to native system call translation			Linux to native system call translation
				.It
				Linux-specific system calls
	.El			.El
	.Pp			.Pp
	It is important to note that the Linux ABI support			Note that dynamically linked Linux executables
	it not provided through an emulator.			will require a suitable environment in
	Rather, a true (albeit limited) ABI implementation is provided.			.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			.Pp
	The following			.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			.Xr sysctl 8
	tunable variables are available:			variables and
	.Bl -tag -width compat.linux.oss_version			.Xr loader 8
	.It compat.linux.osname			tunables:
				.Bl -tag -width indent
				.It Va compat.linux.osname
	Linux kernel operating system name.			Linux kernel operating system name.
	.It compat.linux.osrelease			.It Va compat.linux.osrelease
	Linux kernel operating system release.			Linux kernel operating system release.
	Changing this to something else is discouraged on non-development systems,			Changing this to something else is discouraged on non-development systems,
	because it may change the way Linux programs work.			because it may change the way Linux programs work.
	Recent versions of GNU libc are known to use different syscalls depending			Recent versions of GNU libc are known to use different syscalls depending
	on the value of this sysctl.			on the value of this sysctl.
	.It compat.linux.oss_version			.It Va compat.linux.oss_version
	Linux Open Sound System version.			Linux Open Sound System version.
	.El
	.Pp
	The
	.Nm
	module can be linked into the kernel statically with the
	.Dv COMPAT_LINUX
	kernel configuration option
	or loaded as required.
	The following command will load the module
	if it is neither linked into the kernel
	nor already loaded as a module:
	.Bd -literal -offset indent
	if ! kldstat -v \| grep -E 'linux(aout\|elf)' > /dev/null; then
	kldload linux > /dev/null 2>&1
	fi
	.Ed
	.Pp
	Note that dynamically linked Linux executables
	will require a suitable environment in
	.Pa /compat/linux .
	Specifically, the Linux run-time linker's hints files
	should be correctly initialized.
	For this reason, it is common to execute the following commands
	to prepare the system to correctly run Linux executables:
	.Bd -literal -offset indent
	if [ -x /compat/linux/sbin/ldconfig ]; then
	/compat/linux/sbin/ldconfig -r /compat/linux
	fi
	.Ed
	.Pp
	For information on loading the
	.Nm
	kernel loadable module automatically on system startup,
	see
	.Xr rc.conf 5 .
	This information applies
	regardless of whether the
	.Nm
	module is statically linked into the kernel
	or loaded as a module.
	.Sh FILES			.Sh FILES
	.Bl -tag -width /compat/linux/dev/fd -compact			.Bl -tag -width /compat/linux/dev/shm -compact
	.It Pa /compat/linux			.It Pa /compat/linux
	minimal Linux run-time environment			minimal Linux run-time environment
	.It Pa /compat/linux/dev/fd			.It Pa /compat/linux/dev/fd
	limited Linux file-descriptor file system			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			.It Pa /compat/linux/proc
	limited Linux process file system			Linux process file system, see
				.Xr linprocfs 5
	.It Pa /compat/linux/sys			.It Pa /compat/linux/sys
	limited Linux system file system			Linux kernel objects file system, see
				.Xr linsysfs 5
	.El			.El
	.Sh SEE ALSO			.Sh SEE ALSO
	.Xr brandelf 1 ,			.Xr brandelf 1 ,
	.Xr pty 4 ,			.Xr pty 4 ,
	.Xr elf 5 ,			.Xr elf 5 ,
	.Xr fdescfs 5 ,			.Xr fdescfs 5 ,
	.Xr linprocfs 5 ,			.Xr linprocfs 5 ,
	.Xr linsysfs 5			.Xr linsysfs 5 ,
				.Xr tmpfs 5
	.Sh HISTORY			.Sh HISTORY
	Linux ABI support first appeared in			Linux ABI support first appeared in
	.Fx 2.1 .			.Fx 2.1 .
				.Sh BUGS
				Support for some of the Linux-specific system calls and system call arguments
				is missing.