.\" Copyright (c) 2012 Konstantin Belousov <kib@FreeBSD.org>
.\"
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.Dd August 14, 2012
.Dt AOUT 4
.Os
.Sh NAME
.Nm aout
.Nd kernel support for executing binary files in legacy a.out format
.Sh SYNOPSIS
.Bd -literal -offset indent
kldload a.out
.Ed
.Sh DESCRIPTION
The
.Xr a.out 5
executable format was used before the release of
.Fx 3.0 .
Since i386 was the only supported architecture at that time,
.Xr a.out 5
executables can only be activated on platforms that support
execution of i386 code, such as i386 and amd64.
.Pp
To add kernel support for old syscalls and old syscall invocation methods,
place the following options in the kernel configuration file:
.Bd -ragged -offset indent
.Cd "options COMPAT_43"
.br
.Cd "options COMPAT_FREEBSD32"
.Ed
.Pp
The
.Va COMPAT_FREEBSD32
option is only required on 64-bit CPU architectures.
.Pp
The
.Va aout.ko
module needs to be loaded with the
.Xr kldload 8
utility in order to support the
.Xr a.out 5
image activator:
.Bd -ragged -offset indent
.Ic kldload aout
.Ed
.Pp
Alternatively, to load the module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
aout_load="YES"
.Ed
.Pp
The
.Xr a.out 5
format was mainstream quite a long time ago.
Reasonable default settings and security requirements of
modern operating systems today contradict the default environment of that time
and require adjustments of the system to mimic natural environment for
old binaries.
.Pp
The following
.Xr sysctl 8
tunables are useful for this:
.Bl -tag -offset indent -width "XXXXXXXXXXXXXXXXXXXXXXXXX"
.It Xo Va security.bsd.map_at_zero
.Xc
Set to 1 to allow mapping of process pages at address 0.
Some very old
.Va ZMAGIC
executable images require text mapping at address 0.
.It Xo Va kern.pid_max
.Xc
Old versions of
.Fx
used signed 16-bit type for
.Vt pid_t .
Current kernels use 32-bit type for
.Vt pid_t ,
and allow process id's up to 99999.
Such values cannot be represented by old
.Vt pid_t ,
mostly causing issues for processes using
.Xr wait 2
syscalls, for example shells.
Set the sysctl to 30000 to work around the problem.
.It Xo Va kern.elf32.read_exec
.Xc
Set to 1 to force any accessible memory mapping performed by 32-bit
process to allow execution, see
.Xr mmap 2 .
Old i386 CPUs did not have a bit in PTE which disallowed execution
from the page, so many old programs did not specify
.Va PROT_EXEC
even for mapping of executable code.
The sysctl forces
.Va PROT_EXEC
if mapping has any access allowed at all.
The setting is only needed if the host architecture allows
non-executable mappings.
.El
.Sh SEE ALSO
.Xr execve 2 ,
.Xr a.out 5 ,
.Xr elf 5 ,
.Xr sysctl 8
.Sh HISTORY
The
.Xr a.out 5
executable format was used on ancient
.At
and served as the main executable format for
.Fx
from the beginning up to
.Fx 2.2.9 .
In
.Fx 3.0
it was superseded by
.Xr elf 5 .
.Sh AUTHORS
The
.Nm
manual page was written by
.An Konstantin Belousov Aq Mt kib@FreeBSD.org .
.Sh BUGS
On 64bit architectures, not all wrappers for older syscalls are implemented.