Ubuntu Manpage: aout — kernel support for executing binary files in legacy a.out format

Provided by: freebsd-manpages_12.2-1_all

NAME

       aout — kernel support for executing binary files in legacy a.out format

SYNOPSIS

             kldload a.out

DESCRIPTION

The a.out(5) executable format was used before the release of FreeBSD 3.0. Since i386 was the only
supported architecture at that time, a.out(5) executables can only be activated on platforms that support
execution of i386 code, such as i386 and amd64.

To add kernel support for old syscalls and old syscall invocation methods, place the following options in
the kernel configuration file:

options COMPAT_43
options COMPAT_FREEBSD32

The COMPAT_FREEBSD32 option is only required on 64-bit CPU architectures.

The aout.ko module needs to be loaded with the kldload(8) utility in order to support the a.out(5) image
activator:

kldload aout

Alternatively, to load the module at boot time, place the following line in loader.conf(5):

aout_load="YES"

The 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.

The following sysctl(8) tunables are useful for this:

security.bsd.map_at_zero Set to 1 to allow mapping of process pages at address 0. Some very old
ZMAGIC executable images require text mapping at address 0.

kern.pid_max Old versions of FreeBSD used signed 16-bit type for pid_t. Current
kernels use 32-bit type for pid_t, and allow process id's up to 99999.
Such values cannot be represented by old pid_t, mostly causing issues
for processes using wait(2) syscalls, for example shells. Set the
sysctl to 30000 to work around the problem.

kern.elf32.read_exec Set to 1 to force any accessible memory mapping performed by 32-bit
process to allow execution, see 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 PROT_EXEC even for mapping of executable code.
The sysctl forces PROT_EXEC if mapping has any access allowed at all.
The setting is only needed if the host architecture allows non-
executable mappings.

HISTORY

       The a.out(5) executable format was used on ancient AT&T UNIX and served as the main executable format for
       FreeBSD from the beginning up to FreeBSD 2.2.9.  In FreeBSD 3.0 it was superseded by elf(5).

AUTHORS

       The aout manual page was written by Konstantin Belousov <kib@FreeBSD.org>.

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