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.\"	David E. O'Brien <obrien@FreeBSD.org>.  All rights reserved.
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.\" $FreeBSD$
.\"
.Dd January 28, 2016
.Dt FILEMON 4
.Os
.Sh NAME
.Nm filemon
.Nd the filemon device
.Sh SYNOPSIS
.In dev/filemon/filemon.h
.Sh DESCRIPTION
The
.Nm
device allows a process to collect file operations data of its children.
The device
.Pa /dev/filemon
responds to two
.Xr ioctl 2
calls.
.Pp
.Nm
is not intended to be a security auditing tool.
Many syscalls are not tracked and binaries of foreign ABI will not be fully
audited.
It is intended for auditing of processes for the purpose of determining its
dependencies in an efficient and easily parsable format.
An example of this is
.Xr make 1
which uses this module with
.Sy .MAKE.MODE=meta
to handle incremental builds more smartly.
.Pp
System calls are denoted using the following single letters:
.Pp
.Bl -tag -width indent -compact
.It Ql C
.Xr chdir 2
.It Ql D
.Xr unlink 2
.It Ql E
.Xr exec 2
.It Ql F
.Xr fork 2 ,
.Xr vfork 2
.It Ql L
.Xr link 2 ,
.Xr linkat 2 ,
.Xr symlink 2 ,
.Xr symlinkat 2
.It Ql M
.Xr rename 2
.It Ql R
.Xr open 2
for read
.It Ql S
.Xr stat 2
.It Ql W
.Xr open 2
for write
.It Ql X
.Xr _exit 2
.El
.Pp
Note that
.Ql R
following
.Ql W
records can represent a single
.Xr open 2
for R/W,
or two separate
.Xr open 2
calls, one for
.Ql R
and one for
.Ql W .
Note that only successful system calls are captured.
.Sh IOCTLS
User mode programs communicate with the
.Nm
driver through a number of ioctls which are described below.
Each takes a single argument.
.Bl -tag -width ".Dv FILEMON_SET_PID"
.It Dv FILEMON_SET_FD
Write the internal tracing buffer to the supplied open file descriptor.
.It Dv FILEMON_SET_PID
Child process ID to trace.
.El
.Sh RETURN VALUES
.\" .Rv -std ioctl
The
.Fn ioctl
function returns the value 0 if successful;
otherwise the value \-1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh FILES
.Bl -tag -width ".Pa /dev/filemon"
.It Pa /dev/filemon
.El
.Sh EXAMPLES
.Bd -literal
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <dev/filemon/filemon.h>
#include <fcntl.h>
#include <err.h>
#include <unistd.h>

static void
open_filemon(void)
{
	pid_t child;
	int fm_fd, fm_log;

	if ((fm_fd = open("/dev/filemon", O_RDWR | O_CLOEXEC)) == -1)
		err(1, "open(\e"/dev/filemon\e", O_RDWR)");
	if ((fm_log = open("filemon.out",
	    O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC, DEFFILEMODE)) == -1)
		err(1, "open(filemon.out)");

	if (ioctl(fm_fd, FILEMON_SET_FD, &fm_log) == -1)
		err(1, "Cannot set filemon log file descriptor");

	if ((child = fork()) == 0) {
		child = getpid();
		if (ioctl(fm_fd, FILEMON_SET_PID, &child) == -1)
			err(1, "Cannot set filemon PID");
		/* Do something here. */
	} else {
		wait(&child);
		close(fm_fd);
	}
}
.Ed
.Pp
Creates a file named
.Pa filemon.out
and configures the
.Nm
device to write the
.Nm
buffer contents to it.
.Sh SEE ALSO
.Xr dtrace 1 ,
.Xr ktrace 1 ,
.Xr truss 1 ,
.Xr ioctl 2
.Sh HISTORY
A
.Nm
device appeared in
.Fx 9.1 .
.Sh BUGS
Loading
.Nm
may reduce system performance for the noted syscalls.
.Pp
Only children of the set process are logged.
Processes can escape being traced by double forking.
This is not seen as a problem as the intended use is build monitoring, which
does not make sense to have daemons for.