FILEMON(4)             FreeBSD Kernel Interfaces Manual             FILEMON(4)

NAME
     filemon - the filemon device

SYNOPSIS
     #include <dev/filemon/filemon.h>

DESCRIPTION
     The filemon device allows a process to collect file operations data of
     its children.  The device /dev/filemon responds to two ioctl(2) calls.

     filemon is not intended to be a security auditing tool.  Many system
     calls are not tracked and binaries using a non-native ABI may not be
     fully audited.  It is intended for auditing of processes for the purpose
     of determining their dependencies using an efficient and easily parsable
     format.  An example of this is make(1) which uses this module with
     .MAKE.MODE=meta to handle incremental builds more smartly.

     System calls are denoted using the following single letters:

     `A'     openat(2).  The next log entry may be lacking an absolute path or
             be inaccurate.
     `C'     chdir(2)
     `D'     unlink(2)
     `E'     exec(2)
     `F'     fork(2), vfork(2)
     `L'     link(2), linkat(2), symlink(2)
     `M'     rename(2)
     `R'     open(2) or openat(2) for read
     `W'     open(2) or openat(2) for write
     `X'     _exit(2)

     Note that `R' following `W' records can represent a single open(2) for
     R/W, or two separate open(2) calls, one for `R' and one for `W'.  Note
     that only successful system calls are captured.

IOCTLS
     User mode programs communicate with the filemon driver through a number
     of ioctls which are described below.  Each takes a single argument.

     FILEMON_SET_FD       Write the internal tracing buffer to the supplied
                          open file descriptor.

     FILEMON_SET_PID      Child process ID to trace.  This should normally be
                          done under the control of a parent in the child
                          after fork(2) but before anything else.  See the
                          example below.

RETURN VALUES
     The ioctl() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The ioctl() system call with FILEMON_SET_FD will fail if:

     [EEXIST]           The filemon handle is already associated with a file
                        descriptor.

     [EINVAL]           The file descriptor has an invalid type and cannot be
                        used for tracing.

     [EBADF]            The file descriptor is invalid or not opened for
                        writing.

     The ioctl() system call with FILEMON_SET_PID will fail if:

     [ESRCH]            No process having the specified process ID exists.

     [EBUSY]            The process ID specified is already being traced and
                        was not the current process.

     The close() system call on the filemon file descriptor may fail with the
     errors from write(2) if any error is encountered while writing the log.
     It may also fail if:

     [EFAULT]           An invalid address was used for a traced system call
                        argument, resulting in no log entry for the system
                        call.

     [ENAMETOOLONG]     An argument for a traced system call was too long,
                        resulting in no log entry for the system call.

FILES
     /dev/filemon

EXAMPLES
     #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(\"/dev/filemon\", 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);
             }
     }

     Creates a file named filemon.out and configures the filemon device to
     write the filemon buffer contents to it.

SEE ALSO
     dtrace(1), ktrace(1), script(1), truss(1), ioctl(2)

HISTORY
     A filemon device appeared in FreeBSD 9.1.

BUGS
     Unloading the module may panic the system, thus requires using kldunload
     -f.

FreeBSD 13.1-RELEASE-p6        February 1, 2022        FreeBSD 13.1-RELEASE-p6