Index: share/man/man4/filemon.4
===================================================================
--- share/man/man4/filemon.4
+++ share/man/man4/filemon.4
@@ -31,7 +31,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd June 15, 2019
+.Dd February 1, 2022
 .Dt FILEMON 4
 .Os
 .Sh NAME
@@ -51,10 +51,10 @@
 .Pp
 .Nm
 is not intended to be a security auditing tool.
-Many system calls 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.
+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
 .Xr make 1
 which uses this module with
@@ -145,6 +145,11 @@
 The
 .Nm
 handle is already associated with a file descriptor.
+.It Bq Er EINVAL
+The file descriptor has an invalid type and cannot be used for
+tracing.
+.It Bq Er EBADF
+The file descriptor is invalid or not opened for writing.
 .El
 .Pp
 The
Index: sys/dev/filemon/filemon.c
===================================================================
--- sys/dev/filemon/filemon.c
+++ sys/dev/filemon/filemon.c
@@ -359,9 +359,10 @@
 filemon_ioctl(struct cdev *dev, u_long cmd, caddr_t data, int flag __unused,
     struct thread *td)
 {
-	int error = 0;
 	struct filemon *filemon;
+	struct file *fp;
 	struct proc *p;
+	int error;
 
 	if ((error = devfs_get_cdevpriv((void **) &filemon)) != 0)
 		return (error);
@@ -376,12 +377,21 @@
 			break;
 		}
 
-		error = fget_write(td, *(int *)data,
-		    &cap_pwrite_rights,
-		    &filemon->fp);
-		if (error == 0)
+		error = fget_write(td, *(int *)data, &cap_pwrite_rights, &fp);
+		if (error == 0) {
+			/*
+			 * The filemon handle may be passed to another process,
+			 * so the underlying file handle must support this.
+			 */
+			if ((fp->f_ops->fo_flags & DFLAG_PASSABLE) == 0) {
+				fdrop(fp, curthread);
+				error = EINVAL;
+				break;
+			}
+			filemon->fp = fp;
 			/* Write the file header. */
 			filemon_write_header(filemon);
+		}
 		break;
 
 	/* Set the monitored process ID. */