Index: sys/compat/cloudabi/cloudabi_fd.c
===================================================================
--- sys/compat/cloudabi/cloudabi_fd.c
+++ sys/compat/cloudabi/cloudabi_fd.c
@@ -26,6 +26,13 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <sys/param.h>
+#include <sys/filedesc.h>
+#include <sys/proc.h>
+#include <sys/syscallsubr.h>
+#include <sys/sysproto.h>
+#include <sys/unistd.h>
+
 #include <compat/cloudabi/cloudabi_proto.h>
 
 int
@@ -32,8 +39,7 @@
 cloudabi_sys_fd_close(struct thread *td, struct cloudabi_sys_fd_close_args *uap)
 {
 
-	/* Not implemented. */
-	return (ENOSYS);
+	return (kern_close(td, uap->fd));
 }
 
 int
@@ -58,9 +64,12 @@
 cloudabi_sys_fd_datasync(struct thread *td,
     struct cloudabi_sys_fd_datasync_args *uap)
 {
+	struct fsync_args fsync_args = {
+		.fd = uap->fd
+	};
 
-	/* Not implemented. */
-	return (ENOSYS);
+	/* Call into fsync(), as FreeBSD lacks fdatasync(). */
+	return (sys_fsync(td, &fsync_args));
 }
 
 int
@@ -67,8 +76,7 @@
 cloudabi_sys_fd_dup(struct thread *td, struct cloudabi_sys_fd_dup_args *uap)
 {
 
-	/* Not implemented. */
-	return (ENOSYS);
+	return (kern_dup(td, 0, uap->from, 0));
 }
 
 int
@@ -75,17 +83,48 @@
 cloudabi_sys_fd_replace(struct thread *td,
     struct cloudabi_sys_fd_replace_args *uap)
 {
+	int error;
 
-	/* Not implemented. */
-	return (ENOSYS);
+	/*
+	 * CloudABI's equivalent to dup2(). CloudABI processes should
+	 * not depend on hardcoded file descriptor layouts, but simply
+	 * use the file descriptor numbers that are allocated by the
+	 * kernel. Duplicating file descriptors to arbitrary numbers
+	 * should not be done.
+	 *
+	 * Invoke kern_dup() with FDDUP_MUSTREPLACE, so that we return
+	 * EBADF when duplicating to a nonexistent file descriptor. Also
+	 * clear the return value, as this system call yields no return
+	 * value.
+	 */
+	error = kern_dup(td, FDDUP_MUSTREPLACE, uap->from, uap->to);
+	td->td_retval[0] = 0;
+	return (error);
 }
 
 int
 cloudabi_sys_fd_seek(struct thread *td, struct cloudabi_sys_fd_seek_args *uap)
 {
+	struct lseek_args lseek_args = {
+		.fd	= uap->fd,
+		.offset	= uap->offset
+	};
 
-	/* Not implemented. */
-	return (ENOSYS);
+	switch (uap->whence) {
+	case CLOUDABI_WHENCE_CUR:
+		lseek_args.whence = SEEK_CUR;
+		break;
+	case CLOUDABI_WHENCE_END:
+		lseek_args.whence = SEEK_END;
+		break;
+	case CLOUDABI_WHENCE_SET:
+		lseek_args.whence = SEEK_SET;
+		break;
+	default:
+		return (EINVAL);
+	}
+
+	return (sys_lseek(td, &lseek_args));
 }
 
 int
@@ -109,7 +148,9 @@
 int
 cloudabi_sys_fd_sync(struct thread *td, struct cloudabi_sys_fd_sync_args *uap)
 {
+	struct fsync_args fsync_args = {
+		.fd = uap->fd
+	};
 
-	/* Not implemented. */
-	return (ENOSYS);
+	return (sys_fsync(td, &fsync_args));
 }
Index: sys/kern/kern_descrip.c
===================================================================
--- sys/kern/kern_descrip.c
+++ sys/kern/kern_descrip.c
@@ -102,7 +102,6 @@
 
 static int	closefp(struct filedesc *fdp, int fd, struct file *fp,
 		    struct thread *td, int holdleaders);
-static int	kern_dup(struct thread *td, int flags, int old, int new);
 static int	fd_first_free(struct filedesc *fdp, int low, int size);
 static int	fd_last_used(struct filedesc *fdp, int size);
 static void	fdgrowtable(struct filedesc *fdp, int nfd);
@@ -111,11 +110,6 @@
 static void	fdused(struct filedesc *fdp, int fd);
 static int	getmaxfd(struct thread *td);
 
-/* Flags for kern_dup() */
-#define	FDDUP_FIXED	0x1	/* Force fixed allocation. */
-#define	FDDUP_FCNTL	0x2	/* fcntl()-style errors. */
-#define	FDDUP_CLOEXEC	0x4	/* Atomically set FD_CLOEXEC. */
-
 /*
  * Each process has:
  *
@@ -794,7 +788,7 @@
 /*
  * Common code for dup, dup2, fcntl(F_DUPFD) and fcntl(F_DUP2FD).
  */
-static int
+int
 kern_dup(struct thread *td, int flags, int old, int new)
 {
 	struct filedesc *fdp;
@@ -808,6 +802,8 @@
 	fdp = p->p_fd;
 
 	MPASS((flags & ~(FDDUP_FIXED | FDDUP_FCNTL | FDDUP_CLOEXEC)) == 0);
+	MPASS((flags & (FDDUP_FIXED | FDDUP_MUSTREPLACE)) !=
+	    (FDDUP_FIXED | FDDUP_MUSTREPLACE));
 
 	/*
 	 * Verify we have a valid descriptor to dup from and possibly to
@@ -828,7 +824,7 @@
 		return (EBADF);
 	}
 	oldfde = &fdp->fd_ofiles[old];
-	if (flags & FDDUP_FIXED && old == new) {
+	if (flags & (FDDUP_FIXED | FDDUP_MUSTREPLACE) && old == new) {
 		td->td_retval[0] = new;
 		if (flags & FDDUP_CLOEXEC)
 			fdp->fd_ofiles[new].fde_flags |= UF_EXCLOSE;
@@ -843,7 +839,16 @@
 	 * table is large enough to hold it, and grab it.  Otherwise, just
 	 * allocate a new descriptor the usual way.
 	 */
-	if (flags & FDDUP_FIXED) {
+	if (flags & FDDUP_MUSTREPLACE) {
+		/* Target file descriptor must exist. */
+		if (new >= fdp->fd_nfiles ||
+		    fdp->fd_ofiles[new].fde_file == NULL) {
+			FILEDESC_XUNLOCK(fdp);
+			fdrop(fp, td);
+			return (EBADF);
+		}
+		newfde = &fdp->fd_ofiles[new];
+	} else if (flags & FDDUP_FIXED) {
 		if (new >= fdp->fd_nfiles) {
 			/*
 			 * The resource limits are here instead of e.g.
Index: sys/sys/filedesc.h
===================================================================
--- sys/sys/filedesc.h
+++ sys/sys/filedesc.h
@@ -134,6 +134,12 @@
 					    SX_NOTRECURSED)
 #define	FILEDESC_UNLOCK_ASSERT(fdp)	sx_assert(&(fdp)->fd_sx, SX_UNLOCKED)
 
+/* Flags for kern_dup(). */
+#define	FDDUP_FIXED		0x1	/* Force fixed allocation. */
+#define	FDDUP_FCNTL		0x2	/* fcntl()-style errors. */
+#define	FDDUP_CLOEXEC		0x4	/* Atomically set FD_CLOEXEC. */
+#define	FDDUP_MUSTREPLACE	0x8	/* Target must exist. */
+
 struct thread;
 
 void	filecaps_init(struct filecaps *fcaps);
Index: sys/sys/syscallsubr.h
===================================================================
--- sys/sys/syscallsubr.h
+++ sys/sys/syscallsubr.h
@@ -85,6 +85,7 @@
 int	kern_close(struct thread *td, int fd);
 int	kern_connectat(struct thread *td, int dirfd, int fd,
 	    struct sockaddr *sa);
+int	kern_dup(struct thread *td, int flags, int old, int new);
 int	kern_execve(struct thread *td, struct image_args *args,
 	    struct mac *mac_p);
 int	kern_fchmodat(struct thread *td, int fd, char *path,