Index: head/sys/compat/cloudabi/cloudabi_fd.c
===================================================================
--- head/sys/compat/cloudabi/cloudabi_fd.c
+++ head/sys/compat/cloudabi/cloudabi_fd.c
@@ -94,7 +94,8 @@
 	case CLOUDABI_FILETYPE_SHARED_MEMORY:
 		cap_rights_init(&fcaps.fc_rights, CAP_FSTAT, CAP_FTRUNCATE,
 		    CAP_MMAP_RWX);
-		return (kern_shm_open(td, SHM_ANON, O_RDWR, 0, &fcaps));
+		return (kern_shm_open(td, SHM_ANON, O_RDWR | O_CLOEXEC, 0,
+		    &fcaps));
 	default:
 		return (EINVAL);
 	}
Index: head/sys/kern/uipc_shm.c
===================================================================
--- head/sys/kern/uipc_shm.c
+++ head/sys/kern/uipc_shm.c
@@ -729,7 +729,14 @@
 	fdp = td->td_proc->p_fd;
 	cmode = (mode & ~fdp->fd_cmask) & ACCESSPERMS;
 
-	error = falloc_caps(td, &fp, &fd, O_CLOEXEC, fcaps);
+	/*
+	 * shm_open(2) created shm should always have O_CLOEXEC set, as mandated
+	 * by POSIX.  We allow it to be unset here so that an in-kernel
+	 * interface may be written as a thin layer around shm, optionally not
+	 * setting CLOEXEC.  For shm_open(2), O_CLOEXEC is set unconditionally
+	 * in sys_shm_open() to keep this implementation compliant.
+	 */
+	error = falloc_caps(td, &fp, &fd, flags & O_CLOEXEC, fcaps);
 	if (error)
 		return (error);
 
@@ -844,7 +851,8 @@
 sys_shm_open(struct thread *td, struct shm_open_args *uap)
 {
 
-	return (kern_shm_open(td, uap->path, uap->flags, uap->mode, NULL));
+	return (kern_shm_open(td, uap->path, uap->flags | O_CLOEXEC, uap->mode,
+	    NULL));
 }
 
 int