Add a new 'void closefrom(int lowfd)' system call. When called, it closes

any open file descriptors >= 'lowfd'.  It is largely identical to the same
function on other operating systems such as Solaris, DFly, NetBSD, and
OpenBSD.  One difference from other *BSD is that this closefrom() does not
fail with any errors.  In practice, while the manpages for NetBSD and
OpenBSD claim that they return EINTR, they ignore internal errors from
close() and never return EINTR.  DFly does return EINTR, but for the common
use case (closing fd's prior to execve()), the caller really wants all
fd's closed and returning EINTR just forces callers to call closefrom() in
a loop until it stops failing.

Note that this implementation of closefrom(2) does not make any effort to
resolve userland races with open(2) in other threads.  As such, it is not
multithread safe.

Submitted by:	rwatson (initial version)
Reviewed by:	rwatson
MFC after:	2 weeks

1 files changed, 36 insertions, 0 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 1221825..30bb84a 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -1128,6 +1128,42 @@ kern_close(td, fd)
 	return (error);
 }
 
+/*
+ * Close open file descriptors.
+ */
+#ifndef _SYS_SYSPROTO_H_
+struct closefrom_args {
+	int	lowfd;
+};
+#endif
+/* ARGSUSED */
+int
+closefrom(struct thread *td, struct closefrom_args *uap)
+{
+	struct filedesc *fdp;
+	int fd;
+
+	fdp = td->td_proc->p_fd;
+	AUDIT_ARG(fd, uap->lowfd);
+
+	/*
+	 * Treat negative starting file descriptor values identical to
+	 * closefrom(0) which closes all files.
+	 */
+	if (uap->lowfd < 0)
+		uap->lowfd = 0;
+	FILEDESC_SLOCK(fdp);
+	for (fd = uap->lowfd; fd < fdp->fd_nfiles; fd++) {
+		if (fdp->fd_ofiles[fd] != NULL) {
+			FILEDESC_SUNLOCK(fdp);
+			(void)kern_close(td, fd);
+			FILEDESC_SLOCK(fdp);
+		}
+	}
+	FILEDESC_SUNLOCK(fdp);
+	return (0);
+}
+
 #if defined(COMPAT_43)
 /*
  * Return status information about a file descriptor.