dup3 syscall - atomic set O_CLOEXEC with dup2

[Eitan Adler]

This is an implementation of dup3 for FreeBSD:
man page here (with a FreeBSD patch coming soon):
https://www.kernel.org/doc/man-pages/online/pages/man2/dup.2.html

Is this implementation correct? If so any objection to adding this as
a supported syscall?


Index: sys/kern/kern_descrip.c
===================================================================
--- sys/kern/kern_descrip.c	(revision 229830)
+++ sys/kern/kern_descrip.c	(working copy)
@@ -110,6 +110,7 @@
 /* Flags for do_dup() */
 #define DUP_FIXED	0x1	/* Force fixed allocation */
 #define DUP_FCNTL	0x2	/* fcntl()-style errors */
+#define DUP_CLOEXEC	0x4	/* Enable O_CLOEXEC on the new fs */

 static int do_dup(struct thread *td, int flags, int old, int new,
     register_t *retval);
@@ -307,7 +308,39 @@
 	return (0);
 }

+#ifndef _SYS_SYSPROTO_H_
+struct dup3_args {
+	u_int	from;
+	u_int	to;
+	int	flags;
+};
+#endif
+
 /*
+ * Duplicate a file descriptor and allow for O_CLOEXEC
+ */
+
+/* ARGSUSED */
+int
+sys_dup3(struct thread * const td, struct dup3_args * const uap) {
+
+	KASSERT(td != NULL, ("%s: td == NULL", __func__));
+	KASSERT(uap != NULL, ("%s: uap == NULL", __func__));
+
+	if (uap->from == uap->to)
+		return EINVAL;
+
+	if (uap->flags & ~O_CLOEXEC)
+		return EINVAL;
+
+	const int dupflags = (uap->flags == O_CLOEXEC) ?
DUP_FIXED|DUP_CLOEXEC : DUP_FIXED;
+
+	return (do_dup(td, dupflags, (int)uap->from, (int)uap->to,
+		    td->td_retval));
+	return (0);
+}
+
+/*
  * Duplicate a file descriptor to a particular value.
  *
  * Note: keep in mind that a potential race condition exists when closing
@@ -912,6 +945,9 @@
 		fdp->fd_lastfile = new;
 	*retval = new;

+	if (flags & DUP_CLOEXEC)
+		fdp->fd_ofileflags[new] |= UF_EXCLOSE;
+
 	/*
 	 * If we dup'd over a valid file, we now own the reference to it
 	 * and must dispose of it using closef() semantics (as if a
Index: sys/kern/syscalls.master
===================================================================
--- sys/kern/syscalls.master	(revision 229830)
+++ sys/kern/syscalls.master	(working copy)
@@ -951,5 +951,6 @@
 				    off_t offset, off_t len); }
 531	AUE_NULL	STD	{ int posix_fadvise(int fd, off_t offset, \
 				    off_t len, int advice); }
+532	AUE_NULL	STD	{ int dup3(u_int from, u_int to, int flags); }
 ; Please copy any additions and changes to the following compatability tables:
 ; sys/compat/freebsd32/syscalls.master
Index: sys/compat/freebsd32/syscalls.master
===================================================================
--- sys/compat/freebsd32/syscalls.master	(revision 229830)
+++ sys/compat/freebsd32/syscalls.master	(working copy)
@@ -997,3 +997,4 @@
 				    uint32_t offset1, uint32_t offset2,\
 				    uint32_t len1, uint32_t len2, \
 				    int advice); }
+532	AUE_NULL	STD	{ int dup3(u_int from, u_int to, int flags); }


-- 
Eitan Adler