Add accept4() system call.

The accept4() function, compared to accept(), allows setting the new file
descriptor atomically close-on-exec and explicitly controlling the
non-blocking status on the new socket. (Note that the latter point means
that accept() is not equivalent to any form of accept4().)

The linuxulator's accept4 implementation leaves a race window where the new
file descriptor is not close-on-exec because it calls sys_accept(). This
implementation leaves no such race window (by using falloc() flags). The
linuxulator could be fixed and simplified by using the new code.

Like accept(), accept4() is async-signal-safe, a cancellation point and
permitted in capability mode.

lib/libc/sys/Makefile.inc

@@ -270,6 +270,7 @@ MAN+=	sctp_generic_recvmsg.2 \
 	wait.2 \
 	write.2
 
+MLINKS+=accept.2 accept4.2
 MLINKS+=access.2 eaccess.2 \
 	access.2 faccessat.2
 MLINKS+=brk.2 sbrk.2

lib/libc/sys/Symbol.map

@@ -378,6 +378,7 @@ FBSD_1.2 {
 };
 
 FBSD_1.3 {
+	accept4;
 	bindat;
 	cap_fcntls_get;
 	cap_fcntls_limit;
@@ -461,6 +462,8 @@ FBSDprivate_1.0 {
 	__sys_abort2;
 	_accept;
 	__sys_accept;
+	_accept4;
+	__sys_accept4;
 	_access;
 	__sys_access;
 	_acct;

lib/libc/sys/accept.2

@@ -41,6 +41,8 @@
 .In sys/socket.h
 .Ft int
 .Fn accept "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen"
+.Ft int
+.Fn accept4 "int s" "struct sockaddr * restrict addr" "socklen_t * restrict addrlen" "int flags"
 .Sh DESCRIPTION
 The argument
 .Fa s
@@ -66,6 +68,26 @@ and
 signals from the original socket
 .Fa s .
 .Pp
+The
+.Fn accept4
+system call is similar,
+but the
+.Dv O_NONBLOCK
+property of the new socket is instead determined by the
+.Dv SOCK_NONBLOCK
+flag in the
+.Fa flags
+argument,
+the
+.Dv O_ASYNC
+property is cleared,
+the signal destination is cleared
+and the close-on-exec flag on the new file descriptor can be set via the
+.Dv SOCK_CLOEXEC
+flag in the
+.Fa flags
+argument.
+.Pp
 If no pending connections are
 present on the queue, and the original socket
 is not marked as non-blocking,
@@ -141,13 +163,15 @@ properties and the signal destination being inherited,
 but should set them explicitly using
 .Xr fcntl 2 .
 .Sh RETURN VALUES
-The call returns \-1 on error.
-If it succeeds, it returns a non-negative
+These calls return \-1 on error.
+If they succeed, they return a non-negative
 integer that is a descriptor for the accepted socket.
 .Sh ERRORS
 The
 .Fn accept
-system call will fail if:
+and
+.Fn accept4
+system calls will fail if:
 .Bl -tag -width Er
 .It Bq Er EBADF
 The descriptor is invalid.
@@ -176,6 +200,16 @@ are present to be accepted.
 A connection arrived, but it was closed while waiting
 on the listen queue.
 .El
+.Pp
+The
+.Fn accept4
+system call will also fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+The
+.Fa flags
+argument is invalid.
+.El
 .Sh SEE ALSO
 .Xr bind 2 ,
 .Xr connect 2 ,
@@ -190,3 +224,8 @@ The
 .Fn accept
 system call appeared in
 .Bx 4.2 .
+.Pp
+The
+.Fn accept4
+system call appeared in
+.Fx 10.0 .

lib/libthr/pthread.map

@@ -181,6 +181,7 @@ FBSDprivate_1.0 {
 	___wait;
 	___waitpid;
 	__accept;
+	__accept4;
 	__aio_suspend;
 	__close;
 	__connect;
@@ -408,3 +409,7 @@ FBSD_1.2 {
 	setcontext;
 	swapcontext;
 };
+
+FBSD_1.3 {
+	accept4;
+};

lib/libthr/thread/thr_syscalls.c

@@ -101,6 +101,7 @@ extern pid_t	__waitpid(pid_t, int *, int);
 extern int	__sys_aio_suspend(const struct aiocb * const[], int,
 			const struct timespec *);
 extern int	__sys_accept(int, struct sockaddr *, socklen_t *);
+extern int	__sys_accept4(int, struct sockaddr *, socklen_t *, int);
 extern int	__sys_connect(int, const struct sockaddr *, socklen_t);
 extern int	__sys_fsync(int);
 extern int	__sys_msync(void *, size_t, int);
@@ -129,6 +130,7 @@ int	___usleep(useconds_t useconds);
 pid_t	___wait(int *);
 pid_t	___waitpid(pid_t, int *, int);
 int	__accept(int, struct sockaddr *, socklen_t *);
+int	__accept4(int, struct sockaddr *, socklen_t *, int);
 int	__aio_suspend(const struct aiocb * const iocbs[], int,
 		const struct timespec *);
 int	__close(int);
@@ -176,6 +178,26 @@ __accept(int s, struct sockaddr *addr, socklen_t *addrlen)
  	return (ret);
 }
 
+__weak_reference(__accept4, accept4);
+
+/*
+ * Cancellation behavior:
+ *   If thread is canceled, no socket is created.
+ */
+int
+__accept4(int s, struct sockaddr *addr, socklen_t *addrlen, int flags)
+{
+	struct pthread *curthread;
+	int ret;
+
+	curthread = _get_curthread();
+	_thr_cancel_enter(curthread);
+	ret = __sys_accept4(s, addr, addrlen, flags);
+	_thr_cancel_leave(curthread, ret == -1);
+
+ 	return (ret);
+}
+
 __weak_reference(__aio_suspend, aio_suspend);
 
 int

sys/compat/freebsd32/syscalls.master

@@ -1022,3 +1022,7 @@
 				    int namelen); }
 540	AUE_CHFLAGSAT	NOPROTO	{ int chflagsat(int fd, const char *path, \
 				    u_long flags, int atflag); }
+541	AUE_ACCEPT	NOPROTO	{ int accept4(int s, \
+				    struct sockaddr * __restrict name, \
+				    __socklen_t * __restrict anamelen, \
+				    int flags); }

sys/kern/capabilities.conf

@@ -78,6 +78,7 @@ abort2
 ## relies on existing bindings on a socket, subject to capability rights.
 ##
 accept
+accept4
 
 ##
 ## Allow AIO operations by file descriptor, subject to capability rights.

sys/kern/syscalls.master

@@ -972,5 +972,9 @@
 				    int namelen); }
 540	AUE_CHFLAGSAT	STD	{ int chflagsat(int fd, const char *path, \
 				    u_long flags, int atflag); }
+541	AUE_ACCEPT	STD	{ int accept4(int s, \
+				    struct sockaddr * __restrict name, \
+				    __socklen_t * __restrict anamelen, \
+				    int flags); }
 ; Please copy any additions and changes to the following compatability tables:
 ; sys/compat/freebsd32/syscalls.master

sys/kern/uipc_syscalls.c

@@ -97,10 +97,18 @@ __FBSDID("$FreeBSD$");
 #endif /* SCTP */
 #endif /* INET || INET6 */
 
+/*
+ * Flags for accept1() and kern_accept4(), in addition to SOCK_CLOEXEC
+ * and SOCK_NONBLOCK.
+ */
+#define	ACCEPT4_INHERIT	0x1
+#define	ACCEPT4_COMPAT	0x2
+
 static int sendit(struct thread *td, int s, struct msghdr *mp, int flags);
 static int recvit(struct thread *td, int s, struct msghdr *mp, void *namelenp);
 
-static int accept1(struct thread *td, struct accept_args *uap, int compat);
+static int accept1(struct thread *td, int s, struct sockaddr *uname,
+		   socklen_t *anamelen, int flags);
 static int do_sendfile(struct thread *td, struct sendfile_args *uap, int compat);
 static int getsockname1(struct thread *td, struct getsockname_args *uap,
 			int compat);
@@ -317,49 +325,46 @@ sys_listen(td, uap)
  * accept1()
  */
 static int
-accept1(td, uap, compat)
+accept1(td, s, uname, anamelen, flags)
 	struct thread *td;
-	struct accept_args /* {
-		int	s;
-		struct sockaddr	* __restrict name;
-		socklen_t	* __restrict anamelen;
-	} */ *uap;
-	int compat;
+	int s;
+	struct sockaddr *uname;
+	socklen_t *anamelen;
+	int flags;
 {
 	struct sockaddr *name;
 	socklen_t namelen;
 	struct file *fp;
 	int error;
 
-	if (uap->name == NULL)
-		return (kern_accept(td, uap->s, NULL, NULL, NULL));
+	if (uname == NULL)
+		return (kern_accept4(td, s, NULL, NULL, flags, NULL));
 
-	error = copyin(uap->anamelen, &namelen, sizeof (namelen));
+	error = copyin(anamelen, &namelen, sizeof (namelen));
 	if (error)
 		return (error);
 
-	error = kern_accept(td, uap->s, &name, &namelen, &fp);
+	error = kern_accept4(td, s, &name, &namelen, flags, &fp);
 
 	/*
 	 * return a namelen of zero for older code which might
 	 * ignore the return value from accept.
 	 */
 	if (error) {
-		(void) copyout(&namelen,
-		    uap->anamelen, sizeof(*uap->anamelen));
+		(void) copyout(&namelen, anamelen, sizeof(*anamelen));
 		return (error);
 	}
 
-	if (error == 0 && name != NULL) {
+	if (error == 0 && uname != NULL) {
 #ifdef COMPAT_OLDSOCK
-		if (compat)
+		if (flags & ACCEPT4_COMPAT)
 			((struct osockaddr *)name)->sa_family =
 			    name->sa_family;
 #endif
-		error = copyout(name, uap->name, namelen);
+		error = copyout(name, uname, namelen);
 	}
 	if (error == 0)
-		error = copyout(&namelen, uap->anamelen,
+		error = copyout(&namelen, anamelen,
 		    sizeof(namelen));
 	if (error)
 		fdclose(td->td_proc->p_fd, fp, td->td_retval[0], td);
@@ -371,6 +376,13 @@ accept1(td, uap, compat)
 int
 kern_accept(struct thread *td, int s, struct sockaddr **name,
     socklen_t *namelen, struct file **fp)
+{
+	return (kern_accept4(td, s, name, namelen, ACCEPT4_INHERIT, fp));
+}
+
+int
+kern_accept4(struct thread *td, int s, struct sockaddr **name,
+    socklen_t *namelen, int flags, struct file **fp)
 {
 	struct filedesc *fdp;
 	struct file *headfp, *nfp = NULL;
@@ -400,7 +412,7 @@ kern_accept(struct thread *td, int s, struct sockaddr **name,
 	if (error != 0)
 		goto done;
 #endif
-	error = falloc(td, &nfp, &fd, 0);
+	error = falloc(td, &nfp, &fd, (flags & SOCK_CLOEXEC) ? O_CLOEXEC : 0);
 	if (error)
 		goto done;
 	ACCEPT_LOCK();
@@ -441,7 +453,10 @@ kern_accept(struct thread *td, int s, struct sockaddr **name,
 
 	TAILQ_REMOVE(&head->so_comp, so, so_list);
 	head->so_qlen--;
-	so->so_state |= (head->so_state & SS_NBIO);
+	if (flags & ACCEPT4_INHERIT)
+		so->so_state |= (head->so_state & SS_NBIO);
+	else
+		so->so_state |= (flags & SOCK_NONBLOCK) ? SS_NBIO : 0;
 	so->so_qstate &= ~SQ_COMP;
 	so->so_head = NULL;
 
@@ -454,9 +469,15 @@ kern_accept(struct thread *td, int s, struct sockaddr **name,
 	/* connection has been removed from the listen queue */
 	KNOTE_UNLOCKED(&head->so_rcv.sb_sel.si_note, 0);
 
-	pgid = fgetown(&head->so_sigio);
-	if (pgid != 0)
-		fsetown(pgid, &so->so_sigio);
+	if (flags & ACCEPT4_INHERIT) {
+		pgid = fgetown(&head->so_sigio);
+		if (pgid != 0)
+			fsetown(pgid, &so->so_sigio);
+	} else {
+		fflag &= ~(FNONBLOCK | FASYNC);
+		if (flags & SOCK_NONBLOCK)
+			fflag |= FNONBLOCK;
+	}
 
 	finit(nfp, fflag, DTYPE_SOCKET, so, &socketops);
 	/* Sync socket nonblocking/async state with file flags */
@@ -527,7 +548,18 @@ sys_accept(td, uap)
 	struct accept_args *uap;
 {
 
-	return (accept1(td, uap, 0));
+	return (accept1(td, uap->s, uap->name, uap->anamelen, ACCEPT4_INHERIT));
+}
+
+int
+sys_accept4(td, uap)
+	struct thread *td;
+	struct accept4_args *uap;
+{
+	if (uap->flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
+		return (EINVAL);
+
+	return (accept1(td, uap->s, uap->name, uap->anamelen, uap->flags));
 }
 
 #ifdef COMPAT_OLDSOCK
@@ -537,7 +569,8 @@ oaccept(td, uap)
 	struct accept_args *uap;
 {
 
-	return (accept1(td, uap, 1));
+	return (accept1(td, uap->s, uap->name, uap->anamelen,
+	    ACCEPT4_INHERIT | ACCEPT4_COMPAT));
 }
 #endif /* COMPAT_OLDSOCK */
 

sys/sys/socket.h

@@ -634,6 +634,7 @@ int	accept(int, struct sockaddr * __restrict, socklen_t * __restrict);
 int	bind(int, const struct sockaddr *, socklen_t);
 int	connect(int, const struct sockaddr *, socklen_t);
 #if __BSD_VISIBLE
+int	accept4(int, struct sockaddr * __restrict, socklen_t * __restrict, int);
 int	bindat(int, int, const struct sockaddr *, socklen_t);
 int	connectat(int, int, const struct sockaddr *, socklen_t);
 #endif

sys/sys/syscallsubr.h

@@ -60,6 +60,8 @@ int	kern___getcwd(struct thread *td, u_char *buf, enum uio_seg bufseg,
 	    u_int buflen);
 int	kern_accept(struct thread *td, int s, struct sockaddr **name,
 	    socklen_t *namelen, struct file **fp);
+int	kern_accept4(struct thread *td, int s, struct sockaddr **name,
+	    socklen_t *namelen, int flags, struct file **fp);
 int	kern_access(struct thread *td, char *path, enum uio_seg pathseg,
 	    int flags);
 int	kern_accessat(struct thread *td, int fd, char *path,