Make openat(2) a cancellation point.

This is required by POSIX and matches open(2).

Reviewed by:	kib, jhb
MFC after:	1 month

lib/libc/include/namespace.h

@@ -80,6 +80,7 @@
 #define		listen				_listen
 #define		nanosleep			_nanosleep
 #define		open				_open
+#define		openat				_openat
 #define		poll				_poll
 #define		pthread_atfork			_pthread_atfork
 #define		pthread_attr_destroy		_pthread_attr_destroy

lib/libc/include/un-namespace.h

@@ -61,6 +61,7 @@
 #undef		listen
 #undef		nanosleep
 #undef		open
+#undef		openat
 #undef		poll
 #undef		pthread_atfork
 #undef		pthread_attr_destroy

lib/libc/sys/Symbol.map

@@ -769,6 +769,8 @@ FBSDprivate_1.0 {
 	__sys_olio_listio;
 	_open;
 	__sys_open;
+	_openat;
+	__sys_openat;
 	_pathconf;
 	__sys_pathconf;
 	_pipe;

lib/libthr/pthread.map

@@ -195,6 +195,7 @@ FBSDprivate_1.0 {
 	__msync;
 	__nanosleep;
 	__open;
+	__openat;
 	__poll;
 	__pthread_cond_timedwait;
 	__pthread_cond_wait;
@@ -406,3 +407,7 @@ FBSD_1.1 {
 	pthread_mutex_setspinloops_np;
 	pthread_mutex_setyieldloops_np;
 };
+
+FBSD_1.2 {
+	openat;
+};

lib/libthr/thread/thr_private.h

@@ -668,6 +668,7 @@ void	_pthread_cleanup_pop(int);
 #ifdef  _SYS_FCNTL_H_
 int     __sys_fcntl(int, int, ...);
 int     __sys_open(const char *, int, ...);
+int     __sys_openat(int, const char *, int, ...);
 #endif
 
 /* #include <signal.h> */

lib/libthr/thread/thr_syscalls.c

@@ -139,6 +139,7 @@ int	__fsync(int);
 int	__msync(void *, size_t, int);
 int	__nanosleep(const struct timespec *, struct timespec *);
 int	__open(const char *, int,...);
+int	__openat(int, const char *, int,...);
 int	__poll(struct pollfd *, unsigned int, int);
 ssize_t	__read(int, void *buf, size_t);
 ssize_t	__readv(int, const struct iovec *, int);
@@ -341,6 +342,33 @@ __open(const char *path, int flags,...)
 	return ret;
 }
 
+__weak_reference(__openat, openat);
+
+int
+__openat(int fd, const char *path, int flags, ...)
+{
+	struct pthread *curthread = _get_curthread();
+	int	ret;
+	int	mode = 0;
+	va_list	ap;
+
+	_thr_cancel_enter(curthread);
+	
+	/* Check if the file is being created: */
+	if (flags & O_CREAT) {
+		/* Get the creation mode: */
+		va_start(ap, flags);
+		mode = va_arg(ap, int);
+		va_end(ap);
+	}
+	
+	ret = __sys_openat(fd, path, flags, mode);
+
+	_thr_cancel_leave(curthread);
+
+	return ret;
+}
+
 __weak_reference(__poll, poll);
 
 int