Add O_SYMLINK emulation

for MacOSX partial compatibility, defined as O_PATH | O_SYNC | O_DIRECT.

libc openat() wrapper is modified to fstat() the descriptor and re-open
in the normal mode if the type is not symlink.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56365

lib/libc/include/libc_private.h

@@ -384,4 +384,7 @@ struct uexterror;
 int __uexterr_format(const struct uexterror *ue, char *buf, size_t bufsz);
 int __libc_uexterr_gettext(char *buf, size_t bufsz);
 
+int __impl_openat(int fd, const char *path, int flags, ...);
+int __openat_symlink(int fd, const char *path, int flags, int interposed);
+
 #endif /* _LIBC_PRIVATE_H_ */

lib/libc/sys/Symbol.map

@@ -75,3 +75,7 @@ FBSD_1.7 {
 FBSD_1.9 {
 	pdwait;
 };
+
+FBSDprivate_1.0 {
+	__openat_symlink;
+};

lib/libc/sys/open.c

@@ -29,9 +29,11 @@
  * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include "namespace.h"
 #include <sys/types.h>
 #include <sys/fcntl.h>
 #include <stdarg.h>
+#include "un-namespace.h"
 #include "libc_private.h"
 
 #pragma weak open
@@ -48,5 +50,5 @@ open(const char *path, int flags, ...)
 	} else {
 		mode = 0;
 	}
-	return (INTERPOS_SYS(openat, AT_FDCWD, path, flags, mode));
+	return (__impl_openat(AT_FDCWD, path, flags, mode));
 }

lib/libc/sys/openat.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014 The FreeBSD Foundation.
+ * Copyright 2014, 2026 The FreeBSD Foundation.
  *
  * Portions of this software were developed by Konstantin Belousov
  * under sponsorship from the FreeBSD Foundation.
@@ -29,11 +29,46 @@
  * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include "namespace.h"
 #include <sys/types.h>
 #include <sys/fcntl.h>
+#include <sys/stat.h>
+#include <errno.h>
 #include <stdarg.h>
+#include <unistd.h>
+#include "un-namespace.h"
 #include "libc_private.h"
 
+static int
+do_openat(int fd, const char *path, int flags, int interposed)
+{
+	if (interposed)
+		return (__sys_openat(fd, path, flags | O_PATH, 0));
+	return (INTERPOS_SYS(openat, fd, path, flags | O_PATH, 0));
+}
+
+int
+__openat_symlink(int fd, const char *path, int flags, int interposed)
+{
+	struct stat st;
+	int rfd, xfd, saved_errno;
+
+	flags &= ~O_SYMLINK;
+	rfd = do_openat(fd, path, flags | O_PATH | O_NOFOLLOW, interposed);
+	if (rfd != -1 && _fstat(rfd, &st) != -1 && !S_ISLNK(st.st_mode)) {
+		xfd = do_openat(rfd, "", flags | O_EMPTY_PATH, interposed);
+		saved_errno = errno;
+		/* dup to rfd to guarantee lowest fd number value */
+		if (_dup2(xfd, rfd) == -1) {
+			_close(rfd);
+			rfd = -1;
+		}
+		_close(xfd);
+		errno = saved_errno;
+	}
+	return (rfd);
+}
+
 __sym_compat(openat, __impl_openat, FBSD_1.1);
 __weak_reference(openat, __impl_openat);
 __sym_default(openat, openat, FBSD_1.2);
@@ -45,12 +80,19 @@ openat(int fd, const char *path, int flags, ...)
 	va_list ap;
 	int mode;
 
+	if (__predict_false((flags & (O_SYMLINK | O_CREAT)) ==
+	    (O_SYMLINK | O_CREAT))) {
+		errno = EINVAL;
+		return (-1);
+	}
 	if ((flags & O_CREAT) != 0) {
 		va_start(ap, flags);
 		mode = va_arg(ap, int);
 		va_end(ap);
 	} else {
 		mode = 0;
+		if (__predict_false((flags & O_SYMLINK) == O_SYMLINK))
+			return (__openat_symlink(fd, path, flags, 0));
 	}
 	return (INTERPOS_SYS(openat, fd, path, flags, mode));
 }

lib/libthr/thread/thr_syscalls.c

@@ -298,7 +298,10 @@ __thr_openat(int fd, const char *path, int flags, int mode)
 
 	curthread = _get_curthread();
 	_thr_cancel_enter(curthread);
-	ret = __sys_openat(fd, path, flags, mode);
+	if (__predict_false((flags & O_SYMLINK) == O_SYMLINK))
+		ret = __openat_symlink(fd, path, flags, 1);
+	else
+		ret = __sys_openat(fd, path, flags, mode);
 	_thr_cancel_leave(curthread, ret == -1);
 
 	return (ret);

sys/sys/fcntl.h

@@ -145,10 +145,10 @@ typedef	__pid_t		pid_t;
 
 /*
  * Emulate MacOSX compatibility flag without consuming a flags bit.
- * It is not fully correct since reads over regular files opened with
- * this definition fail.
+ * Selected bits set does not define a useful open request and is
+ * unlikely to be specified by reasonable code.
  */
-#define	O_SYMLINK	(O_PATH | O_NOFOLLOW)
+#define	O_SYMLINK	(O_PATH | O_DSYNC | O_DIRECT)
 #endif
 
 #if __POSIX_VISIBLE >= 202405