libc: Improve POSIX conformance of dirfd()

POSIX states that dirfd() should set errno to EINVAL and return -1 if
dirp does not refer to a valid directory stream.  Our interpretation is
that this applies if dirp is null or the file descriptor associated
with it is negative.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D55025

lib/libc/gen/directory.3

@@ -256,7 +256,9 @@ The
 function returns 0 on success and -1 on failure.
 The
 .Fn fdclosedir
-function returns an open file descriptor on success and -1 on failure.
+and
+.Fn dirfd
+functions return an open file descriptor on success and -1 on failure.
 .Sh ERRORS
 The
 .Fn opendir
@@ -327,6 +329,16 @@ function may also fail and set
 .Va errno
 for any of the errors specified for the routine
 .Xr close 2 .
+.Pp
+The
+.Fn dirfd
+function will fail if:
+.Bl -tag -width Er
+.It Bq Er EINVAL
+The
+.Fa dirp
+argument does not refer to a valid directory stream.
+.El
 .Sh SEE ALSO
 .Xr close 2 ,
 .Xr lseek 2 ,

lib/libc/gen/dirfd.c

@@ -28,6 +28,7 @@
 
 #include "namespace.h"
 #include <dirent.h>
+#include <errno.h>
 #include "un-namespace.h"
 
 #include "gen-private.h"
@@ -35,5 +36,9 @@
 int
 dirfd(DIR *dirp)
 {
+	if (dirp == NULL || _dirfd(dirp) < 0) {
+		errno = EINVAL;
+		return (-1);
+	}
 	return (_dirfd(dirp));
 }