verify that ttyname refers to the same file as the fd

linux containers use separate mount namespace so the /proc
symlink might not point to the right device if the fd was
opened in the parent namespace, in this case return ENOENT.

src/unistd/ttyname_r.c

@@ -1,10 +1,12 @@
 #include <unistd.h>
 #include <errno.h>
+#include <sys/stat.h>
 
 void __procfdname(char *, unsigned);
 
 int ttyname_r(int fd, char *name, size_t size)
 {
+	struct stat st1, st2;
 	char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
 	ssize_t l;
 
@@ -15,8 +17,13 @@ int ttyname_r(int fd, char *name, size_t size)
 
 	if (l < 0) return errno;
 	else if (l == size) return ERANGE;
-	else {
-		name[l] = 0;
-		return 0;
-	}
+
+	name[l] = 0;
+
+	if (stat(name, &st1) || fstat(fd, &st2))
+		return errno;
+	if (st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino)
+		return ENOENT;
+
+	return 0;
 }