8374945: Avoid fstat in os::open

Reviewed-by: dholmes, jsjolen, redestad

src/hotspot/os/linux/os_linux.cpp

@@ -4963,9 +4963,14 @@ int os::open(const char *path, int oflag, int mode) {
   oflag |= O_CLOEXEC;
 
   int fd = ::open(path, oflag, mode);
-  if (fd == -1) return -1;
+  // No further checking is needed if open() returned an error or
+  // access mode is not read only.
+  if (fd == -1 || (oflag & O_ACCMODE) != O_RDONLY) {
+    return fd;
+  }
 
-  //If the open succeeded, the file might still be a directory
+  // If the open succeeded and is read only, the file might be a directory
+  // which the JVM doesn't allow to be read.
   {
     struct stat buf;
     int ret = ::fstat(fd, &buf);