why is openat() needed to avoid a two-step race condition with stat and then open()?

Question

The explanation at

http://man7.org/linux/man-pages/man2/open.2.html

about why openat is needed, reads in part:

openat() allows an application to avoid race conditions that
   could occur when using open() to open files in directories other than
   the current working directory.  These race conditions result from the
   fact that some component of the directory prefix given to open()
   could be changed in parallel with the call to open().  Suppose, for
   example, that we wish to create the file path/to/xxx.dep if the file
   path/to/xxx exists.  The problem is that between the existence check
   and the file creation step, path or to (which might be symbolic
   links) could be modified to point to a different location.

I don't understand why this race is a problem. If an app wants to check for the existence of some file and if so, create a different file, then, of course these are two steps, and the app should and can ensure that nothing interferes in between. Only if a single call to open() could cause a race condition, might some other syscall, such as openat() be needed. Otherwise, this is not for syscalls to solve, but it is an application's responsibility.

What am I not understanding here?

Answer 1

The race is referring only to files that are not in the current directory. The relative path that you are passing to openat() could contain a symlink that points to a different directory than the one you are expecting.

If you only use open() with files in the current directory (after making sure you are where you want to be), you avoid this problem.