Merge tag 'for-linus-2020-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux

Pull thread fix from Christian Brauner:
 "This contains a single fix for a regression which was introduced when
  we introduced the ability to select a specific pid at process creation
  time.

  When this feature is requested, the error value will be set to -EPERM
  after exiting the pid allocation loop. This caused EPERM to be
  returned when e.g. the init process/child subreaper of the pid
  namespace has already died where we used to return ENOMEM before.

  The first patch here simply fixes the regression by unconditionally
  setting the return value back to ENOMEM again once we've successfully
  allocated the requested pid number. This should be easy to backport to
  v5.5.

  The second patch adds a comment explaining that we must keep returning
  ENOMEM since we've been doing it for a long time and have explicitly
  documented this behavior for userspace. This seemed worthwhile because
  we now have at least two separate example where people tried to change
  the return value to something other than ENOMEM (The first version of
  the regression fix did that too and the commit message links to an
  earlier patch that tried to do the same.).

  I have a simple regression test to make sure we catch this regression
  in the future but since that introduces a whole new selftest subdir
  and test files I'll keep this for v5.7"

* tag 'for-linus-2020-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
  pid: make ENOMEM return value more obvious
  pid: Fix error return value in some cases

kernel/pid.c

@@ -247,6 +247,16 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 		tmp = tmp->parent;
 	}
 
+	/*
+	 * ENOMEM is not the most obvious choice especially for the case
+	 * where the child subreaper has already exited and the pid
+	 * namespace denies the creation of any new processes. But ENOMEM
+	 * is what we have exposed to userspace for a long time and it is
+	 * documented behavior for pid namespaces. So we can't easily
+	 * change it even if there were an error code better suited.
+	 */
+	retval = -ENOMEM;
+
 	if (unlikely(is_child_reaper(pid))) {
 		if (pid_ns_prepare_proc(ns))
 			goto out_free;