[hyperman1]

Following the man pages:

wait4's man page points to waitpid for details, and notes wait4 is deprecated in favor of waitpid.

So see the linux notes of this: https://man7.org/linux/man-pages/man2/waitpid.2.html

  The following Linux-specific options [..] can also, since Linux 4.7, be used with waitid():
  __WCLONE  [...] This option is ignored if __WALL is also specified.
  __WALL

So to trigger this:

* You have to call a deprecated function

* With a flag that was at that time illegal (linux < 4.7)

* And a second illegal flag that is cancelled out by the first illegal flag.

This is something any userspace process can do, but no sane process should ever do.

[stareatgoats]

Ok thanks, that clinches it I think!