Kees Cook reviews the security-related enhancements in the 5.1 kernel release. "Now /proc/$pid can be opened and used as an argument for sending signals with the new pidfdsendsignal() syscall. This handle will only refer to the original process at the time the open() happened, and not to any later 'reused' pid if the process dies and a new process is assigned the same pid. Using this method, it’s now possible to racelessly send signals to exactly the intended process without having to worry about pid reuse. (BTW, this commit wins the 2019 award for Most Well Documented Commit Log Justification.)"