Cook: security things in Linux v5.0

Kees Cook reviewssome of the security-related enhancements in the 5.0 kernel."While the C language has a statement to indicate the end of a switchcase ('break'), it doesn't have a statement to indicate that executionshould fall through to the next case statement (just the lack of a 'break'is used to indicate it should fall through - but this is not always thecase), and such 'implicit fall-through' may lead to bugs. Gustavo Silva hasbeen the driving force behind fixing these since at least v4.14, with wellover 300 patches on the topic alone (and over 20 missing break statementsfound and fixed as a result of the work). The goal is to be able to add-Wimplicit-fallthrough to the build so that the kernel will stay entirelyfree of this class of bug going forward. From roughly 2300 warnings, thekernel is now down to about 200. It's also worth noting that with StephenRothwell's help, this bug has been kept out of linux-next by him sendingwarning emails to any tree maintainers where a new instance is introduced(for example, here's a bug introduced on Feb 20th and fixed on Feb21st)."