[StackProtector] attribute __stack_chk_fail as NoReturn
When GCC added support for stack smashing protections, it was defined that: > This hook returns a CALL_EXPR that alerts the runtime that the stack > protect guard variable has been modified. This expression should > involve a call to a noreturn function. > The default version of this hook invokes a function called > ‘__stack_chk_fail’, taking no arguments. Do so as well for __stack_smash_handler for OpenBSD. Every libc implementation I could find has __stack_chk_fail marked noreturn, or the implementation calls abort, exit, or panic (which themselves are noreturn). Glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=debug/stack_chk_fail.c Musl: https://git.musl-libc.org/cgit/musl/tree/src/env/__stack_chk_fail.c Bionic: https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/bionic/__stack_chk_fail.cpp FreeBSD: https://cgit.freebsd.org/src/tree/lib/libc/secure/stack_protector.c OpenBSD: https://github.com/openbsd/src/blob/master/lib/libc/sys/stack_protector.c NetBSD: https://github.com/NetBSD/src/blob/trunk/lib/libc/misc/stack_protector.c Linux Kernel: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/panic.c Apple: https://opensource.apple.com/source/Libc/Libc-1439.40.11/sys/OpenBSD/stack_protector.c.auto.html Link: https://gcc.gnu.org/onlinedocs/gccint/Stack-Smashing-Protection.html#Stack-Smashing-Protection This will later help us diagnose functions that fall through to other functions vs end in calls to functions that are noreturn. Reviewed By: efriedma Differential Revision: https://reviews.llvm.org/D146339
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