aarch64: enable tagged pointers (TBI — Top Byte Ignored)

[val_packett.cool]

Summary

TBI is a very useful feature of ARMv8 that allows the top byte of pointers to be used as a tag. This is useful for various JIT compilers, HWASAN etc. This has been enabled on Linux since 2013, but not on FreeBSD yet. (upd: also on Linux it's per-process toggleable via procctl or something now?)

Linux clears the bits on EL0 data abort, so I've done the same (??) here — not sure about that actually.

Test Plan

This example

#include <stdio.h>
#include <stdint.h>

static inline char *tag(char *ptr) {
        return (char*)(((uintptr_t)ptr & 0xffffffffffff) | (69L << 56));
}

int main() {
        char v = 'x';
        printf("ptr %p val %c\n", &v, *(&v));
        printf("ptr %p\n", tag(&v));
        printf("val %c\n", *tag(&v));
        return 0;
}

no longer segfaults on the last dereference.

UPD: also, I got HWASAN working :) my current llvm80 patches are here (I'll try to keep the link here up to date)

[val_packett.cool]

Rebased on top of the ASID changes

[val_packett.cool]

Rebased

[me_mko.io]

TBI seems like a very useful feature for mitigate ROP. thanks for your work

[val_packett.cool]

Rebased.