Add a BTI sysarch

[andrew]

Summary

This is used to enable the guard page when an elf binary is built with
BTI instructions.

Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation

[andrew]

• Rebase on D42328
• Move ARM64_GUARD_PAGE from 0 to ensure userspace intends to set it

[markj]

Where will BTI be documented? Perhaps bti.3 and mitigations.7?

sys/arm64/arm64/sys_machdep.c

49

75

IMO it is clearer to write this in terms of a range:

sva = trunc_page(gp_args.addr);
eva = round_page(gp_args.addr + gp_args.len);
if (sva > eva || eva > VM_MAX_USER_ADDRESS)
    return (EINVAL);
error = pmap_pti_set(pmap, sva, eva);

77

Shouldn't the pmap should be derived from td, not using pcpu? This looks strange, I don't know why PKRU does it this way.

78

pmap_bti_set() returns 0 if the executable didn't have BTI configured. Don't we want to return an error here in that case?

sys/arm64/include/sysarch.h

42

What is this constant for?