Re: [PATCH v2] arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS)

* Dave Martin:

IIUC the default compiler options when using PAC will only use the
A key, and only use the PAC instructions that execute as NOPs when the
affected key is disabled (precisely so that the code still runs on non-
PAC supporting hardware).  But you can't simply flip it on and off while
there are function frames on the stack that assume it's either on or off.

I think we can do the switch at the top-most frame, at least in ld.so.
I have not thought about statically linked binaries. 8-/