It used to be mapped at the top of the UVA.
If the randomization is enabled any address above .data section will be
randomly chosen and a guard page will be inserted in the shared page
default location.
The shared page is now mapped in exec_map_stack, instead of
exec_new_vmspace. The latter function is called before image activator
has a chance to parse ASLR related flags.
The KERN_PROC_VM_LAYOUT sysctl was extended to provide shared page
address.
The feature is enabled by default for 64 bit applications on all
architectures.
It can be toggled kern.elf64.aslr.shared_page sysctl.
Approved by: mw(mentor)
Sponsored by: Stormshield
Obtained from: Semihalf
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D35349
939f0b6323