On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu [off-list ref] wrote:

When a task does fork(), its shadow stack (SHSTK) must be duplicated
for the child.  This patch implements a flow similar to copy-on-write
of an anonymous page, but for SHSTK.

A SHSTK PTE must be RO and dirty.  This dirty bit requirement is used
to effect the copying.  In copy_one_pte(), clear the dirty bit from a
SHSTK PTE to cause a page fault upon the next SHSTK access.  At that
time, fix the PTE and copy/re-use the page.

Is using VM_SHSTK and special-casing all of this really better than
using a special mapping or other pseudo-file-backed VMA and putting
all the magic in the vm_operations?

--Andy