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There is currently no clean and efficient way to apply mprotect to all of a
program's stack, i.e. to the moving edge of a GROWSDOWN or GROWSUP mapping.
Some processes want to change these protections, particularly to set or
clear the PROT_EXEC bits on stack space.  As it is, an mprotect done to
cover the precise edge page of the mapping will have the desired effect of
changing the protection for existing pages and having that new protection
carried over to new pages grown later.  But there is no very reasonable way
of ascertaining where the edge of the mapping is if it might have grown in
the past beyond the usage at the moment.  An mprotect call that doesn't
cover the edge page splits the mapping and doesn't do what we need.

This patch adds flags that can be OR'd into the protection bits in an
mprotect system call.  PROT_GROWSDOWN means the memory lies in a GROWSDOWN
mapping and the start address of the region to be changed should be
extended down t...