https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219399

--- Comment #216 from Don Lewis <truck...@freebsd.org> ---
LOL ... 

Prior to the fix in r321899, the top page of user memory for amd64 executables
was used by the shared page, the contents of which are controlled by the
kernel.  This page does contain the signal trampoline, which contains a SYSCALL
instruction, which made me very suspicious based on my experiments with
executing code in this page.  The SYSCALL instruction is located well away from
the top of the page, though.  I may try playing with this instruction if I ever
have the time.

After r321899, the shared page is moved lower and we don't allow the top page
to be used at all, similar to Linux.  CloudABI64 got a similar fix.

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