Limiting malloc to the low 2GB?

[Valery Ushakov <uwe%stderr.spb.ru@localhost>]

Do we have a way to tell malloc on a 32-bit system to allocate memory
only below the 2GB boundary (on i386, including when run under amd64)?
I'm trying to port a(n old) program that wants to use the sign bit for
its internal purposes.  I guess one option would be to prevent malloc
from using mmap (and disable alsr?) so that only sbrk (in the low 2GB)
is used.

Suggestions are appreciated.

-uwe