We were using 0x08000000 instead of TASK_UNMAPPED_BASE
so that running something like "/lib/ld-linux.so.2 emacs"
would work.

The issue there was that wherever /lib/ld-linux.so.2 gets
mapped (controlled by ELF_ET_DYN_BASE), that is where the
BSS start for the process ends up.  Now, emacs allocates
dynamic memory for LISP objects from the BSS, and needs
the top 4 bits of the virtual address to be clear so that
it can encode LISP type and GC marking information there.

But making this obscure emacs case work breaks lots of other
stuff.  For example, programs with a reasonably large data
section fail to load via direct ld.so interpreter execution
because the data section is large enough to begin overlapping
with the ELF_ET_DYN_BASE area.

The /lib/ld-linux.so.2 emacs case does not work on a lot of
platforms due to this issue, including i386, so it is not
worth making work on sparc either.  It is indeed useful
sometimes when debugging a new experimental build of glibc
for example, but people doing that can hack the value of
ELF_ET_DYN_BASE in their kernels.  Perhaps at some point
we will make a sysctl controllable value.
Signed-off-by: David S. Miller <davem@davemloft.net>