This patch, by Jake Moilanen with some further hacking from me, adds a
real execute permission bit to the linux PTEs on PPC64, and connects
that into the kernel infrastructure for implementing non-executable
stacks and heaps.  This means that on any PPC64 cpu since the POWER4
(i.e. POWER4, PPC970, PPC970FX, POWER4+, POWER5) you will get a
segfault if you try to execute instructions from a region that doesn't
have PROT_EXEC permission.  The patch also marks the pages of the
linear mapping that aren't part of the kernel text as non-executable.

Andrew and Linus, could you try this on your G5s?  I have tried it
here on a Debian system and a SLES9 system and everything runs fine,
but I haven't been able to try it on YDL, FC or RHEL4.

With this patch we default to executable stack and read-implies-exec
behaviour when there is no PT_GNU_STACK program header entry, or when
there is one and it indicates the stack is executable.  For 32-bit
processes, the heap is always executable, because the PLT contains
instructions and it ends up in the bss segment.
Signed-off-by: Jake Moilanen <moilanen@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>