Hi All,
I've been researching some aspects of the PowerPC chip with respect to
W^X (write xor execute). I have some questions that I could discuss
offline or leave open to the general list.

In looking over the Programming Environment Manual for 32 bit
Microprocessors, there are specific references to being able to set
segments to read, read-write, and executable-only (Chapter 7).
Additionally, it appears that the PPC can handle segments combined with
paging. I found a discuss at

http://www.pantek.com/library/general/lists/openbsd.org/tech/msg00718.html

where this was discussed on an OpenBSD mailing list. I don't fully
follow the conclusion, which is that because the segments have to be
256MB each, W^X can't be done on PPC. Part of that also had to do with
certain sections of ELF executables, which I am not knowledgeable
enough to fully grok.

My immediate question is: would it be possible to set certain
256M segments(via BATS?) to be read-write and other segments to
executable-only, and during the loading of an executable image place the
.text section in the latter and the .data and .bss in the former (using
xcoff terminology)? I know that this would require the kernel to alter
the priviledges of the executable during image loading, but it would
ideally limit the damage of user-land buffer overflows because those
overflows would write into a no-execute segment.

Would the bulk of the issue reside in the ld linker, or in the memory
management? Would the ld linker need to be rewired? If so, and I would
guess that this would break existing compiled applications, is achieving
this worth the loss of backwards compatibility? 

Is it written somewhere that in segment page combinations all parts must
be within the same segment?

I know that 4.4BSD is purely paged. However, would it be possible to
shim prior to pmap such that pmap gets fooled into thinking it is purely
paged? Can pmap differentiate between one virtual address space and
another if both are established prior to pmap? Would it be possible at
the page level to exclude two processes in the same segment from seeing
each other?

Although OpenBSD explored this issue, I have perceived a bias against
the PowerPC CPU and I wonder if the problems are not because of the cpu
but because of the specific, immutable approach they wanted to take.
Given that there is a bit more room to branch within NetBSD, I wondered
if there may be a higher chance for success in doing this here.

Comments and feedback appreciated, offline or online.

thanks,
tim