I'm trying to do something a bit unorthodox: I want to share the
address space between threads, but I want a certain region of the
address space to be writeable only for a particular thread -- for all
other threads this region is read-only.
I've considered several approaches. I'll only go over the one I think
is the best solution. I would appreciate any comments folks might have:
When linking the userspace program, I instruct the linker to avoid the
region from VMA 0 to VMA 0x400000. Then, in the userspace code, I
duplicate the GDT entries for CS, DS, ES, SS into the LDT, except that
the new segments begin at 0x400000, instead of 0. I load the segment
registers with these LDT entries. In the "write-allowed" thread I
create an LDT entry to address VMA 0 through VMA 0x400000, in
read-write mode. In all the other threads I create an LDT entry to
address the same area in read-only mode. This gives me a 4M region at
the bottom of memory that is accessible in different ways to different
threads.
I can also use the LDT to point to thread-specific segments. IMHO this
is much better than the stack trick used by linuxthreads. The problem
is, I don't fully understand how to use modify_ldt(). Is anyone
knowledgeable about this?
If anyone has any other suggestions, please let me know. If you are
confused as to why I would ever want to do this in the first place, I'd
be willing to go over it off the list.
Thanks,
Scott
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