Executing from readablee, no-exec pages

[Scott Wood]

As revealed by the recent "Prevent data exception in kernel space" 
patch, versions of glibc prior to 2.4[1] assume that, on powerpc32, they 
can execute out of any readable mapping, regardless of whether it is 
marked for execution.  This happens in the elf_machine_load_address() 
function.

To maintain compatibility with these versions, we could change the test 
in do_page_fault() to include VM_READ as well as VM_EXEC on targets that 
don't have a separate exec-bit in hardware (are there any powerpc mmus 
that do?).  However, Segher suggested on IRC that we may want to drop 
compatibility with those old versions of glibc, and that I should seek 
your input.

Personally, I'd rather stick the VM_READ in there, partially for selfish 
reasons (our root filesystems are based on older glibcs), and because it 
seems a little too soon to deprecate glibc 2.3, but also because in the 
absence of hardware support, the VM_EXEC check will be nondeterministic, 
kicking in only when the first fault for a page is to execute.

-Scott

[1] It's possible that there are other instances of this in 2.4 and that 
the actual version is newer; I ran into obnoxious cross compilation 
issues trying to try it.  However,

<rant>
Glibc already has target-specific code/headers; if you need to know 
something that you'd otherwise need a runs-on-the-target autoconf test 
for, why not just stick it in such a target-specific header?  In this 
case, it was trying to figure out the size of "long double".
</rant>