Re: Comparing PROT_EXEC-only pages on different CPUs

[Richard Curnow <Richard.Curnow@superh.com>]

Hi Jamie

* Jamie Lokier <jamie@shareable.org> [2004-07-02]:
> 
> Richard Henderson wrote:
> > > Richard raises an interesting point: exec-only pages are useless if
> > > the code needs to read jump tables and constant pools.  It seems very
> > > likely Alpha and IA64 have these.
> > 
> > Only if the processor is crippled enough that mixing jump tables and
> > constant pools in the same pages as code is considered reasonable.
> 
> That's a good point, if the i-cache and/or DTLB are separate from
> d-cache and/or DTLB.  Then it makes more sense to put tables in a
> separate address block.
> 
> However if the caches are unified then it makes sense to put them
> together.  Somehow I doubt if any of these 64-bit chips have unified
> i- and d-caches though :)

For the SH-4, and the SH-5 in SHcompact mode, the insertion of constant
pools in amongst the code is essential.  The SH-4 instruction set has
specific "load word/longword from 2N/4N bytes forward of current PC"
instructions (mov.[wl] @(disp,PC),Rm)  for this.  It seems a reasonable
enough way of handling large immediates in a 16-bit encoding.

For SH-5 in SHmedia mode, it probably isn't really necessary to put
switch tables inside the .text section, but it's what the current
toolchains do.

> > Anyway, that's a strawman -- it's the toolchain's job to get the bits
> > on the pt_load segments correct.
> > 
> > If the pt_load segment or the mmap prot argument says execute-only,
> > then you should honor it.
> 
> In other words, PA-RISC and SH64 kernels _should_ create exec-only
> pages if requested, as the hardware can do it, right?
> 
> And the toolchain _should_ ask read permssion for code segments, if
> (and only if) the compiler has generated code which needs that, right?

As it turns out, it looks like the SH-5 toolchains already do that.  The
kernel was forcing readability onto mappings unnecessarily.  I've done
some local fixes to sh64/2.6 today, and the results of the program are
now coming out:

Requested PROT | ---    R--    -W-    RW-    --X    R-X    -WX    RWX
========================================================================
MAP_SHARED     | ---    r--    -w-    rw-    --x    r-x    -wx    rwx
MAP_PRIVATE    | ---    r--    -w-    rw-    --x    r-x    -wx    rwx

-- 
Richard \\\ SH-4/SH-5 Core & Debug Architect
Curnow  \\\         SuperH (UK) Ltd, Bristol