Re: @MLton load-word and MacOSX 10.7 (Lion)

On Tue, Jun 19, 2012 at 6:19 AM, Wesley W. Terpstra <[email protected]> wrote:
> On Mon, Jun 18, 2012 at 5:16 PM, Matthew Fluet <[email protected]> wrote:
>> I agree that (a) makes the most sense.  I'll add a note to the
>> documentation on MLton.World that saving and loading worlds won't work
>> with position-independent executables.  As I noted before, '-link-opt
>> -fno-PIE' seems to do the trick on MacOSX Lion.
>
> I believe -fno-PIE just clears the MH_PIE bit at link time. So, the
> code remains position independent, but the loader just doesn't try to
> relocate it.

O.k., that makes sense.  At link time, one wouldn't expect significant
rewriting of the object files.  If they were position independent when
compiled/assembled into the object file, then the executable code
would remain position independent in the executable.

> How will this play out if you use a MLton-built library? It seems to
> me that we could run into this problem even without ASLR. If another
> library the program links changes size, it might cause the MLton
> library to get pushed out of the way and cause the same breakage. On
> the other hand, I guess one doesn't expect "load-world" between
> recompiles of a program and loading new libraries is just a step
> worse.

Indeed, a MLton compiled executable gets a (random) magic number that
is stored with a saved heap.  "load-world" fails if the magic number
in the saved heap does not match the magic number in the executable.

It is, however, true that a MLton-built shared library could be loaded
at different addresses, even if the main executable is not a PIE.  I
think the best option in such a situation would be to build a static
library and link that into a non-PIE.

But, at the same time, I'm not sure of the utility of MLton.World.save
for such a library.  If the SML code isn't "in control", then it seems
that there is likely to be a lot of C-side state in the the program,
which wouldn't be saved/loaded.

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