[MirageOS-devel] Fwd: [OCaml 0006167]: OCAMLPARAM Support for Disabling Position-Independent Code (PIC) Generation

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

Thanks to Gabor Pali, the upstream OCaml 4.03 tree now has support for 
disabling PIC code, which was one of the features needed to compile kFreeBSD 
kernel modules in Mirage.  Thanks for pushing this through, Gabor!

-anil

> Begin forwarded message:
> 
> To: anil@xxxxxxxxxx
> Subject: [OCaml 0006167]: OCAMLPARAM Support for Disabling 
> Position-Independent Code (PIC) Generation
> Date: 24 January 2015 16:36:12 GMT
> From: Mantis Bug Tracker <nouser@xxxxxxxx>
> 
> 
> The following issue has been RESOLVED. 
> ====================================================================== 
> http://caml.inria.fr/mantis/view.php?id=6167 
> ====================================================================== 
> Reported By:                pgj
> Assigned To:                gasche
> ====================================================================== 
> Project:                    OCaml
> Issue ID:                   6167
> Category:                   OCaml backend (code generation)
> Reproducibility:            N/A
> Severity:                   feature
> Priority:                   normal
> Status:                     resolved
> Resolution:                 fixed
> Fixed in Version:           4.03.0+dev
> ====================================================================== 
> Date Submitted:             2013-09-10 16:20 CEST
> Last Modified:              2015-01-24 17:36 CET
> ====================================================================== 
> Summary:                    OCAMLPARAM Support for Disabling
> Position-Independent Code (PIC) Generation
> Description: 
> Add support for disabling Position-Independent Code (PIC) generation through 
> the
> OCAMLPARAM environment variable.  This change would make it possible to 
> specify
> a "nopic" token which, when set, instructs the native code generator to emit
> position-dependent code independently of what was requested at the command 
> line.
> 
> The rationale of this option is similar to the other flags already supported 
> by
> OCAMLPARAM: Allow the user to force generation of position-dependent code
> without the need for modifying build system files or sources.
> 
> This patch was directly motivated by the need for making cross-compilation
> possible of the Mirage unikernel [1] to the "kFreeBSD" backend.  This backend
> basically compiles Mirage (OCaml) programs into FreeBSD kernel modules, where
> certain expectations must be satisfied.  One of those expectations is that the
> generated code must not be position-independent as the FreeBSD kernel dynamic
> loader (kld(4)) does not support ELF relocations of that type.  Hence PIC
> generation has to be disabled globally for all the compiled code, regardless
> what the original upstream packages were instructed for.  The "nodynlink" flag
> is also used in this case.
> 
> The attached patch implements this functionality by moving the `pic_code`
> variable to the `Clflags` module for both the AMD64 and the ARM native code
> generator backends, while it retains the default settings for both.  I have
> tested it on FreeBSD/amd64, but it should work on the ARM platform and on 
> other
> operating systems.
> 
> 
> [1] http://openmirage.org/
> 
> ====================================================================== 
> 
> ---------------------------------------------------------------------- 
> (0011174) doligez (administrator) - 2014-04-02 17:11
> http://caml.inria.fr/mantis/view.php?id=6167#c11174 
> ---------------------------------------------------------------------- 
> One small remark: if we add a "nopic" flag we should also add a "pic" flag for
> symmetry (and because all the hard work is already done). 
> 
> ---------------------------------------------------------------------- 
> (0012912) whitequark (developer) - 2014-12-21 14:19
> http://caml.inria.fr/mantis/view.php?id=6167#c12912 
> ---------------------------------------------------------------------- 
> @gasche, can you please take a look at the attached patch? 
> 
> ---------------------------------------------------------------------- 
> (0012913) gasche (developer) - 2014-12-21 16:07
> http://caml.inria.fr/mantis/view.php?id=6167#c12913 
> ---------------------------------------------------------------------- 
> It would be more natural, I think, to use pic=0 and pic=1 to disable or 
> disable
> position-independent code. Given the fact that the key-value-handling logic is
> already present in the OCAMLPARAM code, the current patch allows nopic=1 and
> nopic=0 to respectively disable and enable position-independent code (with
> "nopic" being a shortcut for "nopic=1"), which is a rather obscure interface. 
> If
> people prefer using "nopic" to "pic=0", we could at least support both pic and
> nopic as keys as Damien suggests.
> 
> The other issue I have with the patch is the default-setting logic:
> 
> +let pic_code = ref (String.compare "amd64" Config.architecture == 0)
> +  (* -fPIC (only true by default on amd64) *)
> 
> if you allow me to nitpick (no pun intended), this code would not scale to the
> addition of other architectures. We should pattern-match on 
> Config.architecture,
> with at least a case for each architecture on which the option makes sense
> (having a catch-all pattern with the default value for the others is
> reasonable).
> 
> Is anyone willing to improve the patch on these two aspects? Given the
> non-stellar reaction time, I would assume that the original contributor pgj 
> may
> not be willing to do the extra work. If nobody comes forward by the time of 
> the
> release, I could do it myself. 
> 
> ---------------------------------------------------------------------- 
> (0012914) pgj (reporter) - 2014-12-21 17:26
> http://caml.inria.fr/mantis/view.php?id=6167#c12914 
> ---------------------------------------------------------------------- 
> Sorry, folks, I was busy with other stuff and I did not come to respond to the
> notes.  I think you are right, and yes, I think I could adapt the patch
> according to the recommendations. 
> 
> ---------------------------------------------------------------------- 
> (0012915) gasche (developer) - 2014-12-21 17:39
> http://caml.inria.fr/mantis/view.php?id=6167#c12915 
> ---------------------------------------------------------------------- 
> Nice! I certainly implied no criticism of you, I find it natural to could 
> drift
> a bit after 6 months with no reply. You can take your time to prepare a 
> patch. 
> 
> ---------------------------------------------------------------------- 
> (0012916) pgj (reporter) - 2014-12-21 17:48
> http://caml.inria.fr/mantis/view.php?id=6167#c12916 
> ---------------------------------------------------------------------- 
> Sure, no problem.  Thanks for reminding me :-)  I did not have time for the
> original project either (for which the patch was originally written) so it
> simply got slipped, indeed.
> 
> 
> 
> ---------------------------------------------------------------------- 
> (0013134) pgj (reporter) - 2015-01-17 11:03
> http://caml.inria.fr/mantis/view.php?id=6167#c13134 
> ---------------------------------------------------------------------- 
> Please find a revisited version attached.  It is now adapted to the trunk
> version as of today and theoretically has also the requested changes.  I 
> tested
> it on FreeBSD/amd64, it builds and works for me.  Sorry for the delay :-) 
> 
> ---------------------------------------------------------------------- 
> (0013169) gasche (developer) - 2015-01-24 17:35
> http://caml.inria.fr/mantis/view.php?id=6167#c13169 
> ---------------------------------------------------------------------- 
> Merged in trunk, thanks! 
> 
> Issue History 
> Date Modified    Username       Field                    Change               
> ====================================================================== 
> 2013-09-10 16:20 pgj            New Issue                                    
> 2013-09-10 16:20 pgj            File Added: nopic.diff                       
> 2013-09-11 22:07 doligez        Tag Attached: patch                          
> 2014-04-02 17:09 doligez        Status                   new => confirmed    
> 2014-04-02 17:09 doligez        Target Version            => 4.02.0+dev      
> 2014-04-02 17:11 doligez        Note Added: 0011174                          
> 2014-08-21 12:02 doligez        Target Version           4.02.0+dev =>       
> 2014-12-21 14:19 whitequark     Note Added: 0012912                          
> 2014-12-21 16:07 gasche         Note Added: 0012913                          
> 2014-12-21 16:07 gasche         Tag Attached: junior_job                     
> 2014-12-21 17:26 pgj            Note Added: 0012914                          
> 2014-12-21 17:39 gasche         Note Added: 0012915                          
> 2014-12-21 17:48 pgj            Note Added: 0012916                          
> 2014-12-21 17:48 pgj            Note Edited: 0012916                         
> 2015-01-17 11:01 pgj            File Added: ocamlparam_pic.diff               
>   
> 
> 2015-01-17 11:03 pgj            Note Added: 0013134                          
> 2015-01-24 17:35 gasche         Note Added: 0013169                          
> 2015-01-24 17:36 gasche         Status                   confirmed => resolved
> 2015-01-24 17:36 gasche         Fixed in Version          => 4.03.0+dev      
> 2015-01-24 17:36 gasche         Resolution               open => fixed       
> 2015-01-24 17:36 gasche         Assigned To               => gasche          
> ======================================================================
> 


_______________________________________________
MirageOS-devel mailing list
MirageOS-devel@xxxxxxxxxxxxxxxxxxxx
http://lists.xenproject.org/cgi-bin/mailman/listinfo/mirageos-devel