[Caml-list] Segmentationfault on 64bit when called from C

[Caml-list] Segmentationfault on 64bit when called from C

[Georg Martius]

Dear all,

I experienced a very intricate bug in the 64bit runtime that took me some days 
to nail down. I submitted a bug report #5707. 
Here is what I found: When ocaml code is compiled as as a libary and called 
from C++ under 64bit I get a broken stack for functions with more than 8 
parameters. I compiled ocaml 3.12.1 version with -fPIC. BTW: everthing works 
fine on 32bit.

The problem occurs when a function with more than 8 parameters is called. The 
last parameters do have the right value. If afterwards a reference is accessed 
then a segmentation fault occurs, see the code below.

Here the code: (in the bug report I also attached a tar file)
---- m1.ml ------------------
module M2 = struct
  let v      = ref 0;;

  let foo p1 p2 p3 p4 p5 p6 p7 p8 p9 =
    prerr_endline "start";
    print_endline (string_of_int p1);
    print_endline (string_of_int p2);
    print_endline (string_of_int p3);
    print_endline (string_of_int p4);
    print_endline (string_of_int p5);
    print_endline (string_of_int p6);
    print_endline (string_of_int p7);
    print_endline (string_of_int p8);
    print_endline (string_of_int p9);
    print_endline (string_of_int !v);
    prerr_endline "end";
  ;;
end;;


let test i : unit =
  print_endline ("Initialising: " ^ (string_of_int i));
  M2.foo 1 2 3 4 5 6 7 8 9;
;;

(* test 10;;*)

let _ = Callback.register "test"         test;;
----- end m1.ml

----- interface.c
#include <caml/mlvalues.h>
#include <caml/callback.h>

#include "interface.h"

void ocaml_initialize(char** argv){
  caml_main(argv);
}

void ocaml_test(){
  value* test_pointer = caml_named_value("test");
  caml_callback(*test_pointer, Val_int(11));
}
------ end interface.c

----- interface.h
void ocaml_initialize(char** argv);

void ocaml_test();
----- end interface.h

You need ocaml to be compiled with -fPIC on order to be able to generate a 
shared libary on 64bit, as written in the INSTALL
./configure -cc "gcc -fPIC" -aspp "gcc -c -fPIC"

ocamlopt -o libmytest.so -ccopt -shared interface.c m1.ml
g++ -Wall main.cpp  -lmytest -o test
./test
---- output:
Initialising: 11
start
1
2
3
4
5
6
7
8
70367718054944
Segmentation fault (core dumped)
--- end outpuyt

Some Observations:
Calling it natively as an ocaml executable works.
Commenting the access to !v removes the segmentation fault but still the 
values are wrong.
Uncommenting line "test 10", causes the error already at this call and the 
subsequent call from C runs fine!

Best regards,
  Georg

Re: [Caml-list] Segmentationfault on 64bit when called from C

[Gerd Stolpmann]

Just a rough guess: The frametable is not initialized. Quite funny that
it crashes so quickly. (Maybe more is not initialized, maybe the heap
register, so it crashes at the first allocation.)

You cannot just link Ocaml code without adding the frametable. This is a
description of how the stack and the CPU registers are used at runtime.
If it is missing the OCaml runtime thinks that the Ocaml code is C code,
and applies the wrong conventions.

The -shared switch generates a library that is prepared to be loaded
with Dynlink. Dynlink registers the frametable.

You should consider using -output-obj to create a library that can be
loaded without such tricks (see section 18.7.5 of the Ocaml manual).

Gerd

Am Dienstag, den 31.07.2012, 14:17 +0200 schrieb Georg Martius:
> Dear all,
> 
> I experienced a very intricate bug in the 64bit runtime that took me some days 
> to nail down. I submitted a bug report #5707. 
> Here is what I found: When ocaml code is compiled as as a libary and called 
> from C++ under 64bit I get a broken stack for functions with more than 8 
> parameters. I compiled ocaml 3.12.1 version with -fPIC. BTW: everthing works 
> fine on 32bit.
> 
> The problem occurs when a function with more than 8 parameters is called. The 
> last parameters do have the right value. If afterwards a reference is accessed 
> then a segmentation fault occurs, see the code below.
> 
> Here the code: (in the bug report I also attached a tar file)
> ---- m1.ml ------------------
> module M2 = struct
>   let v      = ref 0;;
> 
>   let foo p1 p2 p3 p4 p5 p6 p7 p8 p9 =
>     prerr_endline "start";
>     print_endline (string_of_int p1);
>     print_endline (string_of_int p2);
>     print_endline (string_of_int p3);
>     print_endline (string_of_int p4);
>     print_endline (string_of_int p5);
>     print_endline (string_of_int p6);
>     print_endline (string_of_int p7);
>     print_endline (string_of_int p8);
>     print_endline (string_of_int p9);
>     print_endline (string_of_int !v);
>     prerr_endline "end";
>   ;;
> end;;
> 
> 
> let test i : unit =
>   print_endline ("Initialising: " ^ (string_of_int i));
>   M2.foo 1 2 3 4 5 6 7 8 9;
> ;;
> 
> (* test 10;;*)
> 
> let _ = Callback.register "test"         test;;
> ----- end m1.ml
> 
> ----- interface.c
> #include <caml/mlvalues.h>
> #include <caml/callback.h>
> 
> #include "interface.h"
> 
> void ocaml_initialize(char** argv){
>   caml_main(argv);
> }
> 
> void ocaml_test(){
>   value* test_pointer = caml_named_value("test");
>   caml_callback(*test_pointer, Val_int(11));
> }
> ------ end interface.c
> 
> ----- interface.h
> void ocaml_initialize(char** argv);
> 
> void ocaml_test();
> ----- end interface.h
> 
> You need ocaml to be compiled with -fPIC on order to be able to generate a 
> shared libary on 64bit, as written in the INSTALL
> ./configure -cc "gcc -fPIC" -aspp "gcc -c -fPIC"
> 
> ocamlopt -o libmytest.so -ccopt -shared interface.c m1.ml
> g++ -Wall main.cpp  -lmytest -o test
> ./test
> ---- output:
> Initialising: 11
> start
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 70367718054944
> Segmentation fault (core dumped)
> --- end outpuyt
> 
> Some Observations:
> Calling it natively as an ocaml executable works.
> Commenting the access to !v removes the segmentation fault but still the 
> values are wrong.
> Uncommenting line "test 10", causes the error already at this call and the 
> subsequent call from C runs fine!
> 
> Best regards,
>   Georg
> 
> 

-- 
------------------------------------------------------------
Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
Creator of GODI and camlcity.org.
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
*** Searching for new projects! Need consulting for system
*** programming in Ocaml? Gerd Stolpmann can help you.
------------------------------------------------------------

Re: [Caml-list] Segmentationfault on 64bit when called from C

[Anil Madhavapeddy]

Interesting; I'm seeing a similar issue that could be to do with the frametable
not being initialised soon enough on OSX Mountain Lion.  It only manifests when
recording backtraces in native mode, but I haven't quite had the time to chase
it down fully yet.  However, the below bug report does show where you can init
the frametable earlier to work around the bug.

http://caml.inria.fr/mantis/view.php?id=5700

-anil

On 31 Jul 2012, at 13:17, Georg Martius <georg.martius@web.de> wrote:

> Dear all,
> 
> I experienced a very intricate bug in the 64bit runtime that took me some days 
> to nail down. I submitted a bug report #5707. 
> Here is what I found: When ocaml code is compiled as as a libary and called 
> from C++ under 64bit I get a broken stack for functions with more than 8 
> parameters. I compiled ocaml 3.12.1 version with -fPIC. BTW: everthing works 
> fine on 32bit.
> 
> The problem occurs when a function with more than 8 parameters is called. The 
> last parameters do have the right value. If afterwards a reference is accessed 
> then a segmentation fault occurs, see the code below.
> 
> Here the code: (in the bug report I also attached a tar file)
> ---- m1.ml ------------------
> module M2 = struct
>  let v      = ref 0;;
> 
>  let foo p1 p2 p3 p4 p5 p6 p7 p8 p9 =
>    prerr_endline "start";
>    print_endline (string_of_int p1);
>    print_endline (string_of_int p2);
>    print_endline (string_of_int p3);
>    print_endline (string_of_int p4);
>    print_endline (string_of_int p5);
>    print_endline (string_of_int p6);
>    print_endline (string_of_int p7);
>    print_endline (string_of_int p8);
>    print_endline (string_of_int p9);
>    print_endline (string_of_int !v);
>    prerr_endline "end";
>  ;;
> end;;
> 
> 
> let test i : unit =
>  print_endline ("Initialising: " ^ (string_of_int i));
>  M2.foo 1 2 3 4 5 6 7 8 9;
> ;;
> 
> (* test 10;;*)
> 
> let _ = Callback.register "test"         test;;
> ----- end m1.ml
> 
> ----- interface.c
> #include <caml/mlvalues.h>
> #include <caml/callback.h>
> 
> #include "interface.h"
> 
> void ocaml_initialize(char** argv){
>  caml_main(argv);
> }
> 
> void ocaml_test(){
>  value* test_pointer = caml_named_value("test");
>  caml_callback(*test_pointer, Val_int(11));
> }
> ------ end interface.c
> 
> ----- interface.h
> void ocaml_initialize(char** argv);
> 
> void ocaml_test();
> ----- end interface.h
> 
> You need ocaml to be compiled with -fPIC on order to be able to generate a 
> shared libary on 64bit, as written in the INSTALL
> ./configure -cc "gcc -fPIC" -aspp "gcc -c -fPIC"
> 
> ocamlopt -o libmytest.so -ccopt -shared interface.c m1.ml
> g++ -Wall main.cpp  -lmytest -o test
> ./test
> ---- output:
> Initialising: 11
> start
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 70367718054944
> Segmentation fault (core dumped)
> --- end outpuyt
> 
> Some Observations:
> Calling it natively as an ocaml executable works.
> Commenting the access to !v removes the segmentation fault but still the 
> values are wrong.
> Uncommenting line "test 10", causes the error already at this call and the 
> subsequent call from C runs fine!
> 
> Best regards,
>  Georg
> 
> 
> -- 
> Caml-list mailing list.  Subscription management and archives:
> https://sympa-roc.inria.fr/wws/info/caml-list
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> Bug reports: http://caml.inria.fr/bin/caml-bugs
> 

Re: [Caml-list] Segmentationfault on 64bit when called from C

[Georg Martius]

On Tuesday 31 July 2012 14:59:14 Gerd Stolpmann wrote:
> Just a rough guess: The frametable is not initialized. Quite funny that
> it crashes so quickly. (Maybe more is not initialized, maybe the heap
> register, so it crashes at the first allocation.)
> 
> You cannot just link Ocaml code without adding the frametable. This is a
> description of how the stack and the CPU registers are used at runtime.
> If it is missing the OCaml runtime thinks that the Ocaml code is C code,
> and applies the wrong conventions.
> 
> The -shared switch generates a library that is prepared to be loaded
> with Dynlink. Dynlink registers the frametable.
> 
> You should consider using -output-obj to create a library that can be
> loaded without such tricks (see section 18.7.5 of the Ocaml manual).
ocamlopt -output-obj -o camlcode.o interface.c m1.ml
gcc -fPIC -g -Wall  -c -I/usr/local/lib/ocaml interface.c
gcc -fPIC -o test main.c camlcode.o  interface.o -L/usr/local/lib/ocaml -lunix 
-lm -lasmrun -ldl

works, but 
ocamlopt -o libmytest.so -output-obj  interface.c m1.ml
gcc -Wall main.c  -lmytest -o test
does not! Is it clear why it doesn't work?

So I get away with static linking for now.

Thanks!
  Georg

-- 
---- Georg Martius,  Tel: +49 177 6413311  -----
------- http://www.flexman.homeip.net ----------

Re: [Caml-list] Segmentationfault on 64bit when called from C

[Xavier Leroy]

On Tuesday 31 July 2012 14:59:14 Gerd Stolpmann wrote:

> The -shared switch generates a library that is prepared to be loaded
> with Dynlink. Dynlink registers the frametable.

You're right that .so libraries produced by -shared should only be
used in conjunction with Dynlink.  However, in this case the frame
table isn't the culprit, as the crash occurs before the GC even has a
chance to run.

On 07/31/2012 04:03 PM, Georg Martius wrote:

> ocamlopt -output-obj -o camlcode.o interface.c m1.ml
> gcc -fPIC -g -Wall  -c -I/usr/local/lib/ocaml interface.c
> gcc -fPIC -o test main.c camlcode.o  interface.o -L/usr/local/lib/ocaml -lunix 
> -lm -lasmrun -ldl
> 
> works, but 
> ocamlopt -o libmytest.so -output-obj  interface.c m1.ml
> gcc -Wall main.c  -lmytest -o test
> does not! Is it clear why it doesn't work?

See my comments on PR#5707.  In the latter case, the dynamic loader is
invoked and trashes registers r10 and r11, which ocamlopt uses to pass
arguments to your "foo" function.  We'll have to come up with a better
choice of parameter registers.

> So I get away with static linking for now.

Please do if at all possible.  ocamlopt was designed for static
linking, and the relatively recent addition of dynamic linking support
isn't completely mature yet.

- Xavier Leroy