Re: [Caml-list] memory corruption with Weak.t in Gc.finalise

[Goswin von Brederlow <goswin-v-b@web.de>]

On Tue, Mar 08, 2016 at 11:00:33AM -0500, Gabriel Scherer wrote:
> It's great that you have a minimal reproduction example. Could you
> create an issue on the issue tracker?
>   http://caml.inria.fr/mantis/
> 
> Issues on the bugtracker are easier to keep track of than mails on the
> caml-list.

If it is a bug in the compiler and not just my code then sure.

MfG
	Goswin
 
> On Tue, Mar 8, 2016 at 10:49 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> > Hi,
> >
> > I found a memory corruption in Gc.finalise when sharing a Waek.t with
> > C stubs. I made a small test case and it's gotten worse. The code now
> > segfaults:
> >
> >   https://github.com/mrvn/ocam-problems/tree/weak-finalise
> >
> > The problem I had was the address of the obj stored in the Weak.t
> > changes (which might be the GC moving it?) and also the tag changes
> > from 248 (Object_tag) to 168 (random constructor).
> >
> > With my smaller test case I further get:
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x000000000043c9f8 in caml_format_exception ()
> > (gdb) bt
> > #0  0x000000000043c9f8 in caml_format_exception ()
> > #1  0x000000000043cbc1 in caml_fatal_uncaught_exception ()
> > #2  0x000000000043b1a7 in caml_main ()
> > #3  0x000000000042c45d in main ()
> >
> > Ideas what is going on there?
> >
> > MfG
> >         Goswin
> >
> > ------------------------------------------------------------------------
> > ==> Makefile <==
> > all: test
> >         ./test
> >
> > test: main.cmx stubs.o
> >         ocamlopt -o $@ $+
> >
> > %.cmx: %.ml
> >         ocamlopt -g -c -o $@ $<
> >
> > %.o: %.c
> >         gcc -O2 -W -Wall -g -c -o $@ $<
> >
> > clean:
> >         rm -f test *.o *.cmx *.cmi *~
> >
> > ==> main.ml <==
> > external setup : 'a Weak.t -> unit = "setup"
> > external test : unit -> unit = "test"
> >
> > let cleanup obj =
> >   Printf.printf "cleanup\n%!";
> >   test ()
> >
> > class obj = object(self)
> >   val weak = Weak.create 1
> >   initializer Weak.set weak 0 (Some self)
> >   initializer setup weak
> >   initializer Gc.finalise cleanup self
> > end
> >
> > let () =
> >   Printf.printf "Creating object\n%!";
> >   ignore (new obj);
> >   test ()
> >
> > let () =
> >   Printf.printf "Garbage collection\n%!";
> >   Gc.full_major ()
> >
> > let () =
> >   Printf.printf "checking after GC\n%!";
> >   test ()
> >
> > ==> stubs.c <==
> > #include <stdio.h>
> > #include <caml/mlvalues.h>
> > #include <caml/memory.h>
> >
> > value weak;
> >
> > void setup(value ml_weak) {
> >     CAMLparam1(ml_weak);
> >     printf("%s(0x%lx)\n", __PRETTY_FUNCTION__, ml_weak);
> >     weak = ml_weak;
> >     caml_register_generational_global_root(&weak);
> >     CAMLreturn0;
> > }
> >
> > void test(void) {
> >     CAMLparam0();
> >     CAMLlocal1(obj);
> >     printf("%s\n", __PRETTY_FUNCTION__);
> >     printf("  weak = 0x%lx\n", weak);
> >     if (weak != 0) {
> >         obj = Field(weak, 1);
> >         printf("  obj = 0x%lx\n", obj);
> >         if (obj != 0) {
> >             printf("  tag = %d\n", Tag_val(obj));
> >         }
> >     }
> >     printf("%s done\n", __PRETTY_FUNCTION__);
> >     CAMLreturn0;
> > }
> >
> > --
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> 
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