Re: [Xen-devel] [PATCH 11/28] libxl: ocaml: propagate the libxl return error code in exceptions

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-03-26 at 11:33 +0000, Dave Scott wrote:
> On 25/03/13 14:45, Rob Hoes wrote:
> > +static void failwith_xl(int error, char *fname)
> > +{
> > +       CAMLlocal1(arg);
> >          value *exc = caml_named_value("Xenlight.Error");
> > +
> >          if (!exc)
> >                  caml_invalid_argument("Exception Xenlight.Error not 
> > initialized, please link xl.cma");
> > -       caml_raise_with_string(*exc, fname);
> > +
> > +       arg = caml_alloc_small(2, 0);
> > +
> > +       Field(arg, 0) = Val_error(error);
> > +       Field(arg, 1) = caml_copy_string(fname);
> 
> I think this violates Rule 5 in the OCaml FFI manual[*]. In the 
> low-level interface when you allocate a block with "caml_alloc_small" 
> all the fields contain random values. The assignment:
> 
>    Field(arg, 1) = caml_copy_string(fname);
> 
> will first call "caml_copy_string" which performs an allocation before 
> setting the field to a valid value. Any function which performs an 
> allocation can trigger a GC which will segfault if it sees the random 
> data in field 1.

I think this answers my earlier query on another patch about the
redundant looking store to Field ...

Ian.


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