[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 11/28] libxl: ocaml: propagate the libxl return error code in exceptions
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 strongly recommend using the "simple interface" i.e. caml_alloc() caml_alloc_tuple() Store_field() If you look in the definition of "caml_alloc" [**] it does this: CAMLexport value caml_alloc (mlsize_t wosize, tag_t tag) { value result; mlsize_t i; Assert (tag < 256); Assert (tag != Infix_tag); if (wosize == 0){ result = Atom (tag); }else if (wosize <= Max_young_wosize){ Alloc_small (result, wosize, tag); if (tag < No_scan_tag){ for (i = 0; i < wosize; i++) Field (result, i) = 0; }^^^^^ -- it sets the fields to 0 preventing the GC seeing a random value Whereas "caml_alloc_small" just does the "Alloc_small". + + caml_raise_with_arg(*exc, arg); +} [*] http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual033.html [**] https://github.com/ocaml/ocaml/blob/trunk/byterun/alloc.c _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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