From cf3b6c9450c252fd16774e606b03c1730c216a18 Mon Sep 17 00:00:00 2001

From: "Richard W.M. Jones" <rjones@redhat.com>

Date: Thu, 25 Jun 2015 18:00:02 +0100

Subject: [PATCH] v2v: Free XML objects in the correct order.

If you free an xmlDocPtr before any xmlXPathObjectPtrs that reference

the doc, you'll get valgrind errors like this:

  ==7390== Invalid read of size 4

  ==7390==    at 0x4EB8BC6: xmlXPathFreeNodeSet (xpath.c:4185)

  ==7390==    by 0x4EB8CC5: xmlXPathFreeObject (xpath.c:5492)

  ==7390==    by 0x400A56: main (in /tmp/test)

  ==7390==  Address 0x60c0928 is 8 bytes inside a block of size 120 free'd

  ==7390==    at 0x4C29D2A: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)

  ==7390==    by 0x4E8784F: xmlFreeNodeList (tree.c:3683)

  ==7390==    by 0x4E87605: xmlFreeDoc (tree.c:1242)

  ==7390==    by 0x400A4A: main (in /tmp/test)

The following simple test program demonstrates the problem:

  #include <stdio.h>

  #include <stdlib.h>

  #include <assert.h>

  #include <libxml/xpath.h>

  int

  main (int argc, char *argv[])

  {

    xmlDocPtr doc;

    xmlXPathContextPtr xpathctx;

    xmlXPathObjectPtr xpathobj;

    doc = xmlReadMemory ("<test/>", 7, NULL, NULL, XML_PARSE_NONET);

    assert (doc);

    xpathctx = xmlXPathNewContext (doc);

    assert (xpathctx);

    xpathobj = xmlXPathEvalExpression (BAD_CAST "/test", xpathctx);

    assert (xpathobj);

    xmlFreeDoc (doc);

    xmlXPathFreeObject (xpathobj);

    xmlXPathFreeContext (xpathctx);

    exit (EXIT_SUCCESS);

  }

In virt-v2v we were not freeing up objects in the correct order,

because we didn't express the dependency between objects at the C

level into the OCaml, where the OCaml garbage collector could see

those dependencies.  For example code like:

  let doc = ... in

  let xpathctx = xpath_new_context doc in

  let xpathobj = xpath_eval_expression xpathctx "/foo" in

might end up freeing the 'doc' (xmlDocPtr) if, say, there were no

further references to it in the code, even though the 'xpathobj'

(xmlXPathObjectPtr) remains live.

To avoid this, we change the OCaml-level representation of objects

like xpathobj so they contain a reference back to the higher-level

objects (xpathctx & doc).  Therefore holding an xpathobj means that

the doc cannot be freed.

However that alone is not quite sufficient.  There is a further

problem when the program calls Gc.full_major, Gc.compact etc., or even

just when xpathctx & doc happen to be freed at the same time.  The GC

won't necessarily free them in the right order as it knows both need

to be freed but doesn't know that one must be freed before the other.

To solve this we have to move the finalisers into OCaml code, since

the OCaml Gc.finalise function comes with an explicit ordering

guarantee (that finalisers are always called in reverse order that

they were created), which the C-level finaliser does not.

(cherry picked from commit 3888582da89c757d0740d11c3a62433d748c85aa)

---

 v2v/input_libvirtxml.ml |  18 ++++-----

 v2v/input_ova.ml        |  10 ++---

 v2v/output_libvirt.ml   |   6 +--

 v2v/xml-c.c             | 103 +++++++++++++++++++++++++-----------------------

 v2v/xml.ml              |  77 ++++++++++++++++++++++++------------

 v2v/xml.mli             |   2 +-

 6 files changed, 123 insertions(+), 93 deletions(-)

diff --git a/v2v/input_libvirtxml.ml b/v2v/input_libvirtxml.ml

index 0f21bb3..16c34a6 100644

--- a/v2v/input_libvirtxml.ml

+++ b/v2v/input_libvirtxml.ml

@@ -44,14 +44,14 @@ let parse_libvirt_xml ?conn ~verbose xml =

     let obj = Xml.xpath_eval_expression xpathctx expr in

     if Xml.xpathobj_nr_nodes obj < 1 then default

     else (

-      let node = Xml.xpathobj_node doc obj 0 in

+      let node = Xml.xpathobj_node obj 0 in

       Xml.node_as_string node

     )

   and xpath_to_int expr default =

     let obj = Xml.xpath_eval_expression xpathctx expr in

     if Xml.xpathobj_nr_nodes obj < 1 then default

     else (

-      let node = Xml.xpathobj_node doc obj 0 in

+      let node = Xml.xpathobj_node obj 0 in

       let str = Xml.node_as_string node in

       try int_of_string str

       with Failure "int_of_string" ->

@@ -76,7 +76,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     let obj = Xml.xpath_eval_expression xpathctx "/domain/features/*" in

     let nr_nodes = Xml.xpathobj_nr_nodes obj in

     for i = 0 to nr_nodes-1 do

-      let node = Xml.xpathobj_node doc obj i in

+      let node = Xml.xpathobj_node obj i in

       features := Xml.node_name node :: !features

     done;

     !features in

@@ -87,7 +87,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     if nr_nodes < 1 then None

     else (

       (* Ignore everything except the first <graphics> device. *)

-      let node = Xml.xpathobj_node doc obj 0 in

+      let node = Xml.xpathobj_node obj 0 in

       Xml.xpathctx_set_current_context xpathctx node;

       let keymap =

         match xpath_to_string "@keymap" "" with "" -> None | k -> Some k in

@@ -148,7 +148,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     if nr_nodes < 1 then None

     else (

       (* Ignore everything except the first <sound> device. *)

-      let node = Xml.xpathobj_node doc obj 0 in

+      let node = Xml.xpathobj_node obj 0 in

       Xml.xpathctx_set_current_context xpathctx node;

       match xpath_to_string "@model" "" with

@@ -187,7 +187,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     if nr_nodes < 1 then

       error (f_"this guest has no non-removable disks");

     for i = 0 to nr_nodes-1 do

-      let node = Xml.xpathobj_node doc obj i in

+      let node = Xml.xpathobj_node obj i in

       Xml.xpathctx_set_current_context xpathctx node;

       let controller =

@@ -250,7 +250,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

             let obj = Xml.xpath_eval_expression xpathctx expr in

             if Xml.xpathobj_nr_nodes obj < 1 then default

             else (

-              let node = Xml.xpathobj_node doc obj 0 in

+              let node = Xml.xpathobj_node obj 0 in

               Xml.node_as_string node

             ) in

@@ -281,7 +281,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     let nr_nodes = Xml.xpathobj_nr_nodes obj in

     let disks = ref [] in

     for i = 0 to nr_nodes-1 do

-      let node = Xml.xpathobj_node doc obj i in

+      let node = Xml.xpathobj_node obj i in

       Xml.xpathctx_set_current_context xpathctx node;

       let controller =

@@ -311,7 +311,7 @@ let parse_libvirt_xml ?conn ~verbose xml =

     let nr_nodes = Xml.xpathobj_nr_nodes obj in

     let nics = ref [] in

     for i = 0 to nr_nodes-1 do

-      let node = Xml.xpathobj_node doc obj i in

+      let node = Xml.xpathobj_node obj i in

       Xml.xpathctx_set_current_context xpathctx node;

       let mac = xpath_to_string "mac/@address" "" in

diff --git a/v2v/input_ova.ml b/v2v/input_ova.ml

index 8deedda..ab8c27b 100644

--- a/v2v/input_ova.ml

+++ b/v2v/input_ova.ml

@@ -184,14 +184,14 @@ object

       let obj = Xml.xpath_eval_expression xpathctx expr in

       if Xml.xpathobj_nr_nodes obj < 1 then default

       else (

-        let node = Xml.xpathobj_node doc obj 0 in

+        let node = Xml.xpathobj_node obj 0 in

         Xml.node_as_string node

       )

     and xpath_to_int expr default =

       let obj = Xml.xpath_eval_expression xpathctx expr in

       if Xml.xpathobj_nr_nodes obj < 1 then default

       else (

-        let node = Xml.xpathobj_node doc obj 0 in

+        let node = Xml.xpathobj_node obj 0 in

         let str = Xml.node_as_string node in

         try int_of_string str

         with Failure "int_of_string" ->

@@ -247,7 +247,7 @@ object

       let obj = Xml.xpath_eval_expression xpathctx expr in

       let nr_nodes = Xml.xpathobj_nr_nodes obj in

       for i = 0 to nr_nodes-1 do

-        let n = Xml.xpathobj_node doc obj i in

+        let n = Xml.xpathobj_node obj i in

         Xml.xpathctx_set_current_context xpathctx n;

         (* XXX We assume the OVF lists these in order.

@@ -316,7 +316,7 @@ object

       let obj = Xml.xpath_eval_expression xpathctx expr in

       let nr_nodes = Xml.xpathobj_nr_nodes obj in

       for i = 0 to nr_nodes-1 do

-        let n = Xml.xpathobj_node doc obj i in

+        let n = Xml.xpathobj_node obj i in

         Xml.xpathctx_set_current_context xpathctx n;

         let id = xpath_to_int "rasd:ResourceType/text()" 0 in

         assert (id = 14 || id = 15 || id = 16);

@@ -350,7 +350,7 @@ object

     let obj = Xml.xpath_eval_expression xpathctx "/ovf:Envelope/ovf:VirtualSystem/ovf:VirtualHardwareSection/ovf:Item[rasd:ResourceType/text()=10]"  in

     let nr_nodes = Xml.xpathobj_nr_nodes obj in

     for i = 0 to nr_nodes-1 do

-      let n = Xml.xpathobj_node doc obj i in

+      let n = Xml.xpathobj_node obj i in

       Xml.xpathctx_set_current_context xpathctx n;

       let vnet = xpath_to_string "rasd:ElementName/text()" (sprintf"eth%d" i) in

       let nic = {

diff --git a/v2v/output_libvirt.ml b/v2v/output_libvirt.ml

index 23e881a..6188aa6 100644

--- a/v2v/output_libvirt.ml

+++ b/v2v/output_libvirt.ml

@@ -52,7 +52,7 @@ let target_features_of_capabilities_doc doc arch =

     warning ~prog (f_"the target hypervisor does not support a %s KVM guest") arch;

     []

   ) else (

-    let node (* first matching <guest> *) = Xml.xpathobj_node doc obj 0 in

+    let node (* first matching <guest> *) = Xml.xpathobj_node obj 0 in

     Xml.xpathctx_set_current_context xpathctx node;

     (* Get guest/features/* nodes. *)

@@ -60,7 +60,7 @@ let target_features_of_capabilities_doc doc arch =

     let features = ref [] in

     for i = 0 to Xml.xpathobj_nr_nodes obj - 1 do

-      let feature_node = Xml.xpathobj_node doc obj i in

+      let feature_node = Xml.xpathobj_node obj i in

       let feature_name = Xml.node_name feature_node in

       features := feature_name :: !features

     done;

@@ -355,7 +355,7 @@ class output_libvirt verbose oc output_pool = object

       let obj = Xml.xpath_eval_expression xpathctx expr in

       if Xml.xpathobj_nr_nodes obj < 1 then default

       else (

-        let node = Xml.xpathobj_node doc obj 0 in

+        let node = Xml.xpathobj_node obj 0 in

         Xml.node_as_string node

       )

     in

diff --git a/v2v/xml-c.c b/v2v/xml-c.c

index 0b619e5..c71ab24 100644

--- a/v2v/xml-c.c

+++ b/v2v/xml-c.c

@@ -42,60 +42,53 @@

 /* xmlDocPtr type */

 #define Doc_val(v) (*((xmlDocPtr *)Data_custom_val(v)))

-static void

-doc_finalize (value docv)

-{

-  xmlDocPtr doc = Doc_val (docv);

-

-  if (doc)

-    xmlFreeDoc (doc);

-}

-

 static struct custom_operations doc_custom_operations = {

   (char *) "doc_custom_operations",

-  doc_finalize,

+  custom_finalize_default,

   custom_compare_default,

   custom_hash_default,

   custom_serialize_default,

   custom_deserialize_default

 };

+value

+v2v_xml_free_doc_ptr (value docv)

+{

+  CAMLparam1 (docv);

+  xmlDocPtr doc = Doc_val (docv);

+

+  xmlFreeDoc (doc);

+  CAMLreturn (Val_unit);

+}

+

 /* xmlXPathContextPtr type */

-#define Xpathctx_val(v) (*((xmlXPathContextPtr *)Data_custom_val(v)))

+#define Xpathctx_ptr_val(v) (*((xmlXPathContextPtr *)Data_custom_val(v)))

-static void

-xpathctx_finalize (value xpathctxv)

-{

-  xmlXPathContextPtr xpathctx = Xpathctx_val (xpathctxv);

-

-  if (xpathctx)

-    xmlXPathFreeContext (xpathctx);

-}

-

-static struct custom_operations xpathctx_custom_operations = {

-  (char *) "xpathctx_custom_operations",

-  xpathctx_finalize,

+static struct custom_operations xpathctx_ptr_custom_operations = {

+  (char *) "xpathctx_ptr_custom_operations",

+  custom_finalize_default,

   custom_compare_default,

   custom_hash_default,

   custom_serialize_default,

   custom_deserialize_default

 };

+value

+v2v_xml_free_xpathctx_ptr (value xpathctxv)

+{

+  CAMLparam1 (xpathctxv);

+  xmlXPathContextPtr xpathctx = Xpathctx_ptr_val (xpathctxv);

+

+  xmlXPathFreeContext (xpathctx);

+  CAMLreturn (Val_unit);

+}

+

 /* xmlXPathObjectPtr type */

-#define Xpathobj_val(v) (*((xmlXPathObjectPtr *)Data_custom_val(v)))

+#define Xpathobj_ptr_val(v) (*((xmlXPathObjectPtr *)Data_custom_val(v)))

-static void

-xpathobj_finalize (value xpathobjv)

-{

-  xmlXPathObjectPtr xpathobj = Xpathobj_val (xpathobjv);

-

-  if (xpathobj)

-    xmlXPathFreeObject (xpathobj);

-}

-

-static struct custom_operations xpathobj_custom_operations = {

-  (char *) "xpathobj_custom_operations",

-  xpathobj_finalize,

+static struct custom_operations xpathobj_ptr_custom_operations = {

+  (char *) "xpathobj_ptr_custom_operations",

+  custom_finalize_default,

   custom_compare_default,

   custom_hash_default,

   custom_serialize_default,

@@ -103,6 +96,16 @@ static struct custom_operations xpathobj_custom_operations = {

 };

 value

+v2v_xml_free_xpathobj_ptr (value xpathobjv)

+{

+  CAMLparam1 (xpathobjv);

+  xmlXPathObjectPtr xpathobj = Xpathobj_ptr_val (xpathobjv);

+

+  xmlXPathFreeObject (xpathobj);

+  CAMLreturn (Val_unit);

+}

+

+value

 v2v_xml_parse_memory (value xmlv)

 {

   CAMLparam1 (xmlv);

@@ -124,7 +127,7 @@ v2v_xml_parse_memory (value xmlv)

 }

 value

-v2v_xml_xpath_new_context (value docv)

+v2v_xml_xpath_new_context_ptr (value docv)

 {

   CAMLparam1 (docv);

   CAMLlocal1 (xpathctxv);

@@ -136,21 +139,21 @@ v2v_xml_xpath_new_context (value docv)

   if (xpathctx == NULL)

     caml_invalid_argument ("xpath_new_context: unable to create xmlXPathNewContext");

-  xpathctxv = caml_alloc_custom (&xpathctx_custom_operations,

+  xpathctxv = caml_alloc_custom (&xpathctx_ptr_custom_operations,

                                  sizeof (xmlXPathContextPtr), 0, 1);

-  Xpathctx_val (xpathctxv) = xpathctx;

+  Xpathctx_ptr_val (xpathctxv) = xpathctx;

   CAMLreturn (xpathctxv);

 }

 value

-v2v_xml_xpath_register_ns (value xpathctxv, value prefix, value uri)

+v2v_xml_xpathctx_ptr_register_ns (value xpathctxv, value prefix, value uri)

 {

   CAMLparam3 (xpathctxv, prefix, uri);

   xmlXPathContextPtr xpathctx;

   int r;

-  xpathctx = Xpathctx_val (xpathctxv);

+  xpathctx = Xpathctx_ptr_val (xpathctxv);

   r = xmlXPathRegisterNs (xpathctx, BAD_CAST String_val (prefix), BAD_CAST String_val (uri));

   if (r == -1)

       caml_invalid_argument ("xpath_register_ns: unable to register namespace");

@@ -159,30 +162,30 @@ v2v_xml_xpath_register_ns (value xpathctxv, value prefix, value uri)

 }

 value

-v2v_xml_xpath_eval_expression (value xpathctxv, value exprv)

+v2v_xml_xpathctx_ptr_eval_expression (value xpathctxv, value exprv)

 {

   CAMLparam2 (xpathctxv, exprv);

   CAMLlocal1 (xpathobjv);

   xmlXPathContextPtr xpathctx;

   xmlXPathObjectPtr xpathobj;

-  xpathctx = Xpathctx_val (xpathctxv);

+  xpathctx = Xpathctx_ptr_val (xpathctxv);

   xpathobj = xmlXPathEvalExpression (BAD_CAST String_val (exprv), xpathctx);

   if (xpathobj == NULL)

     caml_invalid_argument ("xpath_eval_expression: unable to evaluate XPath expression");

-  xpathobjv = caml_alloc_custom (&xpathobj_custom_operations,

+  xpathobjv = caml_alloc_custom (&xpathobj_ptr_custom_operations,

                                  sizeof (xmlXPathObjectPtr), 0, 1);

-  Xpathobj_val (xpathobjv) = xpathobj;

+  Xpathobj_ptr_val (xpathobjv) = xpathobj;

   CAMLreturn (xpathobjv);

 }

 value

-v2v_xml_xpathobj_nr_nodes (value xpathobjv)

+v2v_xml_xpathobj_ptr_nr_nodes (value xpathobjv)

 {

   CAMLparam1 (xpathobjv);

-  xmlXPathObjectPtr xpathobj = Xpathobj_val (xpathobjv);

+  xmlXPathObjectPtr xpathobj = Xpathobj_ptr_val (xpathobjv);

   if (xpathobj->nodesetval == NULL)

     CAMLreturn (Val_int (0));

@@ -191,10 +194,10 @@ v2v_xml_xpathobj_nr_nodes (value xpathobjv)

 }

 value

-v2v_xml_xpathobj_get_node_ptr (value xpathobjv, value iv)

+v2v_xml_xpathobj_ptr_get_node_ptr (value xpathobjv, value iv)

 {

   CAMLparam2 (xpathobjv, iv);

-  xmlXPathObjectPtr xpathobj = Xpathobj_val (xpathobjv);

+  xmlXPathObjectPtr xpathobj = Xpathobj_ptr_val (xpathobjv);

   int i = Int_val (iv);

   if (i < 0 || i >= xpathobj->nodesetval->nodeNr)

@@ -215,7 +218,7 @@ value

 v2v_xml_xpathctx_set_node_ptr (value xpathctxv, value nodev)

 {

   CAMLparam2 (xpathctxv, nodev);

-  xmlXPathContextPtr xpathctx = Xpathctx_val (xpathctxv);

+  xmlXPathContextPtr xpathctx = Xpathctx_ptr_val (xpathctxv);

   xmlNodePtr node = (xmlNodePtr) nodev;

   xpathctx->node = node;

diff --git a/v2v/xml.ml b/v2v/xml.ml

index fea8784..16f3f70 100644

--- a/v2v/xml.ml

+++ b/v2v/xml.ml

@@ -18,37 +18,64 @@

 (* Mini interface to libxml2. *)

-type doc

+type doc = doc_ptr

+and doc_ptr

 type node_ptr

-type xpathctx

-type xpathobj

+type xpathctx_ptr

+type xpathobj_ptr

-(* Since node is owned by doc, we have to make that explicit to the

- * garbage collector.

+(* At the C level, various objects "own" other objects.  We have to

+ * make that ownership explicit to the garbage collector, else we could

+ * end up freeing an object before all the C references to it are

+ * freed.

  *)

-type node = doc * node_ptr

-

-external parse_memory : string -> doc = "v2v_xml_parse_memory"

-external xpath_new_context : doc -> xpathctx = "v2v_xml_xpath_new_context"

-external xpath_eval_expression : xpathctx -> string -> xpathobj = "v2v_xml_xpath_eval_expression"

-external xpath_register_ns : xpathctx -> string -> string -> unit = "v2v_xml_xpath_register_ns"

-

-external xpathobj_nr_nodes : xpathobj -> int = "v2v_xml_xpathobj_nr_nodes"

-external xpathobj_get_node_ptr : xpathobj -> int -> node_ptr = "v2v_xml_xpathobj_get_node_ptr"

-let xpathobj_node doc xpathobj i =

-  let n = xpathobj_get_node_ptr xpathobj i in

-  (doc, n)

-

-external xpathctx_set_node_ptr : xpathctx -> node_ptr -> unit = "v2v_xml_xpathctx_set_node_ptr"

-let xpathctx_set_current_context xpathctx (_, node) =

-  xpathctx_set_node_ptr xpathctx node

+type xpathctx = doc_ptr * xpathctx_ptr

+type xpathobj = xpathctx * xpathobj_ptr

+type node = doc_ptr * node_ptr

+

+external free_doc_ptr : doc_ptr -> unit = "v2v_xml_free_doc_ptr"

+external free_xpathctx_ptr : xpathctx_ptr -> unit = "v2v_xml_free_xpathctx_ptr"

+external free_xpathobj_ptr : xpathobj_ptr -> unit = "v2v_xml_free_xpathobj_ptr"

+

+external _parse_memory : string -> doc_ptr = "v2v_xml_parse_memory"

+let parse_memory xml =

+  let doc_ptr = _parse_memory xml in

+  Gc.finalise free_doc_ptr doc_ptr;

+  doc_ptr

+

+external xpath_new_context_ptr : doc_ptr -> xpathctx_ptr = "v2v_xml_xpath_new_context_ptr"

+let xpath_new_context doc_ptr =

+  let xpathctx_ptr = xpath_new_context_ptr doc_ptr in

+  Gc.finalise free_xpathctx_ptr xpathctx_ptr;

+  doc_ptr, xpathctx_ptr

+

+external xpathctx_ptr_register_ns : xpathctx_ptr -> string -> string -> unit = "v2v_xml_xpathctx_ptr_register_ns"

+let xpath_register_ns (_, xpathctx_ptr) prefix uri =

+  xpathctx_ptr_register_ns xpathctx_ptr prefix uri

+

+external xpathctx_ptr_eval_expression : xpathctx_ptr -> string -> xpathobj_ptr = "v2v_xml_xpathctx_ptr_eval_expression"

+let xpath_eval_expression ((_, xpathctx_ptr) as xpathctx) expr =

+  let xpathobj_ptr = xpathctx_ptr_eval_expression xpathctx_ptr expr in

+  Gc.finalise free_xpathobj_ptr xpathobj_ptr;

+  xpathctx, xpathobj_ptr

+

+external xpathobj_ptr_nr_nodes : xpathobj_ptr -> int = "v2v_xml_xpathobj_ptr_nr_nodes"

+let xpathobj_nr_nodes (_, xpathobj_ptr) =

+  xpathobj_ptr_nr_nodes xpathobj_ptr

+

+external xpathobj_ptr_get_node_ptr : xpathobj_ptr -> int -> node_ptr = "v2v_xml_xpathobj_ptr_get_node_ptr"

+let xpathobj_node ((doc_ptr, _), xpathobj_ptr) i =

+  doc_ptr, xpathobj_ptr_get_node_ptr xpathobj_ptr i

+

+external xpathctx_ptr_set_node_ptr : xpathctx_ptr -> node_ptr -> unit = "v2v_xml_xpathctx_set_node_ptr"

+let xpathctx_set_current_context (_, xpathctx_ptr) (_, node_ptr) =

+  xpathctx_ptr_set_node_ptr xpathctx_ptr node_ptr

 external node_ptr_name : node_ptr -> string = "v2v_xml_node_ptr_name"

-let node_name (_, node) = node_ptr_name node

+let node_name (_, node_ptr) = node_ptr_name node_ptr

-external node_ptr_as_string : doc -> node_ptr -> string = "v2v_xml_node_ptr_as_string"

-let node_as_string (doc, node) =

-  node_ptr_as_string doc node

+external node_ptr_as_string : doc_ptr -> node_ptr -> string = "v2v_xml_node_ptr_as_string"

+let node_as_string (doc_ptr, node_ptr) = node_ptr_as_string doc_ptr node_ptr

 type uri = {

   uri_scheme : string option;

diff --git a/v2v/xml.mli b/v2v/xml.mli

index 890fa4e..46c7d3e 100644

--- a/v2v/xml.mli

+++ b/v2v/xml.mli

@@ -34,7 +34,7 @@ val xpath_register_ns : xpathctx -> string -> string -> unit

 val xpathobj_nr_nodes : xpathobj -> int

 (** Get the number of nodes in the nodeset of the xmlXPathObjectPtr. *)

-val xpathobj_node : doc -> xpathobj -> int -> node

+val xpathobj_node : xpathobj -> int -> node

 (** Get the i'th node in the nodeset of the xmlXPathObjectPtr. *)

 val xpathctx_set_current_context : xpathctx -> node -> unit

-- 

1.8.3.1