Re: [xml] C14N issue with digital signature due to pointer comparison


Actually, I have an additional layer to emulate DOM compliant API over libxml. In that layer, I have a global list of xmlNsPtr for all namespaces encountered by the nodes (and referenced in node->ns) of a single XML document. And for each namespace definition (xmlns:xxx) set on a node, a new xmlNsPtr is created and referenced in node->nsDef. So during canonicalization process, when a node looks for a parent namespace definition via xmlSearchNs, it can happen that the xmlNsPtr reference from the node is different from the parent node->nsDef , even if the href and prefix value are the same. Then of course simple pointer comparison doesn't work. So I was asking myself if such situation cannot happen to other people, and maybe change the pointer comparison because depending on how people build and manipulate the document, the test is not always accurate.


Le 20/03/2014 03:16, Aleksey Sanin a écrit :
How do you manipulate the XML tree? If you are using "official" LibXML2
function then I believe this code should work just fine unless there is
a bug in the strings dictionary.


On 3/12/14, 10:28 AM, Frank Gross wrote:

   I'm getting some trouble to verify a XML signature because the
xmlC14NProcessNamespacesAxis() function does a xmlNsPtr pointer
comparison to decide whether a sub node belongs to the same default
namespace as an ancestor. But if the sub node has been manipulated by
program (in my case) to point to another xmlNsPtr with same values, the
canonicalization process breaks. Wouldn't it be better to check the
namespace href values instead as in following patch ?

diff --git a/lib-xmlsoft-libxml2/src/c14n.c
index 9c3cad2..f73e709 100644
--- a/lib-xmlsoft-libxml2/src/c14n.c
+++ b/lib-xmlsoft-libxml2/src/c14n.c
@@ -623,7 +623,7 @@ xmlC14NProcessNamespacesAxis(xmlC14NCtxPtr ctx,
xmlNodePtr cur, int visible)
      for(ns = n->nsDef; ns != NULL; ns = ns->next) {
          tmp = xmlSearchNs(cur->doc, cur, ns->prefix);

-        if((tmp == ns) && !xmlC14NIsXmlNs(ns) && xmlC14NIsVisible(ctx,
ns, cur)) {
+        if((xmlStrEqual(tmp->href,ns->href)) &&
(xmlStrEqual(tmp->prefix,ns->prefix)) && !xmlC14NIsXmlNs(ns) &&
xmlC14NIsVisible(ctx, ns, cur)) {
          already_rendered = xmlC14NVisibleNsStackFind(ctx->ns_rendered,
          if(visible) {
                  xmlC14NVisibleNsStackAdd(ctx->ns_rendered, ns, cur);


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