Re: [libvirt] [PATCH] util: xml: Don't conflict with other libxml2 user callbacks

On Fri, Feb 23, 2018 at 06:21:15PM -0500, Cole Robinson wrote: > lxml is a popular python XML processing library. It uses libxml2 > behind the scenes, and registers custom callbacks via > xmlSetExternalEntityLoader. However this can cause crashes if > if an app uses both lxml and libxml2 together in the same process. > > This is a known limitation of lxml and libxml2 generally. It also > prevents us from using lxml in virt-manager: > > https://bugzilla.redhat.com/show_bug.cgi?id=1544019 > > However it's easy enough to work around in libvirt, by unsetting the > EntityLoader callback to a known state before we ask libxml2 to > parse a file from disk. > > Signed-off-by: Cole Robinson <crobinso(a)redhat.com&gt; > --- > src/util/virxml.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/src/util/virxml.c b/src/util/virxml.c > index 6e87605ea..3e01794f9 100644 > --- a/src/util/virxml.c > +++ b/src/util/virxml.c > @@ -810,9 +810,14 @@ virXMLParseHelper(int domcode, > pctxt->sax->error = catchXMLError; > > if (filename) { > + /* Reset any libxml2 file callbacks, other libs (like python lxml) > + * may have set their own which can get crashy */ > + xmlExternalEntityLoader origloader = xmlGetExternalEntityLoader(); > + xmlSetExternalEntityLoader(xmlNoNetExternalEntityLoader); > xml = xmlCtxtReadFile(pctxt, filename, NULL, > XML_PARSE_NONET | > XML_PARSE_NOWARNING); > + xmlSetExternalEntityLoader(origloader); This does not look thread-safe at all - what if two libvirt threads try to parse some XML at the same time?