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

[Ján Tomko]

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 at redhat.com>
>---
> 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?

Jan

>     } else {
>         xml = xmlCtxtReadDoc(pctxt, BAD_CAST xmlStr, url, NULL,
>                              XML_PARSE_NONET |
>-- 
>2.14.3
>
>--
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