Access xpathCtx properties after NULL-pointer verification is passed

Mykola Mokhnach · facebook-github-bot · commit 3955b657f9ad · 2017-01-27T06:17:30.000-08:00
Summary: It is possible that _xmlXPathNewContext_ returns NULL in case of internal failure and then we'll have some hardcore crash as a result. Closes facebookarchive#469 Differential Revision: D4474571 Pulled By: marekcirkos fbshipit-source-id: 9edd09d81d3b0097d06f86deb807c011bfe65ead
diff --git a/WebDriverAgentLib/Utilities/FBXPath.m b/WebDriverAgentLib/Utilities/FBXPath.m
@@ -167,11 +167,11 @@ + (xmlChar *)xmlCharPtrForInput:(const char *)input
 + (xmlXPathObjectPtr)evaluate:(NSString *)xpathQuery document:(xmlDocPtr)doc
 {
   xmlXPathContextPtr xpathCtx = xmlXPathNewContext(doc);
-  xpathCtx->node = doc->children;
   if (NULL == xpathCtx) {
     [FBLogger logFmt:@"Failed to invoke libxml2>xmlXPathNewContext for XPath query \"%@\"", xpathQuery];
     return NULL;
   }
+  xpathCtx->node = doc->children;
 
   xmlXPathObjectPtr xpathObj = xmlXPathEvalExpression([FBXPath xmlCharPtrForInput:[xpathQuery cStringUsingEncoding:NSUTF8StringEncoding]], xpathCtx);
   if (NULL == xpathObj) {

Original file line number	Diff line number	Diff line change
`@@ -167,11 +167,11 @@ + (xmlChar )xmlCharPtrForInput:(const char )input`
`167`	`167`	`+ (xmlXPathObjectPtr)evaluate:(NSString *)xpathQuery document:(xmlDocPtr)doc`
`168`	`168`	`{`
`169`	`169`	`xmlXPathContextPtr xpathCtx = xmlXPathNewContext(doc);`
`170`		`- xpathCtx->node = doc->children;`
`171`	`170`	`if (NULL == xpathCtx) {`
`172`	`171`	`[FBLogger logFmt:@"Failed to invoke libxml2>xmlXPathNewContext for XPath query \"%@\"", xpathQuery];`
`173`	`172`	`return NULL;`
`174`	`173`	`}`
	`174`	`+ xpathCtx->node = doc->children;`
`175`	`175`
`176`	`176`	`xmlXPathObjectPtr xpathObj = xmlXPathEvalExpression([FBXPath xmlCharPtrForInput:[xpathQuery cStringUsingEncoding:NSUTF8StringEncoding]], xpathCtx);`
`177`	`177`	`if (NULL == xpathObj) {`