liblok: check new methods via macros on nSize, not by de-referencing.

We can't check for NULL from beyond the end of a smaller structure.

Change-Id: Id3754bf747c402cf0d767eda5fd4b5ad6b5789e9

include/LibreOfficeKit/LibreOfficeKit.h

@@ -18,6 +18,13 @@ extern "C"
 typedef struct _LibreOfficeKit LibreOfficeKit;
 typedef struct _LibreOfficeKitDocument LibreOfficeKitDocument;
 
+// Do we have an extended member in this struct ?
+#define LIBREOFFICEKIT_HAS_MEMBER(strct,member,nSize) \
+    ((((int)((unsigned char *)&((strct *) 0)->member) +  \
+      (int)sizeof ((strct *) 0)->member)) <= (nSize))
+
+#define LIBREOFFICEKIT_HAS(pKit,member) LIBREOFFICEKIT_HAS_MEMBER(LibreOfficeKit,member,(pKit)->nSize)
+
 struct _LibreOfficeKit
 {
   int  nSize;
@@ -28,6 +35,8 @@ struct _LibreOfficeKit
   char*                   (*getError)      (LibreOfficeKit *pThis);
 };
 
+#define LIBREOFFICEKIT_DOCUMENT_HAS(pDoc,member) LIBREOFFICEKIT_HAS_MEMBER(LibreOfficeKitDocument,member,(pDoc)->nSize)
+
 struct _LibreOfficeKitDocument
 {
   int  nSize;

include/LibreOfficeKit/LibreOfficeKit.hxx

@@ -45,11 +45,12 @@ public:
     inline bool saveAsWithOptions(const char* pUrl, const char* pFormat = NULL, const char* pFilterOptions = NULL)
     {
         // available since LibreOffice 4.3
-        if (!mpDoc->saveAsWithOptions)
+        if (!LIBREOFFICEKIT_DOCUMENT_HAS(mpDoc, saveAsWithOptions))
             return false;
 
         return mpDoc->saveAsWithOptions(mpDoc, pUrl, pFormat, pFilterOptions);
     }
+    inline LibreOfficeKitDocument *get() { return mpDoc; }
 };
 
 class Office

smoketest/libtest.cxx

@@ -75,6 +75,16 @@ int main (int argc, char **argv)
         return -1;
     }
 
+    if (!LIBREOFFICEKIT_DOCUMENT_HAS(pDocument->get(), saveAsWithOptions))
+    {
+        fprintf( stderr, "using obsolete LibreOffice %d + %d vs. %d\n",
+                 (int)((unsigned char *)&((LibreOfficeKitDocument *) 0)->saveAsWithOptions),
+                 (int)sizeof ((LibreOfficeKitDocument *) 0)->saveAsWithOptions,
+                 pDocument->get()->nSize );
+        return -1;
+    }
+
+
     end = getTimeMS();
     fprintf( stderr, "load time: %ld ms\n", (end-start) );
     start = end;