Permit NULL context to krb5_get_error_message()

Application developers have a tendency to use krb5_get_error_message()
as a drop in replacement for error_message() and under various
circumstances they pass in a NULL context.  This method works fine
for MIT's implementation which ignores the context but in Heimdal
passing in a NULL context would dump core.

This patch set modifies krb5_get_error_message() in order to
permit the passing of a NULL context.  First, if the context
is NULL, an attempt will be made to allocate one locally for
the purpose of evaluating the error code.  Second, if a local
context cannot be allocated, fall back on calling error_message().
If error_message() fails to return a string, then generate an
"unknown error" response.

Only if all of the above fails is NULL returned.

Change-Id: If4baf7d6c428cf0baf11c044b8dfd5c2b3cdf7e4

lib/krb5/error_string.c

@@ -206,29 +206,53 @@ krb5_vprepend_error_message(krb5_context context, krb5_error_code ret,
 KRB5_LIB_FUNCTION const char * KRB5_LIB_CALL
 krb5_get_error_message(krb5_context context, krb5_error_code code)
 {
-    char *str;
+    char *str = NULL;
+    const char *cstr = NULL;
+    char buf[128];
+    int free_context = 0;
 
+    if (code == 0)
+	return strdup("Success");
+
+    /*
+     * The MIT version of this function ignores the krb5_context
+     * and several widely deployed applications call krb5_get_error_message()
+     * with a NULL context in order to translate an error code as a
+     * replacement for error_message().  Another reason a NULL context
+     * might be provided is if the krb5_init_context() call itself
+     * failed.
+     */
+    if (context)
+    {
         HEIMDAL_MUTEX_lock(context->mutex);
         if (context->error_string &&
             (code == context->error_code || context->error_code == 0))
         {
             str = strdup(context->error_string);
-	if (str) {
-	    HEIMDAL_MUTEX_unlock(context->mutex);
-	    return str;
-	}
         }
         HEIMDAL_MUTEX_unlock(context->mutex);
 
-    if (code == 0)
+        if (str)
-	return strdup("Success");
+            return str;
-    {
-	const char *msg;
-	char buf[128];
-	msg = com_right_r(context->et_list, code, buf, sizeof(buf));
-	if (msg)
-	    return strdup(msg);
     }
+    else
+    {
+        if (krb5_init_context(&context) == 0)
+            free_context = 1;
+    }
+
+    if (context)
+        cstr = com_right_r(context->et_list, code, buf, sizeof(buf));
+
+    if (free_context)
+        krb5_free_context(context);
+
+    if (cstr)
+        return strdup(cstr);
+
+    cstr = error_message(code);
+    if (cstr)
+        return strdup(cstr);
 
     if (asprintf(&str, "<unknown error: %d>", (int)code) == -1 || str == NULL)
 	return NULL;