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Re: NSE exceptions: false or nil?
From: David Fifield <david () bamsoftware com>
Date: Sat, 3 Nov 2007 12:41:08 -0700

On Fri, Nov 02, 2007 at 01:00:24PM +0100, Stoiko Ivanov wrote:
On Thu, Nov 01, 2007 at 10:40:22PM -0700, David Fifield wrote:
Are NSE exceptions supposed to return false or nil? At
http://insecure.org/nmap/nse/nse-api.html#nse-exceptions it says that
the first thing returned is a Boolean value. But trying to catch an
exception from connect fails, as the attached example script (test.nse)
shows. It produces this message:

    Initiating SCRIPT ENGINE at 22:26
    false   Name or service not known
    SCRIPT ENGINE: In: nse_nmaplib.cc:474 Trying to finalize a non conforming function. Are you sure you return 
true on success followed by the remaining return values and nil on failure followed by an error string?
    QUITTING!

We see that connect returns (false, "Name or service not known"), but
this is "non conforming." Should NSE's internal functions be modified to
return nil instead of false, or should false be considered an exception
marker? Does the patch I attached do the right thing?

I think the latter would be more convenient. 
Since the network I/O functions return true on success I would expect a false 
value to indicate an exception. 
AFAIK this patch shouldn't cause any problems with the remaining NSE.

Index: nse_nmaplib.cc
===================================================================
--- nse_nmaplib.cc  (revision 6111)
+++ nse_nmaplib.cc  (working copy)
@@ -460,7 +460,7 @@
 }
 
 static int l_exc_finalize(lua_State *l) {
-   if (lua_isnil(l, 1)) {
+   if (lua_isnil(l, 1) || !lua_toboolean(l, 1)) {

This looks good to me, however if I recall correctly the lua_isnil() is not
really needed, because lua_toboolean() should return false if there is nil
at the specified position on the stack.

I attached another patch that's better. It causes an exception if the
first returned value is false or nil, strips the first returned value
only if it's a Boolean true, and shows the "non conforming" message
otherwise.

I want to check before I commit it that this is really what's wanted.
false or nil or true as the first returned value, and anything else is
an error. If that's the case, the online documentation should make it
clear.

David Fifield

Attachment: nse-exception-2.diff
Description:


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