[sr #107515] Objective-C forwarding not working with libffi library

[Jonathan Olson]

URL:
  <http://savannah.gnu.org/support/?107515>

                 Summary: Objective-C forwarding not working with libffi
library
                 Project: GNUstep
            Submitted by: jpolsonaz
            Submitted on: Sat 13 Nov 2010 04:17:03 PM GMT
                Category: Foundation
                Priority: 5 - Normal
                Severity: 4 - Important
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

I recently encountered a serious bug in gnustep-base-1.20.1 when building
using libffi.   This bug would affect any application which proxies methods
using methodSignatureForSelector: and forwardInvocation:

When libffi is used for message forwarding, an object's
methodSignatureForSelector: instance method is never invoked if the caller
invokes the proxy using a call like the following:

[proxy performSelector:aSelector withObject:anObject]

This is because the implementation of [NSObject performSelector:withObject:]
uses the following call to lookup the IMP for the specified selector.

  return get_imp(object_getClass(self), aSelector);

Note that because get_imp() only looks up methods in the object's class, it
cannot possibly invoke methodSignatureForSelector: for the instance to
determine the correct method signature for forwarded methods.

This bug only affects libffi, because FFI forwarding requires a correct
signature to create a valid IMP to return as a trampoline.  In contrast, the
ffcall interface invokes methodSignatureForSelector: later in the call to the
function returned by get_imp().

Attached is a patch which resolves this problem for gnustep-base-1.20.1. 
This patch replaces the calls to get_imp() with objc_msg_lookup() which
correctly handles message forwarding.

Regards,


Jonathan P. Olson




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File Attachments:


-------------------------------------------------------
Date: Sat 13 Nov 2010 04:17:03 PM GMT  Name: nsobject.patch  Size: 1kB   By:
jpolsonaz

<http://savannah.gnu.org/support/download.php?file_id=22002>

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