[lwip-devel] [bug #26027] Chaining netbufs

[Hans-Erik Floryd]

URL:
  <http://savannah.nongnu.org/bugs/?26027>

                 Summary: Chaining netbufs
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: hefloryd
            Submitted on: Fri 27 Mar 2009 13:48:56 GMT
                Category: None
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.3.0

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Details:

When trying to chain two netbufs (identifier and payload) together,
netbuf_chain calls memp_free on the tail. It is unclear if/how the pbuf
associated with the tail can then be reclaimed. Test case:

   identifier = netbuf_new();
   payload = netbuf_new();

   netbuf_ref (identifier, pId, pIdSize);
   netbuf_ref (payload, pData, pDataSize);
   netbuf_chain (identifier, payload);

   netconn_sendto (conn, identifier, &addr, port);
   netbuf_delete (payload);
   netbuf_delete (identifier);

netbuf_chain calls memp_free on the tail (i.e. payload), so when
netbuf_delete(payload) is called, payload is no longer valid. On the other
hand, if netbuf_delete(payload) is not called, a pbuf is lost because the
reference count on the payload data is two, so netbuf_delete(identifier) will
not free the data.

Is it correct for netbuf_chain to free the tail netbuf? If that one line is
removed the code above works fine.

address@hidden suggested this in the lwip-users mailing list:
"[...] from reading the code, netbuf_chain should either call netbuf_delete,
use pbuf_cat instead of pbuf_chain or don't call memp_free. Which is right I
don't know."







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