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[lwip-devel] [bug #26405] Prematurely released semaphore causes lwip_sel

From: Konstantin
Subject: [lwip-devel] [bug #26405] Prematurely released semaphore causes lwip_select() to crash
Date: Thu, 30 Apr 2009 07:19:40 +0000
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; ru; rv: Gecko/2009042316 Firefox/3.0.10


                 Summary: Prematurely released semaphore causes lwip_select()
to crash
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: konstua
            Submitted on: Чтв 30 Апр 2009 07:19:38
                Category: sockets
                Severity: 3 - Normal
              Item Group: Crash Error
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.3.0



While trying to get lwIP to work on a FreeRTOS port for Blackfin BF53x, I
have observed elusive memory corruption. After a lot of debugging I have
traced the cause to the following code:

(sockets.c :: event_callback())

if (scb) {
      scb->sem_signalled = 1;
    } else {

It seems that, at least on my system, a situation occured where lwip_select()
was waiting for selectsem in another thread, and as soon as it was released,
lwip_select() could be exited, leaving scb (pointing at a local variable
inside lwip_select()) an invalid reference.

Suggestion: swapping the two subsequent calls to sys_sem_signal() seems to
resolve the problem:

if (scb) {
      scb->sem_signalled = 1;
      sys_sem_signal(scb->sem); /* -- before selectsem */
      sys_sem_signal(selectsem); /* -- may invalidate scb */
    } else {


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