[lwip-devel] possible issue in sockets.c

[Albert Huitsing]

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Hello members of this list,

Sorry to cut straight to the point, but I think I maybe have a usefull 
contribution to the project.

I've encountered a possible issue in src/api/sockets.c

in the function event_callback() there seems to be a potential problem with 
detecting possible
modification of the list while looping:

      }
      if (do_signal) {
        scb->sem_signalled = 1;
        /* Don't call SYS_ARCH_UNPROTECT() before signaling the semaphore, as 
this might
           lead to the select thread taking itself off the list, invalidagin 
the semaphore. */
        sys_sem_signal(&scb->sem);
      }
    }
    /* unlock interrupts with each step */
    last_select_cb_ctr = select_cb_ctr;


the call to sys_sem_signal() could cause a modification of the list.

so I think the 'select_cb_ctr' has to be stored *before* the call to 
sys_sem_signal()

what do you think?

kindest regards,

Albert Huitsing



  for (scb = select_cb_list; scb != NULL; scb = scb->next) {

    // remember the cb_ctr before anyone can change
    // select_cb_list (note: 'sys_sem_signal()' can cause a context switch!)
    last_select_cb_ctr = select_cb_ctr;

    if (scb->sem_signalled == 0) {
      /* semaphore not signalled yet */
      int do_signal = 0;
      /* Test this select call for our socket */
      if (sock->rcvevent > 0) {
        if (scb->readset && FD_ISSET(s, scb->readset)) {
          do_signal = 1;
        }
      }
      if (sock->sendevent != 0) {
        if (!do_signal && scb->writeset && FD_ISSET(s, scb->writeset)) {
          do_signal = 1;
        }
      }
      if (sock->errevent != 0) {
        if (!do_signal && scb->exceptset && FD_ISSET(s, scb->exceptset)) {
          do_signal = 1;
        }
      }
      if (do_signal) {
        scb->sem_signalled = 1;
        /* Don't call SYS_ARCH_UNPROTECT() before signaling the semaphore, as 
this might
           lead to the select thread taking itself off the list, invalidagin 
the semaphore. */
        sys_sem_signal(&scb->sem);
      }
    }

    /* unlock interrupts with each step */
    SYS_ARCH_UNPROTECT(lev);
    /* this makes sure interrupt protection time is short */
    SYS_ARCH_PROTECT(lev);
    if (last_select_cb_ctr != select_cb_ctr) {
      /* someone has changed select_cb_list, restart at the beginning */
      goto again;
    }
  }


- -- 
Albert Huitsing (address@hidden)
Huitsing Embedded Systems
Dr. Mondenweg 5
7831 JA  Nw. Weerdinge
+31-(0)591-521222
http://www.huitsing.nl
"conformity is the uncomfortable feeling of wearing
 somebody else's clothes; even when they don't fit"
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