preempt race in getppid

With CONFIG_PREEMPT && !CONFIG_SMP, it's possible for sys_getppid to return a bogus value if the parent's task_struct gets reallocated after current->group_leader->real_parent is read:

        asmlinkage long sys_getppid(void)
        {
                int pid;
                struct task_struct *me = current;
                struct task_struct *parent;

                parent = me->group_leader->real_parent;
RACE HERE =>    for (;;) {
                        pid = parent->tgid;       
        #ifdef CONFIG_SMP
        {
                        struct task_struct *old = parent;

                        /*
                         * Make sure we read the pid before re-reading the
                         * parent pointer:
                         */
                        smp_rmb();
                        parent = me->group_leader->real_parent;
                        if (old != parent)
                                continue;
        }
        #endif
                        break;
                }
                return pid;
        }

If the process gets preempted at the indicated point, the parent process can go ahead and call exit() and then get wait()'d on to reap its task_struct. When the preempted process gets resumed, it will not do any further checks of the parent pointer on !CONFIG_SMP: it will read the bad pid and return.

So, the same algorithm used when SMP is enabled should be used when preempt is enabled, which will recheck ->real_parent in this case.

---

 kernel/timer.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

Index: v2.6/kernel/timer.c
===================================================================
@@ -999,7 +999,7 @@ asmlinkage long sys_getppid(void)
 	parent = me->group_leader->real_parent;
 	for (;;) {
 		pid = parent->tgid;
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
 {
 		struct task_struct *old = parent;