[PATCH] 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.

Signed-off-by: David Meybohm <dmeybohmlkml@bellsouth.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/kernel/timer.c b/kernel/timer.c
index f2a1188..5377f40 100644 (file)
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1023,7 +1023,7 @@ 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;