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 <[email protected]>
---
kernel/timer.c | 2 +-
1 files changed, 1 insertion(+), 1 deletion(-)
Index: v2.6/kernel/timer.c
===================================================================
--- v2.6.orig/kernel/timer.c 2005-08-19 07:29:29.000000000 -0400
+++ v2.6/kernel/timer.c 2005-08-19 07:31:25.000000000 -0400
@@ -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;
-
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