Although I'm not sure it's needed for this problem. A getppid() which does
asmlinkage long sys_getppid(void)
{
int pid;
read_lock(&tasklist_lock);
pid = current->group_leader->real_parent->tgid;
read_unlock(&tasklist_lock);
return pid;
}
seems like a fine implementation to me ;)
Why do we need to use ->group_leader? All threads should have the same
->real_parent.
I'm not sure this is true for old LinuxThreads...
Why do we need tasklist_lock? I think rcu_read_lock() is enough.
In other words, do you see any problems with this code
smlinkage long sys_getppid(void)
{
int pid;
rcu_read_lock();
pid = rcu_dereference(current->real_parent)->tgid;
rcu_read_unlock();
return pid;
}
? Yes, we may read a stale value for ->real_parent, but the memory
can't be freed while we are under rcu_read_lock(). And in this case
the returned value is ok because the task could be reparented just
after return anyway.
Your patch doesn't cure the problem.
rcu_read_lock just disables preemtion and rcu_dereference
introduces memory barrier. _None_ of this _prevents_
another CPU from freeing old real_parent in parallel with your dereference.
You can minimize the probability very much by making local_irq_disable()/enable()
around the code in question, but still it won't be a real fix (at least due to NMIs).
Kirill
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