Re: [PATCH] sys_getppid oopses on debug kernel (v2)

From: Kirill Korotaev
Date: Wed Aug 09 2006 - 08:04:44 EST

Although I'm not sure it's needed for this problem. A getppid() which does

asmlinkage long sys_getppid(void)
int pid;

pid = current->group_leader->real_parent->tgid;

return pid;

seems like a fine implementation to me ;)

Why do we need to use ->group_leader? All threads should have the same
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;

pid = rcu_dereference(current->real_parent)->tgid;

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).


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