Re: [RFC PATCH 7/X] ptrace: mv task->parent ptrace_task->pt_tracer

Oleg Nesterov
2009-05-28T02:58:48Z

On 05/27, Roland McGrath wrote:
>
> > OK, lets do this change later ;)
>
> Agreed.

Damn. OTOH, it is so ugly to cache pid/uid for the very unlikely case.

Roland, will you agree with the patch below?

If yes, I'd like to send it separately.

Oleg.

-------------------------------------------------------------------
[PATCH -mm] ptrace: ptrace_signal: fix the usage of ->parent

This patch complicates the code to fix the pure theoretical problems.
But since we are going to change this code, it is better to fix them
anyway.

- If we are not traced any longer after ptrace_stop(), si_pid/si_uid
are not necessary right.

- It is not safe to dereference current->parent without tasklist or
RCU lock. The tracer can detach and exit. ->siglock can't prevent
this, and (in theory) local_irq_disable() doesn't imply RCU lock.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

--- PTRACE/kernel/signal.c~PTRACE_SIGNAL 2009-05-28 04:12:02.000000000 +0200
+++ PTRACE/kernel/signal.c 2009-05-28 04:14:38.000000000 +0200
@@ -1770,11 +1770,22 @@ static int ptrace_signal(int signr, sigi
specific in the siginfo structure then it should
have updated *info via PTRACE_SETSIGINFO. */
if (signr != info->si_signo) {
+ struct task_struct *tracer;
+
info->si_signo = signr;
info->si_errno = 0;
info->si_code = SI_USER;
- info->si_pid = task_pid_vnr(current->parent);
- info->si_uid = task_uid(current->parent);
+
+ rcu_read_lock();
+ tracer = current->parent;
+ if (task_ptrace(current)) {
+ info->si_pid = task_pid_vnr(tracer);
+ info->si_uid = task_uid(tracer);
+ } else {
+ info->si_pid = 0;
+ info->si_uid = 0;
+ }
+ rcu_read_unlock();
}

/* If the (new) signal is now blocked, requeue it. */