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SubjectRe: [PATCH 2/2] fix kill_proc_info() vs fork() theoretical race
On Wed, Feb 15, 2006 at 10:13:26PM +0300, Oleg Nesterov wrote:
> copy_process:
>
> attach_pid(p, PIDTYPE_PID, p->pid);
> attach_pid(p, PIDTYPE_TGID, p->tgid);
>
> What if kill_proc_info(p->pid) happens in between?

Doesn't your patch 1/2 that expanded the scope of siglock in
copy_process() prevent this from happening?

o A new process is being created on CPU 0, and does the first
attach_pid() in copy_process(), but has not yet done
the second attach_pid().

o Meanwhile, on CPU 1, kill_proc_info() successfully looks up the
new process via find_task_by_pid().

o Also on CPU 1, kill_proc_info() calls group_send_sig_info(),
which checks permissions, locates the sighand structure,
then attempts to acquire siglock.

Given your patch 1/2, CPU 1 cannot proceed until CPU 0 gets
done with the remaining attach_pid() calls.

So, what am I missing this time? ;-)

Thanx, Paul

> copy_process() holds current->sighand.siglock, so we are safe
> in CLONE_THREAD case, because current->sighand == p->sighand.
>
> Otherwise, p->sighand is unlocked, the new process is already
> visible to the find_task_by_pid(), but have a copy of parent's
> 'struct pid' in ->pids[PIDTYPE_TGID].
>
> This means that __group_complete_signal() may hang while doing
>
> do ... while (next_thread() != p)
>
> We can solve this problem if we reverse these 2 attach_pid()s:
>
> attach_pid() does wmb()
>
> group_send_sig_info() calls spin_lock(), which
> provides a read barrier. // Yes ?
>
> I don't think we can hit this race in practice, but still.
>
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
>
> --- 2.6.16-rc3/kernel/fork.c~2_HANG 2006-02-15 23:21:51.000000000 +0300
> +++ 2.6.16-rc3/kernel/fork.c 2006-02-16 00:03:20.000000000 +0300
> @@ -1173,8 +1173,6 @@ static task_t *copy_process(unsigned lon
> if (unlikely(p->ptrace & PT_PTRACED))
> __ptrace_link(p, current->parent);
>
> - attach_pid(p, PIDTYPE_PID, p->pid);
> - attach_pid(p, PIDTYPE_TGID, p->tgid);
> if (thread_group_leader(p)) {
> p->signal->tty = current->signal->tty;
> p->signal->pgrp = process_group(current);
> @@ -1184,6 +1182,8 @@ static task_t *copy_process(unsigned lon
> if (p->pid)
> __get_cpu_var(process_counts)++;
> }
> + attach_pid(p, PIDTYPE_TGID, p->tgid);
> + attach_pid(p, PIDTYPE_PID, p->pid);
>
> nr_threads++;
> total_forks++;
>
-
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