Messages in this thread | | | From | "Eric W. Biederman" <> | Date | Wed, 27 Apr 2022 09:20:51 -0500 | Subject | Re: [PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held |
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Oleg Nesterov <oleg@redhat.com> writes:
> On 04/26, Eric W. Biederman wrote: >> >> @@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk, >> } >> >> sighand = parent->sighand; >> - spin_lock_irqsave(&sighand->siglock, flags); >> + lock = tsk->sighand != sighand; >> + if (lock) >> + spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING); > > But why is it safe? > > Suppose we have two tasks, they both trace each other, both call > ptrace_stop() at the same time. Of course this is ugly, they both > will block. > > But with this patch in this case we have the trivial ABBA deadlock, > no?
I was thinking in terms of the process tree (which is fine).
The ptrace parental relationship definitely has the potential to be a graph with cycles. Which as you point out is not fine.
The result is very nice and I don't want to give it up. I suspect something ptrace cycles are always a problem and can simply be forbidden. That is going to take some analsysis and some additional code in ptrace_attach.
I will go look at that.
Eric
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