Peter Zijlstra <pet...@infradead.org> writes:
> On Wed, Apr 27, 2022 at 09:47:10AM -0500, Eric W. Biederman wrote:
>
>> Hmm. If we have the following process tree.
>>
>> A
>> \
>> B
>> \
>> C
>>
>> Process A, B, and C are all in the same process group.
>> Process A and B are setup to receive SIGCHILD when
>> their process stops.
>>
>> Process C traces process A.
>>
>> When a sigstop is delivered to the group we can have:
>>
>> Process B takes siglock(B) siglock(A) to notify the real_parent
>> Process C takes siglock(C) siglock(B) to notify the real_parent
>> Process A takes siglock(A) siglock(C) to notify the tracer
>>
>> If they all take their local lock at the same time there is
>> a deadlock.
>>
>> I don't think the restriction that you can never ptrace anyone
>> up the process tree is going to fly. So it looks like I am back to the
>> drawing board for this one.
>
> I've not had time to fully appreciate the nested locking here, but if it
> is possible to rework things to always take both locks at the same time,
> then it would be possible to impose an arbitrary lock order on things
> and break the cycle that way.
>
> That is, simply order the locks by their heap address or something:
>
> static void double_siglock_irq(struct sighand *sh1, struct sighand2 *sh2)
> {
> if (sh1 > sh2)
> swap(sh1, sh2)
>
> spin_lock_irq(&sh1->siglock);
> spin_lock_nested(&sh2->siglock, SINGLE_DEPTH_NESTING);
> }
You know it might be. Especially given that the existing code is
already dropping siglock and grabbing tasklist_lock.
It would take a potentially triple lock function to lock
the task it's real_parent and it's tracer (aka parent).
That makes this possible to consider is that notifying the ``parents''
is a fundamental part of the operation so we know we are going to
need the lock so we can move it up.
Throw in a pinch of lock_task_sighand and the triple lock function
gets quite interesting.
It is certainly worth trying, and I will.
Eric
_______________________________________________
linux-um mailing list
linux-um@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-um
