Re: [PATCH 1/2] wait/ptrace: always assume __WALL if the child is traced

[Pedro Alves <palves@redhat.com>]

On 10/28/2015 04:11 PM, Oleg Nesterov wrote:
> On 10/26, Pedro Alves wrote:
>>
>> On 10/25/2015 03:54 PM, Oleg Nesterov wrote:
>>>
>>> In any case, the real question is whether we should change the kernel to
>>> fix the problem, or ask the distros to fix their init's. In the former
>>> case 1/2 looks simpler/safer to me than the change in ptrace_traceme(),
>>> and you seem to agree that 1/2 is not that bad.
>>
>> A risk here seems to be that waitpid will start returning unexpected
>> (thread) PIDs to parent processes,
> 
> I don't see how this change can make the things worse,
> 
>> and it's not unreasonable to assume
>> that e.g., a program asserts that waitpid either returns error or a
>> known (process) PID.
> 
> Well. /sbin/init can never assume this, obviously.

Right.  I was actually thinking of !init processes -- basically code
that spawns helper processes, keeps a data structure indexed by pid, then
discards the structure when the child exits.  Something like:

 pid = waitpid(-1, &status, 0);
 if (pid > 0)
 {
   struct child_process *child = find_process(pid);
   assert (child != NULL);
 }

As in, before your change, the child could get stuck forever, but after your
change, the parent could die/assert instead.

But ...

> 
>> That's not an init-only issue,
> 
> Yes. Because we have CLONE_PARENT. So "waitpid either returns error or a
> known (process) PID" is only true if you trust your children.

... OK, that's indeed a good point.

>> (Also, in the original test case, if the child gets/raises a signal or execs
>> before exiting, the bash/init/whatever process won't be issuing PTRACE_CONT,
>> and the child will thus end up stuck (though should be SIGKILLable,
>
> Oh, but if it is killable everything is fine. How does this differ from the
> case when, say, you jusr reparent to init and do kill(getpid(), SIGSTOP) ?

The difference is that if the child called PTRACE_TRACEME, then it goes
to ptrace-stop instead and no amount of SIGCONT unstucks it -- the only way
out is force killing.  I agree it's not a major issue as there's a way out
(and thus made it a parens), but I wouldn't call it nice either.

>> All this because PTRACE_TRACEME is broken by design
>
> Heh. I agree. But we can't fix it now.

Perhaps the man page could document it as deprecated, suggesting
PTRACE_ATTACH/PTRACE_SEIZE instead?

Thanks,
Pedro Alves