linux-kernel - Re: [PATCH v3] do_wait: make PIDTYPE

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Date:   Thu, 11 Mar 2021 10:37:27 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jim Newsome <jnewsome@...project.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] do_wait: make PIDTYPE_PID case O(1) instead of O(n)

Oleg Nesterov <oleg@...hat.com> writes:

> On 03/10, Eric W. Biederman wrote:
>>
>> Jim Newsome <jnewsome@...project.org> writes:
>>
>> > +static int do_wait_pid(struct wait_opts *wo)
>> > +{
>> > +	struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID);
>>                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> This is subtle change in behavior.
>>
>> Today on the task->children list we only place thread group leaders.
>
> Aaah, yes, thanks Eric!
>
>> So the code either needs a thread_group_leader filter on target before
>> the ptrace=0 case or we need to use "pid_task(wo->wo_pid, PIDTYPE_TGID)"
>> and "pid_task(wo->wo_pid, PIDTYPE_PID)" for the "ptrace=1" case.
>
> Agreed,
>
>> I would like to make thread_group_leaders go away
>
> Hmm, why?

Mostly because we have class of very nasty bugs to fix because code
thinks one thread is special.

There has been and I think still is code that mishandles zombie thread
group leaders.

Particularly nasty are zombie thread group leaders after userspace has
called setresuid in a way that changes signal permissions.

Eric