linux-kernel - Re: [RFC][PATCH 07/11] signal: Deliver group signals via PIDTYPE_TGID not PIDTYPE

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Date:   Mon, 16 Jul 2018 09:50:20 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Wen Yang <wen.yang99@....com.cn>,
        majiang <ma.jiang@....com.cn>
Subject: Re: [RFC][PATCH 07/11] signal: Deliver group signals via PIDTYPE_TGID not PIDTYPE_PID

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

> On 07/10, Eric W. Biederman wrote:
>>
>> Now that we can make the distinction use PIDTYPE_TGID rather than
>> PIDTYPE_PID.
>
> Wai, wait, this doesn't look right...
>
>> There is no immediate effect as they point point at the
>> same task,
>
> How so? pid_task(pid, PIDTYPE_TGID) will return NULL unless this pid is actually
> a group leader's pid,
>
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -1315,7 +1315,7 @@ int kill_pid_info(int sig, struct siginfo *info, struct pid *pid)
>>
>>  	for (;;) {
>>  		rcu_read_lock();
>> -		p = pid_task(pid, PIDTYPE_PID);
>> +		p = pid_task(pid, PIDTYPE_TGID);
>>  		if (p)
>>  			error = group_send_sig_info(sig, info, p);
>
> So, currently kill(pid_nr) always works, even if pid_nr is a sub-thread's tid.
>
> After this change kill(2) will always fail with -ESRCH in this case.
>
> Or I am totally confused?

No you are not.

That does at least need to be documented in the description of the
patch.

In practice since glibc does not make thread id's available I don't
expect anyone relies on this behavior.  Since no one relies on it we
can change it without creating a regression.

I believe this can be described as fixing a bug that we were not able to
before the introduction of PIDTYPE_TGID.

I will update my change description.

Eric