linux-kernel - Re: [PATCH] user_ns: use correct check for single-threadedness

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Date:	Tue, 11 Aug 2015 20:17:48 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Ricky Zhou <rickyz@...omium.org>,
	Julien Tinnes <jln@...gle.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness

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

> On 08/06, Oleg Nesterov wrote:
>>
>> On 08/05, Eric W. Biederman wrote:
>> >
>> > So I have to ask.
>>
>> I hope you are asking someone else, not me ;) I never understood what
>> exactly we try to restrict and why.
>>
>> > Is it possible to rework these checks such that we
>> > look at the sighand struct and signal sharing handling sharing instead
>> > of the count on the mm_struct?
>>
>> Then why we can't simply check thread_group_empty() == T ? Why should we
>> worry about CLONE_SIGHAND at all?
>
> The same for clone() actually... I forgot why we decided to check
> CLONE_SIGHAND, iirc I suggested CLONE_THREAD initially then we switched
> to CLONE_SIGHAND "just in case", to make it as strict as possible.

I do agree that making the test be for CLONE_THREAD is safe, makes
sense, and is less confusing than what we have now.x

> How about the patch below?
>
> (note that the "or parent" part of the comment is wrong in any case).

It was correct.  You failed to removed it when you removed CLONE_PARENT
from that test.

Eric

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