linux-kernel - Re: [PATCH v2] unshare: Unsharing a thread does not require unsharing a vm

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Date:	Thu, 13 Aug 2015 18:30:25 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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 v2] unshare: Unsharing a thread does not require
	unsharing a vm

On 08/13, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > On 08/12, Eric W. Biederman wrote:
> >>
> >> +	if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
> >> +		if (atomic_read(&current->sighand->count) > 1)
> >> +			return -EINVAL;
> >> +	}
> >
> > I am still not sure we want this... please the the previous email.
>
> Reading your other email I did not see why you thought this check was
> unnecessary.
>
> > But perhaps I missed something.
>
> In short:
> clone(VM) --> mm_users > 1 && sighand_struct->count == 1
> followed by:
> unshare(SIGHAND)
> the unshare should succeed.
>
> Meanwhile:
> clone(VM|SIGHAND) --> mm_users > 1 && sighand_struct->count > 1
> followed by:
> unshare(SIGHAND)
> the unshare should fail.

Yes, yes, yes.

But once again, I meant we can remove this sighand->count check
if unshare(SIGHAND) checks current_is_single_threaded(). That is
why I suggested to do

	if (unshare_flags & CLONE_SIGHAND)
		unshare_flags |= CLONE_VM;

in sys_unshare(), or change check_unshare_flags() to check
"unshare_flags & (CLONE_VM | CLONE_SIGHAND)" before
current_is_single_threaded().



Damn. And this discussion makes me think that another cleanup makes
sense too. Can't we move all these unshare_flags manipulations into
check_unshare_flags? So that sys_unshare() will only do

	err = check_unshare_flags(&unshare_flags);

and the reader of this code won't need to read 2 functions to understand
whats going on.

Oleg.

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