From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Kees Cook <keescook@chromium.org>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Oleg Nesterov <oleg@redhat.com>,
David Howells <dhowells@redhat.com>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Rik van Riel <riel@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Ricky Zhou <rickyz@chromium.org>, Julien Tinnes <jln@google.com>
Subject: Re: [PATCH] user_ns: use correct check for single-threadedness
Date: Wed, 5 Aug 2015 14:53:19 +0300 [thread overview]
Message-ID: <20150805115319.GB25784@node.dhcp.inet.fi> (raw)
In-Reply-To: <20150805113854.GA9110@gmail.com>
On Wed, Aug 05, 2015 at 01:38:54PM +0200, Ingo Molnar wrote:
>
> * Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> > On Tue, Jul 28, 2015 at 02:35:04PM -0700, Andrew Morton wrote:
> > > On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@chromium.org> wrote:
> > >
> > > > From: Ricky Zhou <rickyz@chromium.org>
> > > >
> > > > Checking mm_users > 1 does not mean a process is multithreaded. For
> > > > example, reading /proc/PID/maps temporarily increments mm_users, allowing
> > > > other processes to (accidentally) interfere with unshare() calls.
> > > >
> > > > This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> > > > returning EINVAL if another processes happened to be simultaneously
> > > > reading the maps file.
> > >
> > > Yikes. current_is_single_threaded() is expensive. Are we sure this
> > > isn't going to kill someone's workload?
> >
> > It's expensive only if mm_users > 1. We will go to for_each_process() only
> > if somebody outside of the process grabs mm_users references (like reading
> > /proc/PID/maps). Or if it called it from multithreaded application.
>
> It's considerably expensive:
>
> rcu_read_lock();
> for_each_process(p) {
> do {
> ...
> } while_each_thread(p, t);
> }
>
>
> 'only' if it's multi-threaded, i.e. when some workload cares so much about
> performance that it uses multiple threads?
>
> Can you see the contradiction there?
I can. man 2 unshare:
CLONE_NEWUSER requires that the calling process is not threaded;
The workload cares so much about performance that it ignores API
requirements. Some slow down looks like a fair price to me.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2015-08-05 11:53 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-28 17:15 [PATCH] user_ns: use correct check for single-threadedness Kees Cook
2015-07-28 18:02 ` Rik van Riel
2015-07-28 18:17 ` Eric W. Biederman
2015-07-28 20:55 ` Ricky Zhou
2015-07-28 21:01 ` Kees Cook
2015-08-05 18:13 ` Eric W. Biederman
2015-08-05 19:40 ` Kees Cook
2015-07-28 21:35 ` Andrew Morton
2015-07-28 21:50 ` Kees Cook
2015-07-28 22:11 ` Kirill A. Shutemov
2015-08-05 11:38 ` Ingo Molnar
2015-08-05 11:53 ` Kirill A. Shutemov [this message]
2015-08-05 13:13 ` Ricky Zhou
2015-08-05 17:23 ` Oleg Nesterov
2015-08-05 18:00 ` Eric W. Biederman
2015-08-05 18:52 ` Eric W. Biederman
2015-08-06 13:06 ` Oleg Nesterov
2015-08-06 13:44 ` Oleg Nesterov
2015-08-12 1:17 ` Eric W. Biederman
2015-08-12 14:40 ` Oleg Nesterov
2015-08-12 15:11 ` Eric W. Biederman
2015-08-12 1:22 ` [PATCH 0/2] userns: Creation logic fixes Eric W. Biederman
2015-08-12 1:24 ` [PATCH 1/2] unshare: Unsharing a thread does not require unsharing a vm Eric W. Biederman
2015-08-12 17:48 ` Oleg Nesterov
2015-08-12 18:39 ` Eric W. Biederman
2015-08-13 12:55 ` Oleg Nesterov
2015-08-13 15:38 ` Eric W. Biederman
2015-08-13 16:17 ` Oleg Nesterov
2015-08-13 16:27 ` Eric W. Biederman
2015-08-13 16:50 ` Oleg Nesterov
2015-08-14 17:59 ` Oleg Nesterov
2015-08-12 19:59 ` [PATCH v2] " Eric W. Biederman
2015-08-13 12:57 ` Oleg Nesterov
2015-08-13 16:01 ` Eric W. Biederman
2015-08-13 16:30 ` Oleg Nesterov
2015-08-13 16:39 ` Eric W. Biederman
2015-08-12 1:25 ` [PATCH 2/2] userns,pidns: Force thread group sharing, not signal handler sharing Eric W. Biederman
2015-08-12 17:24 ` Oleg Nesterov
2015-08-12 6:29 ` [PATCH 0/2] userns: Creation logic fixes Kees Cook
2015-08-06 14:35 ` [PATCH] user_ns: use correct check for single-threadedness Oleg Nesterov
2015-08-06 21:16 ` Eric W. Biederman
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