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From: Oleg Nesterov <oleg@redhat.com>
To: Andrei Vagin <avagin@google.com>
Cc: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
	Christian Brauner <brauner@kernel.org>,
	Shuah Khan <shuah@kernel.org>, Kees Cook <kees@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Jan Kara <jack@suse.cz>, Aleksa Sarai <cyphar@cyphar.com>,
	Kirill Tkhai <tkhai@ya.ru>,
	Alexander Mikhalitsyn <alexander@mihalicyn.com>,
	Adrian Reber <areber@redhat.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v2 1/2] pid_namespace: allow opening pid_for_children before init was created
Date: Tue, 24 Feb 2026 17:09:56 +0100	[thread overview]
Message-ID: <aZ3NVMEpeWKOxFkC@redhat.com> (raw)
In-Reply-To: <CAEWA0a61jLQxULDMXLcpZn2M-cJXxzSMyCS8C4y=YkFfHAEYng@mail.gmail.com>

On 02/24, Andrei Vagin wrote:
>
> On Tue, Feb 24, 2026 at 5:38 AM Pavel Tikhomirov
> >
> > This check here is more for the concurrent alloc_pid() case. When one process
> > in alloc_pid() successfully allocated the pid and than, for instance, hit the
> > pidfs_add_pid() error and is going to free_pid(), but the pid 1 is remains yet
> > allocated from idr and the cursor is on 2 at the moment. At the same time the
> > concurrent process may get to alloc_pid(), and will see cursor on 2, it should
> > not be able to create a process as this process will get pid 2 and will be
> > created before init.
> >
> > And in general (non concurrent case) it makes sense to only allow allocating 1,
> > for the first process.
>
> In this case, there is likely a race condition. Two alloc_pid() calls
> can run concurrently,
> where idr_get_cursor() returns 0 in both instances.

This code runs with pidmap_lock held.

But,

> I think this
> check needs to
> be moved after idr_alloc_cyclic() to verify the actual value that was allocated.

Agreed, see other emails in this thread.

Oleg.



  reply	other threads:[~2026-02-24 16:10 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-02-23 20:01 [PATCH v2 0/2] pid_namespace: make init creation more flexible Pavel Tikhomirov
2026-02-23 20:01 ` [PATCH v2 1/2] pid_namespace: allow opening pid_for_children before init was created Pavel Tikhomirov
2026-02-24  7:02   ` Andrei Vagin
2026-02-24 10:37     ` Pavel Tikhomirov
2026-02-24 15:38       ` Andrei Vagin
2026-02-24 16:09         ` Oleg Nesterov [this message]
2026-02-24 12:09   ` Oleg Nesterov
2026-02-24 13:23     ` Pavel Tikhomirov
2026-02-24 16:03       ` Oleg Nesterov
2026-02-24 16:35         ` Pavel Tikhomirov
2026-02-23 20:01 ` [PATCH v2 2/2] selftests: Add tests for creating pidns init via setns Pavel Tikhomirov

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