From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DA3ACCA6F for ; Sun, 22 Feb 2026 15:38:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771774719; cv=none; b=AVkpJdXpmzdgTrLu1hOAY2Npab0ujaDhFZkIk78kwLeF6aEDc8Ak3BldctLmf0Xw3g5Jigz+sM0YQ91ZKZLFwDWNCkyZa+puQ+JVEDzptEGb3RtvfW7PXw8BaVg+/eNv6DP+6TU98pAK2XVhSdLvrzBwsK/BV/ebuhUoZIl2d5U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771774719; c=relaxed/simple; bh=gokw9VKswMNmDkG640UqNZUw5Y4isY0FCdwYm44ip2k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NSQU4Z+ALX0KbmkGtHf1CfsGpXqIuab+SDmd+sUE2utSyVxvDmit5KfsQBL+NXGXE7jsDdzRxxEVPwbQncgoduEKAyGAsq+F82IzuJp+zCFZLEQvQwQe6O2sUKb+D0kmanicpdzjciUbhLMDT1Oq9A01muCwxEw+Vb+Bd8wLAno= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=MfOfaF+x; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="MfOfaF+x" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1771774717; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=JVwf0mUY9YEcprI5Bl/Pa4G3ZzYnRp/WR44iIwoXjv4=; b=MfOfaF+xEV78H7pAgvKggtqieuxFdiTtHX3+17OMuxVD2yRkCo9FP3lDsp/35yzgsv/EEE MLMxbxz/aZsLMnxIBM4FxLYtAtH15bgGEcOMnh5FoPzkkK+wzp2o4mQKg2USQEmUzpWG2t ahp9/WGU8HWKeUi4uvT2iy0f58bZXWg= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-267-Kbx0EiyrN9SalzJk7UU-Jg-1; Sun, 22 Feb 2026 10:38:33 -0500 X-MC-Unique: Kbx0EiyrN9SalzJk7UU-Jg-1 X-Mimecast-MFC-AGG-ID: Kbx0EiyrN9SalzJk7UU-Jg_1771774711 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6E99C19560B7; Sun, 22 Feb 2026 15:38:31 +0000 (UTC) Received: from fedora (unknown [10.44.32.38]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id 3C68219560A7; Sun, 22 Feb 2026 15:38:26 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Sun, 22 Feb 2026 16:38:31 +0100 (CET) Date: Sun, 22 Feb 2026 16:38:25 +0100 From: Oleg Nesterov To: Pavel Tikhomirov Cc: Christian Brauner , Shuah Khan , Jan Kara , Aleksa Sarai , Andrei Vagin , Kirill Tkhai , Alexander Mikhalitsyn , Adrian Reber , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH 1/2] pid_namespace: allow opening pid_for_children before init was created Message-ID: References: <20260220164559.2465466-1-ptikhomirov@virtuozzo.com> <20260220164559.2465466-2-ptikhomirov@virtuozzo.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260220164559.2465466-2-ptikhomirov@virtuozzo.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Pavel, Your patch doesn't apply to Linus's tree. and in any case... can you avoid read_lock(tasklist) in alloc_pid() ? This is really not good. Oleg. On 02/20, Pavel Tikhomirov wrote: > > This effectively gives us an ability to create the pid namespace init as > a child of the process (setns-ed to the pid namespace) different to the > process which created the pid namespace itself. > > Original problem: > > There is a cool set_tid feature in clone3() syscall, it allows you to > create process with desired pids on multiple pid namespace levels. Which > is useful to restore processes in CRIU for nested pid namespace case. > > In nested container case we can potentially see this kind of pid/user > namespace tree: > > Process > ┌─────────┐ > User NS0 ──▶ Pid NS0 ──▶ Pid p0 │ > │ │ │ │ > ▼ ▼ │ │ > User NS1 ──▶ Pid NS1 ──▶ Pid p1 │ > │ │ │ │ > ... ... │ ... │ > │ │ │ │ > ▼ ▼ │ │ > User NSn ──▶ Pid NSn ──▶ Pid pn │ > └─────────┘ > > So to create the "Process" and set pids {p0, p1, ... pn} for it on all > pid namespace levels we can use clone3() syscall set_tid feature, BUT > the syscall does not allow you to set pid on pid namespace levels you > don't have permission to. So basically you have to be in "User NS0" when > creating the "Process" to actually be able to set pids on all levels. > > It is ok for almost any process, but with pid namespace init this does > not work, as currently we can only create pid namespace init and the pid > namespace itself simultaneously, so to make "Pid NSn" owned by "User > NSn" we have to be in the "User NSn". > > We can't possibly be in "User NS0" and "User NSn" at the same time, > hence the problem. > > Alternative solution: > > Yes, for the case of pid namespace init we can use old and gold > /proc/sys/kernel/ns_last_pid interface on the levels lower than n. But > it is much more complicated and introduces tons of extra code to do. It > would be nice to make clone3() set_tid interface also aplicable to this > corner case. > > Implementation: > > Now when anyone can setns to the pid namespace before the creation of > init, and thus multiple processes can fork children to the pid > namespace, we enforce that the first process created is always the init, > and only allow other processes after the init passed the same steps as > were previously required to allow to do setns. > > Signed-off-by: Pavel Tikhomirov > --- > kernel/pid.c | 10 +++++++++- > kernel/pid_namespace.c | 9 --------- > 2 files changed, 9 insertions(+), 10 deletions(-) > > diff --git a/kernel/pid.c b/kernel/pid.c > index a31771bc89c1..d549f08036ab 100644 > --- a/kernel/pid.c > +++ b/kernel/pid.c > @@ -188,9 +188,15 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid, > pid->level = ns->level; > > for (i = ns->level; i >= 0; i--) { > + bool pidns_ready = true; > int tid = 0; > int pid_max = READ_ONCE(tmp->pid_max); > > + read_lock(&tasklist_lock); > + if (!tmp->child_reaper) > + pidns_ready = false; > + read_unlock(&tasklist_lock); > + > if (set_tid_size) { > tid = set_tid[ns->level - i]; > > @@ -201,7 +207,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid, > * Also fail if a PID != 1 is requested and > * no PID 1 exists. > */ > - if (tid != 1 && !tmp->child_reaper) > + if (tid != 1 && !pidns_ready) > goto out_free; > retval = -EPERM; > if (!checkpoint_restore_ns_capable(tmp->user_ns)) > @@ -221,6 +227,8 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid, > */ > if (nr == -ENOSPC) > nr = -EEXIST; > + } else if (!pidns_ready && idr_get_cursor(&tmp->idr) != 0) { > + nr = -EINVAL; > } else { > int pid_min = 1; > /* > diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c > index e48f5de41361..d36afc58ee1d 100644 > --- a/kernel/pid_namespace.c > +++ b/kernel/pid_namespace.c > @@ -369,15 +369,6 @@ static struct ns_common *pidns_for_children_get(struct task_struct *task) > } > task_unlock(task); > > - if (ns) { > - read_lock(&tasklist_lock); > - if (!ns->child_reaper) { > - put_pid_ns(ns); > - ns = NULL; > - } > - read_unlock(&tasklist_lock); > - } > - > return ns ? &ns->ns : NULL; > } > > -- > 2.53.0 >