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Wed, 25 Feb 2026 09:06:44 +0000 (UTC) Received: from fedora (unknown [10.44.32.38]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id B05BB1800352; Wed, 25 Feb 2026 09:06:37 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Wed, 25 Feb 2026 10:06:44 +0100 (CET) Date: Wed, 25 Feb 2026 10:06:35 +0100 From: Oleg Nesterov To: Andrei Vagin Cc: Pavel Tikhomirov , Christian Brauner , Shuah Khan , Kees Cook , Andrew Morton , David Hildenbrand , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Jan Kara , Aleksa Sarai , Andrei Vagin , Kirill Tkhai , Alexander Mikhalitsyn , Adrian Reber , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 2/4] pid: check init is created first after idr alloc Message-ID: References: <20260224164852.306583-1-ptikhomirov@virtuozzo.com> <20260224164852.306583-3-ptikhomirov@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Mimecast-MFC-PROC-ID: Qgf0tV2lO8c8aHxNGhYAuEzDMvTbDIz-HnBHZvM8K_g_1772010405 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam05 X-Rspam-User: X-Rspamd-Queue-Id: BAE91180004 X-Stat-Signature: 8m8u8yohbipkgn75k6r4eycwa3pfb8ak X-HE-Tag: 1772010411-291824 X-HE-Meta: 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 lcNxlw1o Mwu6c5gmpfQphyNlnsRxrqNLXnE8Vk2DzmvVIe2lOv0uc6U2T6svxMTMPaNAWEJNaN137qlitsq9SvJDvesDVLt+SJw6OTJkbnwdduzvkM3/SEVP2voe5dbYrGysVQ0JzlodBSq8QjhQpM8msVN3geyvk4o8oGDmIR6JC8xaxv8qhYRT/gSjAHDDoKFAidbi5t/OS3pfSoBSkgjSEhGx9G3qQY3d1aQVWhqX+yVJwhbiU9LFj5GuxgJr9uTVU+cXvXTtl+wPcyGgRWVS4bjEsJTh5My+C1R6eC+s46t+Ut5SrGrEO+PgNVRcAg96eCFMTzcf1vkotAq0iyrymiExYR7hdApzov5ViSTWDHmQRSPIun7mOSXbBsNakBo+zTcxKOO5fuEwz/avr680mfxsa1C5wBctp1cUh8hGeKR6n4K/VA4uFjEuKq9/k2g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02/24, Andrei Vagin wrote: > > On Tue, Feb 24, 2026 at 9:02 AM Pavel Tikhomirov > wrote: > > > > This moves the condition (tid != 1 && !tmp->child_reaper) to after idr > > alloc, so it not only covers that first process in pid namespace has pid > > 1 in case of clone3(set_tid) requesting wrong pid, but also if idr > > itself gives wrong pid for some reason. > > > > This could've been the case before this patch, when creating first > > process the alloc_pid()->pidfs_add_pid() code path fails, so that the > > idr->idr_next is non zero anymore and next process calling to > > alloc_pid(), will get 2 as a pid from idr_alloc_cyclic(). Effectively > > leading to init-less pid namespace, which is a bug. > > I don't think this is an issue right now, as we only allow one attempt > to create the init. If that first attempt fails, free_pid will disable > further PID allocations in the target namespace. Thanks, you are right! > This isn't directly related to this patch, but why do we check > "ns->pid_allocated & PIDNS_ADDING" after allocating all pids? Wouldn't > it be more reasonable to do that right after taking the pidmap_lock? Perhaps, but alloc_pid() can drop pidmap_lock and retry, so we will need to re-check PIDNS_ADDING. Oleg.