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Tue, 24 Feb 2026 16:10:04 +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 E12E21800361; Tue, 24 Feb 2026 16:09:57 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Tue, 24 Feb 2026 17:10:04 +0100 (CET) Date: Tue, 24 Feb 2026 17:09:56 +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 , Kirill Tkhai , Alexander Mikhalitsyn , Adrian Reber , 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 Message-ID: References: <20260223200254.4104651-1-ptikhomirov@virtuozzo.com> <20260223200254.4104651-2-ptikhomirov@virtuozzo.com> <4de06792-d8ea-4f2e-848d-0dfccdb64253@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: bR7rUqfenO6qX177hmm0kc3X0Llj3gwSD6ptsrrIngA_1771949405 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 336EF1C0004 X-Stat-Signature: 9qkh9jogbiu1pqb7kdhy31bxw4iewtrr X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1771949412-407124 X-HE-Meta: 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 +SyeHh/3 4bbE8uG843G9vnUIRjL9agMEXoD6bzIOAPmMfAGU8HWU7st2T1EBtxxsR3nQnRZo3jMxnCULGPhW1jypRzOF7h5UaA/S977KTLYdqmKx9EwZADyCPC6oCewlRQig6ZXdglbB2xr5RpKloesjsuU7Mij+MmcMaCAyc8DQfPA9axFr8vkRx/mLXKcms6kEY1eSXUPslaLsz0IuZrpi24YF/99b8nwl5EX9fNSwlrjgm8FlucfevOCubw2xdr8XU1cHmoF3GWKSx93BI2eCdGxt2vT0zK/L17iLyD7Jbo5hlY2A+Y772hOuEl2mqPEylf0wRDflRFNXtjBz3KH6sk2N7wFUuxiHQoyAVGDPR2uxjq2l9ZIm8yoxlt49T4xqUT2t+JwX6ifRxUNh12lO5MDI7legVOni9RForYuX1Iz/GV2Q0qwo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 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 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.