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[PATCH v3 2/4] pid: check init is created first after idr alloc To: Oleg Nesterov , Andrei Vagin Cc: 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 References: <20260224164852.306583-1-ptikhomirov@virtuozzo.com> <20260224164852.306583-3-ptikhomirov@virtuozzo.com> Content-Language: en-US From: Pavel Tikhomirov In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: BE1P281CA0235.DEUP281.PROD.OUTLOOK.COM (2603:10a6:b10:8c::10) To DU0PR08MB9003.eurprd08.prod.outlook.com (2603:10a6:10:471::13) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DU0PR08MB9003:EE_|AS8PR08MB6421:EE_ X-MS-Office365-Filtering-Correlation-Id: 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/25/26 10:06, Oleg Nesterov wrote: > 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! I guess it means that we should change the commit message a bit, e.g.: replace "Effectively leading ..." to "Though thanks to PIDNS_ADDING logic, free_pid() disables further pid allocation in this case and it does not lead to any real problem.". Please advise if I should resend the whole series (v4) for that? > >> 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. Maybe it was this way to consolidate the (->pid_allocated & PIDNS_ADDING) check and the (->pid_allocated++) change in one place for readability. Code reads like: we don't want the increment if the flag is unset. > > Oleg. > -- Best regards, Pavel Tikhomirov Senior Software Developer, Virtuozzo.