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Wed, 25 Feb 2026 03:57:27 -0500 X-MC-Unique: FGNCDzsOM22o-VbrbTemxw-1 X-Mimecast-MFC-AGG-ID: FGNCDzsOM22o-VbrbTemxw_1772009845 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6078F180025C; Wed, 25 Feb 2026 08:57:25 +0000 (UTC) Received: from fedora (unknown [10.44.32.38]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id E428D1955F43; Wed, 25 Feb 2026 08:57:18 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Wed, 25 Feb 2026 09:57:24 +0100 (CET) Date: Wed, 25 Feb 2026 09:57:17 +0100 From: Oleg Nesterov To: Pavel Tikhomirov 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 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 3.0 on 10.30.177.17 X-Mimecast-MFC-PROC-ID: OvzAtXUvY_Fizrybrb4rVjkp32N1aAAVWBd6O4076fk_1772009845 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam09 X-Stat-Signature: uuecrt1pdmmqg9irdmfkkzenmdo1tpmn X-Rspamd-Queue-Id: 056F78000C X-Rspam-User: X-HE-Tag: 1772009851-131798 X-HE-Meta: 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 iWPqT+w0 AYQG7ucltzaUpTP+MCL6/RKgnOLJufB3M+oHUK3590MspnTbNtZdLivBWtg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02/25, Oleg Nesterov wrote: > > On 02/24, 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. > > Yes. > > alloc_pid() does: > > /* On failure to allocate the first pid, reset the state */ > if (ns->pid_allocated == PIDNS_ADDING) > idr_set_cursor(&ns->idr, 0); > > but this logic is broken. > > Suppose that a task P does sys_unshare(CLONE_NEWPID). Then it does > fork(), and fork() fails for any reason after alloc_pid() succeeds. > If P does another fork() to retry, we have a bug. > > So with this patch we can either remove the code above, or (better) > improve this logic. Damn, no, I was wrong ;) As Andrei pointed out, free_pid() will clear PIDNS_ADDING in this case. Still I think the patch is fine. Oleg.