From nobody Sat Jun 26 03:08:31 2021 X-Original-To: jail@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 406FE11E3659 for ; Sat, 26 Jun 2021 03:08:34 +0000 (UTC) (envelope-from jamie@freebsd.org) Received: from gritton.org (gritton.org [199.192.165.131]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4GBf2p0fnVz4jV1 for ; Sat, 26 Jun 2021 03:08:33 +0000 (UTC) (envelope-from jamie@freebsd.org) Received: from gritton.org ([127.0.0.131]) (authenticated bits=0) by gritton.org (8.15.2/8.15.2) with ESMTPA id 15Q38V5E030807; Fri, 25 Jun 2021 20:08:31 -0700 (PDT) (envelope-from jamie@freebsd.org) List-Id: Discussion about FreeBSD jail(8) List-Archive: https://lists.freebsd.org/archives/freebsd-jail List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-jail@freebsd.org MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Fri, 25 Jun 2021 20:08:31 -0700 From: James Gritton To: jail@freebsd.org Cc: Michael Gmelin Subject: Re: POSIX shared memory and dying jails In-Reply-To: <20210625185859.40fead46@bsd64.grem.de> References: <20210625164100.73c71055@bsd64.grem.de> <03809b2655a40134dd802386afa6be7d@freebsd.org> <20210625185859.40fead46@bsd64.grem.de> User-Agent: Roundcube Webmail/1.4.1 Message-ID: <75475234c76c97c67a8bd2525669c650@freebsd.org> X-Sender: jamie@freebsd.org X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.6.2 (gritton.org [127.0.0.131]); Fri, 25 Jun 2021 20:08:31 -0700 (PDT) X-Rspamd-Queue-Id: 4GBf2p0fnVz4jV1 X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-ThisMailContainsUnwantedMimeParts: N On 2021-06-25 09:58, Michael Gmelin wrote: > On Fri, 25 Jun 2021 09:19:05 -0700 > James Gritton wrote: > >> On 2021-06-25 07:41, Michael Gmelin wrote: >> > It seems like non-anonymous POSIX shared memory is not freed >> > automatically when a jail is removed and keeps it in a dying state, >> > until the shared memory segment is deleted manually. >> > >> > See below for the most basic example: >> > >> > [root@jailhost ~]# jail -c path=/ command=/bin/sh >> > # posixshmcontrol create /removeme >> > # exit >> > [root@jailhost ~]# jls -dv -j shmtest dying >> > true >> > >> > So at this point, the jail is stuck in a dying state. >> > >> > Checking POSIX shared memory segments shows the shared memory >> > segment which is stopping the jail from crossing the Styx: >> > >> > [root@jailhost ~]# posixshmcontrol list >> > MODE OWNER GROUP SIZE PATH >> > rw------- root wheel 0 /removeme >> > >> > After removing the shared memory segment manually... >> > >> > [root@jailhost ~]# posixshmcontrol rm /removeme >> > >> > the jail passes away peacefully: >> > >> > [root@jailhost ~]# jls -dv -j shmtest dying >> > jls: jail "shmtest" not found >> > >> > I wonder if it wouldn't make sense to always remove POSIX shared >> > memory created by a jail automatically when it's removed. >> >> That does seem reasonable, though it would take some bookkeeping to do >> right. There is currently no concrete idea of a jail's ownership of a >> POSIX shm object, as it uses only uid and gid for access permissions, >> same as files. The tie to the jail is in the underlying vm_object, >> which holds a cred that references the jail - that seems to be what's >> keeping the jail from going away. > > Interesting - I was wondering how that worked, thanks. Would there by a > way to cut that tie somehow (for use cases that deliberately want to > leave the shared memory segment behind)? It might be possible to change vm_object's cred to one that has the same uid/gid but is outside of the jail. The big argument against that is that I don't know enough about the VM subsystem to go poking about there lightly. From the user perspective, you can keep such objects with a little planning ahead: always create them outside of the jail, though using the jail's path in the name (which is how a non-jailed process would refer to it anyway). Then jailed processes can access the shared memory, but won't own it. - Jamie