Date: Fri, 25 Jun 2021 20:08:31 -0700 From: James Gritton <jamie@freebsd.org> To: jail@freebsd.org Cc: Michael Gmelin <freebsd@grem.de> Subject: Re: POSIX shared memory and dying jails Message-ID: <75475234c76c97c67a8bd2525669c650@freebsd.org> In-Reply-To: <20210625185859.40fead46@bsd64.grem.de> References: <20210625164100.73c71055@bsd64.grem.de> <03809b2655a40134dd802386afa6be7d@freebsd.org> <20210625185859.40fead46@bsd64.grem.de>
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On 2021-06-25 09:58, Michael Gmelin wrote: > On Fri, 25 Jun 2021 09:19:05 -0700 > James Gritton <jamie@freebsd.org> 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
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