Date: Fri, 25 Jun 2021 09:19:05 -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: <03809b2655a40134dd802386afa6be7d@freebsd.org> In-Reply-To: <20210625164100.73c71055@bsd64.grem.de> References: <20210625164100.73c71055@bsd64.grem.de>
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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. Like files, POSIX shared memory is one way a jail may communicate with the rest of the system. So it's theoretically conceivable that shared memory created by a defunct jail my still be in use by a parent jail, in the same way that shared memory created by a defunct process is still visible to other processes, but that may be a rare enough case to disregard. - Jamie
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