Date: Tue, 11 Apr 2017 11:23:57 -0700 From: Maxim Sobolev <sobomax@freebsd.org> To: Kirk McKusick <mckusick@mckusick.com> Cc: FreeBSD Filesystems <freebsd-fs@freebsd.org> Subject: Re: mksnap_ffs(8) is not working while chrooted Message-ID: <CAH7qZfsS61LEO1eR1T3uoYgiuOgLvnPZRpsScR--a3OnxLK3zw@mail.gmail.com> In-Reply-To: <201704111807.v3BI7PGM001952@chez.mckusick.com> References: <CAH7qZfvO9AjiZM0FyHqRBzp-KN4u=7Pq4Y2xNhQ2W4t53rApTQ@mail.gmail.com> <201704111807.v3BI7PGM001952@chez.mckusick.com>
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Kirk, what you are saying in not applying in our case. The whole FS is mounted *inside* the chroot. The reason why we are trying to use mksnap_ffs is to take a clean snapshot to make a compressed version of it without the need to forcefully zero out free space. So it looks like the following: parent# chroot /mnt/chroot /bin/sh chroot# truncate /tmp/diskimage chroot# mdconfig -a -f /tmp/diskimage md0 chroot# newfs md0 chroot# mount /dev/md0 /mnt [...do stuff with /mnt...] chroot# mksnap_ffs /mnt/snap mksnap_ffs: No such file or directory Perhaps, to work around your concern is to add some flag for the statfs(1) to normalize f_mntonname for use inside chroot then? I have not tested it here, but I believe that if I'd strip "/mnt/chroot" prefix inside mksnap_ffs(8) that would work in our scenario just fine. -Max On Tue, Apr 11, 2017 at 11:07 AM, Kirk McKusick <mckusick@mckusick.com> wrote: > > From: Maxim Sobolev <sobomax@freebsd.org> > > Date: Tue, 11 Apr 2017 10:40:58 -0700 > > Subject: mksnap_ffs(8) is not working while chrooted > > To: Kirk McKusick <mckusick@mckusick.com>, > > FreeBSD Filesystems <freebsd-fs@freebsd.org> > > > > Hi Kirk et al, > > > > I've stumbled upon problem that it is impossible to use mksnap_ffs(8) > while > > in the chrooted environment. Other utilities that manipulate fs'es (i.e. > > mount(8) / umount(8)) work just fine. Quick glance through the code shows > > that the problem stems from the fact that mksnap_ffs uses f_mntonname > > returned by the statfs(2) system call to fill fspath parameter for the > > nmount call. And the statfs() returns f_mntonname path outside chroot. As > > far as I can see, there are two options to address this issue. > > > > 1. Adjust statfs(2) system call to substract chroot prefix while > > returning f_mntonname. Similar to what prison_enforce_statfs() function > > does for jails. > > > > 2. Enhance nmount(2) to allow taking FSID in place of mount path to do > > resolution using existing flag MNT_BYFSID and adjust mksnap_ffs to use > that > > instead. This is what umount(8) does to work around the problem. > > > > Which of two approaches would be preferred solution if any? The second > one > > seems a bit simpler to me. Please advise. Thanks! > > > > -Max > > It is not secure to allow mksnap_ffs(8) to work inside a jail. The issue > is that mksnap_ffs takes a snapshot of the entire filesystem. Based on the > way that snapshots work in FFS, it is not possible to snapshot only the > part of the filesystem that is in the jail. Thus, if you take a snapshot > of the entire filesystem and then mount it inside the jail, you will > expose parts of the filesystem from outside of the jail to the jail. > As such, you should only be able to snapshot a filesystem if it is > entirely contained within the jail. > > If snapshots within jails are important to you, I recommend that you use > ZFS which allows you to create per-jail filesystems which you can then > snapshot. > > Kirk McKusick > >
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