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    SubjectRe: /etc/mtab and per-process namespaces
    On Sat, 2005-10-29 at 03:16, Rob Landley wrote:
    > On Friday 28 October 2005 19:06, Ram Pai wrote:
    > > > Mike's comments are very apt. The current situation with mount
    > > > support is untenable. Even working on private development machines it
    > > > gets confusing as to what is or is not mounted in various
    > > > shells/processes. The basic infra-structure is there with process
    > > > specific mount information (/proc/self/mounts) but mount and friends
    > > > are a bit problematic with respect to supporting this.
    > I fairly extensively rewrote busybox mount, and one of my goals was doing the
    > best job with /proc/mounts (only) support that I could. In some ways,
    > busybox's mount is better (such as the fact it can autodetect when you're
    > trying to mount a file and figure out it needs -o loop without being told).
    > If you want try the busybox version of mount/losetup/umount, I hope it does
    > what you want and am willing to fix it if it doesn't. (P.S. To
    > use /proc/mounts either configure it without /etc/mtab support or
    > symlink /etc/mtab to /proc/mounts.)
    > > > I'm working on a namespace toolkit to address these issues. I've got
    > > > a pretty basic tool, similar to sudo, which allows spawning processes
    > > > with a protected namespace. I'm adding a configuration system which
    > > > allow systems administrators to define a setup of bind mounts which
    > > > are automatically executed before the user is given their shell. I'm
    > > > also working up a PAM account module to go along with this. I would
    > > > certainly be open to suggestions as to what else people would consider
    > > > useful in such a toolkit.
    > > >
    > > > I've been pondering the best way to take on the mount problem.
    > > > Current mount binaries seem to fall back to /proc/mounts if /etc/mtab
    > > > is not present. All bets are off of course if the mount binary is
    > > > used for the bind mount since a new /etc/mtab is created.
    > Have you tried having /etc/mtab be a symlink to /proc/mounts?
    > > > I'm willing to whack on the mount binary a bit as part of this. The
    > > > obvious solution is to teach mount to act differently if it is running
    > > > in a private namespace. If anybody knows of a good way to detect this
    > > > I would be interested in knowing that. In newns (the namespace sudo
    > > > tool) I'm setting an environment variable for mount to detect on but a
    > > > system level approach would be more generic.
    > >
    > > actually there is a hackish way for a process to figure out if it is in
    > > a different namespace than the system namespace.
    > >
    > > ls /proc/1/root
    > >
    > > in a system namespace it will allow you to see the content.
    > > And in a per-process-namespace it will fail with permission denied.
    > >
    > > But I think we should figure out a cleaner way to decipher this,
    > > and that would start with clearly defining the requirements, I think.
    > The big thing I've never figured out how to do is make umount -a work in the
    > presence of multiple namespaces. (Should it just umount what it sees? I
    > don't know how to umount everything because I can't find everything...)

    Yes you won't find everything, since some of them are in a different
    namespaces. Instead unmount whatever you see. Or use /proc/mounts
    to unmount whatever is there in its namespace.


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