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    SubjectRe: [RFC][PATCH 00/20] Mount writer count and read-only bind mounts (v2)
    On Sat, Jun 17, 2006 at 02:10:17AM +0200, Grzegorz Kulewski wrote:
    > On Fri, 16 Jun 2006, Dave Hansen wrote:
    > >On Sat, 2006-06-17 at 01:29 +0200, Grzegorz Kulewski wrote:
    > >>Isn't this some kind of security risk (at least in my planned use)? I mean
    > >>- for a small fraction of second somebody seeing /dest can write
    > >>/source... No?
    > >
    > >I assume you're talking about this kind of situation:
    > >
    > >mount --bind /local/writable/dir /chroot/untrusted/area/
    > >mount --o remount,ro /chroot/untrusted/area/
    > Well, actually about some kind of VPS: openvz or something like that.
    > But yes, this is the same kind of scenario.

    yes, Linux-VServer provides this kind of ro --bind mounts
    without the race, as the the flags are passed on the actual

    > >This has no r/w window in the chroot area:
    > >
    > >mount --bind /local/writable/dir /tmp/area/
    > >mount --o remount,ro /tmp/area/
    > >mount --bind /tmp/area/ /chroot/untrusted/area/
    > >umount /tmp/area/
    > Well, it looks a little scarry and complicated at first. And probably
    > requires you to know that semantic of --bind lets you do the last
    > unmount. But if you are saying that this makes kernel smaller, faster
    > and less buggy then you are probably very right.

    well, it makes the kernel more consistant in it's behaviour,
    because especially for --rbind mounts, the logic what is
    changed where and when is not as well defined as one would
    wish ...

    btw, you could get the same result by simply doing:

    mount --bind /local/writable/dir /tmp/area/
    mount --o remount,ro /tmp/area/
    mount --move /tmp/area/ /chroot/untrusted/area/

    without the duplicate mount and the unmount


    > Thank you for your explanation,
    > Grzegorz Kulewski
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