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    SubjectRe: pivot_root(".", ".") and the fchdir() dance
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    Hello Eric,

    A ping on my question below. Could you take a look please?

    Thanks,

    Michael

    >>>> The concern from our conversation at the container mini-summit was that
    >>>> there is a pathology if in your initial mount namespace all of the
    >>>> mounts are marked MS_SHARED like systemd does (and is almost necessary
    >>>> if you are going to use mount propagation), that if new_root itself
    >>>> is MS_SHARED then unmounting the old_root could propagate.
    >>>>
    >>>> So I believe the desired sequence is:
    >>>>
    >>>>>>> chdir(new_root);
    >>>> +++ mount("", ".", MS_SLAVE | MS_REC, NULL);
    >>>>>>> pivot_root(".", ".");
    >>>>>>> umount2(".", MNT_DETACH);
    >>>>
    >>>> The change to new new_root could be either MS_SLAVE or MS_PRIVATE. So
    >>>> long as it is not MS_SHARED the mount won't propagate back to the
    >>>> parent mount namespace.
    >>>
    >>> Thanks. I made that change.
    >>
    >> For what it is worth. The sequence above without the change in mount
    >> attributes will fail if it is necessary to change the mount attributes
    >> as "." is both put_old as well as new_root.
    >>
    >> When I initially suggested the change I saw "." was new_root and forgot
    >> "." was also put_old. So I thought there was a silent danger without
    >> that sequence.
    >
    > So, now I am a little confused by the comments you added here. Do you
    > now mean that the
    >
    > mount("", ".", MS_SLAVE | MS_REC, NULL);
    >
    > call is not actually necessary?
    >
    > Thanks,
    >
    > Michael
    >


    --
    Michael Kerrisk
    Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
    Linux/UNIX System Programming Training: http://man7.org/training/

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