Re: writing to RW-mounted UFS2 snapshots - confirmed.

From: John Kozubik (john_at_kozubik.com)
Date: 07/02/04

  • Next message: Eitarou Kamo: "Re: FreeBSD and MacOS" 
    Date: Thu, 1 Jul 2004 18:16:50 -0700 (PDT)
To: freebsd-hackers@freebsd.org

    On Thu, 1 Jul 2004, Q wrote:

    > > While this may not be "expected" behavior, I am curious why this is
    > > something that should be prevented, rather than verified for
    > > correctness? By "correct" I mean, that the copy on write process is
    > > performed correctly and modifications made to the snapshot don't
    > > modify the underlying filesystem elements also.
    >
    > Ok, I decided to be self sufficient and read Marshall McKusick's paper
    > on background fsck (which also covers snapshots) to answer my own
    > question.
    >
    > Firstly, one of the stated requirements for snapshots to function as
    > outlined in the paper is that they must be read only. The reason they
    > need to be read only is because the support for multiple snapshots
    > assumes that they cannot change, and handles block allocation of freed
    > blocks with this in mind. Supporting snapshot modification is outside
    > the scope of the original implementation and would require further
    > thought.

    This was the main reason that I decided to go back and see if this was
    really happening or not. I read the USENIX paper (McKusick) to be quite
    clear that snapshots must be read-only: "Writes to snapshot files are not
    permitted". However, nothing is said about writing files to, or changing
    files within, a mounted snapshot file.

    My initial assumption is that both activities are equally incorrect (and
    dangerous - especially for subsequent snapshots created after altering the
    first one).

    As for a solution, I suppose a quick fix would be to alter `mount` to
    disallow the read/write mount of a snapshot file. This is similar to the
    exception that is already coded into `unlink` to accomodate snapshot
    files.

    I just submitted a PR for this.

    -----
    John Kozubik - john@kozubik.com - http://www.kozubik.com

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  • Next message: Eitarou Kamo: "Re: FreeBSD and MacOS"

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