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    SubjectRe: 2.6.17-rc3 - fs/namespace.c issue
    has other stuff Herbert Poetzl wrote:
    > On Mon, May 01, 2006 at 02:33:44PM -0700, Andrew Morton wrote:
    >> Valdis.Kletnieks@vt.edu wrote:
    >>> There seems to have been a bug introduced in this changeset:
    >>>
    >>> http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=f6422f17d3a480f21917a3895e2a46b968f56a08
    >>>
    >>> Am running 2.6.17-rc3-mm1. When this changeset is applied, 'mount --bind'
    >>> misbehaves:
    >>>
    >>>> # mkdir /foo
    >>>> # mount -t tmpfs -o rw,nosuid,nodev,noexec,noatime,nodiratime none /foo
    >>>> # mkdir /foo/bar
    >>>> # mount --bind /foo/bar /foo
    >>>> # tail -2 /proc/mounts
    >>>> none /foo tmpfs rw,nosuid,nodev,noexec,noatime,nodiratime 0 0
    >>>> none /foo tmpfs rw 0 0
    >>> Reverting this changeset causes both mounts to have the same options.
    >>>
    >>> (Thanks to Stephen Smalley for tracking down the changeset...)
    >
    > well, IMHO there are several open questions regarding semantics
    >
    > first, what do we expect from --bind mounts regarding
    > vfs (mount) level flags like noatime, noexec, nodev?
    >
    > - should they be propagated from the original mfs/mount?
    > - should they only restrict the original set?
    > - should they allow to modify the existing flags?

    What does it mean if the flags can be modified? If I mount a tree ro, do
    I want to open the can of worms from allowing part of it to be rw
    elsewhere? And what checking is done, or should be done? If I do a ro
    mount with something like NFS, what should happen if I mount part of it
    rw? Substitute any of the other above flags, is there a security issue
    here, and can I shoot myself in the foot?

    Can I apply the "user" attribute in fstab to a bind mount? If I let a
    user bind /foo/bar to /bazfaz/zot, what happens if I have the wrong
    thing mounted on /foo? Or if /bazfaz is NFS exported read only?
    >
    > IMHO, it makes perfect sense to mount something noatime
    > and change that rule later for a subtree like this:
    >
    > mkdir /foo
    > mount -t tmpfs -o rw,noatime none /foo
    > mkdir /foo/bar
    > mount --bind -o atime /foo/bar /foo/bar
    >
    > second, has the kernel to decide what flags userspace
    > can request and/or change, depending on the original?
    >
    > and finally, how to handle --rbind mounts at a level
    > deeper than the top?
    >
    > so I do not consider the example above a misbehaviour.
    > what I consider a misbehaviour is that mount (userspace)
    > blindly assumes that --bind mounts are independant, so
    > it does not check the existing flags, and thus, does not
    > preserve them (instead it replaces them with the default)
    >
    > removing the mnt->mnt_flags = mnt_flags; assignment
    > is sufficient to _only_ allow the identical attributes
    > of the original mount, as they are copied in the
    > clone_mnt() operation, of course, this also makes it
    > impossible to have any flags/changes to the --bind mounts
    > over the original

    That certainly is a lot less likely to violate Plauger's law of least
    astonishment.
    >
    > as this patch was torn out of a much larger patch set
    > to allow for such attribute changes at --bind mount time
    > I'd sugegst the following untested 'fix'
    >
    > best,
    > Herbert

    --
    -bill davidsen (davidsen@tmr.com)
    "The secret to procrastination is to put things off until the
    last possible moment - but no longer" -me
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