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    SubjectRe: [PATCH] New System call unshare (try 2)
    Jamie Lokier wrote:
    > Janak Desai wrote:
    >> Don't allow namespace unsharing, if sharing fs (CLONE_FS)
    > Makes sense. clone() has the same test at the start. (I think
    > namespace should be a property of fs, not task, anyway. Or completely
    > eliminated because it's implied by the task's root dentry+vfsmnt).
    >> Don't allow sighand unsharing if not unsharing vm
    > Why not? It's permitted to clone with unshared sighand and shared vm,
    > and it's useful too.
    > It's the combination shared sighand + unshared vm which is not
    > allowed by clone - so I think that's what you should refuse.

    Yes, thanks. I had that backwards. I am checking for the illegal
    combination of "shared sighand + unshared vm" later but I will
    allow (and test) unsharing of sighand while keeping shared vm.

    >> Don't allow vm unsharing if task cloned with CLONE_THREAD
    > It would be better to do what clone does, and say "don't allow sighand
    > unsharing if task cloned with CLONE_THREAD". This is because
    > CLONE_THREAD tasks must have shared signals.
    > In combination with the rule above for sighand (my rule, not yours),
    > that implies "don't allow vm unsharing.." as a consequence.

    Ok. Basically, with my current logic you are forced to unshare
    both vm and sighand. I will update so you can unshare just sighand
    and tie the CLONE_THREAD restriction to sighand.

    >> Don't allow vm unsharing if the task is performing async io
    > Why not?
    > Async ios are tied to an mm (see lookup_ioctx in fs/aio.c), which may
    > be shared among tasks. I see no reason why the async ios can't
    > continue and be waited in on in other tasks that may be using the old mm.
    > The new mm, if vm is unshared, would simply not see the outstanding
    > aios - in the same way as if a vm was unshared by fork().

    Yes, I did see that async ios are tied to an mm and that aio context is
    cleared when a new mm is created by clone/exec. It just appeared odd to
    me that a task that setup aio is unable to continue performing aio
    after unsharing mm, but the task that it was sharing mm with, can. I
    don't have any problem making this change. It's just that I wasn't sure
    about aio usage and thought it was better to not allow an mm unshare if
    aio is being performed.


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