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    SubjectRe: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks.
    * Peter Zijlstra <peterz@infradead.org> [2011-06-16 14:00:26]:

    > On Thu, 2011-06-16 at 08:56 +0530, Srikar Dronamraju wrote:
    > > * Peter Zijlstra <peterz@infradead.org> [2011-06-15 20:11:26]:
    > >
    > > > On Tue, 2011-06-07 at 18:29 +0530, Srikar Dronamraju wrote:
    > > > > + up_write(&mm->mmap_sem);
    > > > > + mutex_lock(&uprobes_mutex);
    > > > > + down_read(&mm->mmap_sem);
    > > >
    > > > egads, and all that without a comment explaining why you think that is
    > > > even remotely sane.
    > > >
    > > > I'm not at all convinced, it would expose the mmap() even though you
    > > > could still decide to tear it down if this function were to fail, I bet
    > > > there's some funnies there.
    > >
    > > The problem is with lock ordering. register/unregister operations
    > > acquire uprobes_mutex (which serializes register unregister and the
    > > mmap_hook) and then holds mmap_sem for read before they insert a
    > > breakpoint.
    > >
    > > But the mmap hook would be called with mmap_sem held for write. So
    > > acquiring uprobes_mutex can result in deadlock. Hence we release the
    > > mmap_sem, take the uprobes_mutex and then again hold the mmap_sem.
    >
    > Sure, I saw why you wanted to do it, I'm just not quite convinced its
    > safe to do and something like this definitely wants a comment explaining
    > why its safe to drop mmap_sem.
    >
    > > After we re-acquire the mmap_sem, we do check if the vma is valid.
    >
    > But you don't on the return path, and if !ret
    > mmap_region():unmap_and_free_vma will be touching vma again to remove
    > it.
    >

    Agree.

    > > Do we have better solutions?
    >
    > /me kicks the brain into gear and walks off to get a fresh cup of tea.
    >
    > So the reason we take uprobes_mutex there is to avoid probes from going
    > away while you're installing them, right?

    It serializes register/unregister/mmap operations.

    >
    > So we start by doing this add_to_temp_list() thing (horrid name), which
    > iterates the probes on this inode under uprobes_treelock and adds them
    > to a list.
    >
    > Then we iterate the list, installing the probles.
    >
    > How about we make the initial pass under uprobes_treelock take a
    > references on the probe, and then after install_breakpoint() succeeds we
    > again take uprobes_treelock and validate the uprobe still exists in the
    > tree and drop the extra reference, if not we simply remove the
    > breakpoint again and continue like it never existed.
    >
    > That should avoid the need to take uprobes_mutex and not require
    > dropping mmap_sem, right?

    Now since a register and mmap operations can run in parallel, we could
    have subtle race conditions like this:

    1. register_uprobe inserts the uprobe in RB tree.
    2. register_uprobe loops thro vmas and inserts breakpoints.

    3. mmap is called for same inode, mmap_uprobe() takes reference;
    4. mmap completes insertion and releases reference.

    5. register uprobe tries to install breakpoint on one vma fails and not
    due to -ESRCH or -EEXIST.
    6. register_uprobe rolls back all install breakpoints except the one
    inserted by mmap.

    We end up with breakpoints that we have inserted by havent cleared.

    Similarly unregister_uprobe might be looping to remove the breakpoints
    when mmap comes in installs the breakpoint and returns.
    unregister_uprobe might erase the uprobe from rbtree after mmap is done.

    --
    Thanks and Regards
    Srikar


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