Re: [PATCH v4 3.0-rc2-tip 7/22] 7: uprobes: mmap and fork hooks.

Peter Zijlstra
2011-06-17T08:04:47Z

On Fri, 2011-06-17 at 10:20 +0530, Srikar Dronamraju wrote:
> >
> > void __unregister_uprobe(...)
> > {
> > uprobe = find_uprobe(); // ref++
> > if (delete_consumer(...)); // includes tree removal on last consumer
> > // implies we own the last ref
> > return; // consumers
> >
> > vma_prio_tree_foreach() {
> > // create list
> > }
> >
> > list_for_each_entry_safe() {
> > // remove from list
> > remove_breakpoint(); // unconditional, if it wasn't there
> > // its a nop anyway, can't get any new
> > // new probes on account of holding
> > // uprobes_mutex and mmap() doesn't see
> > // it due to tree removal.
> > }
> > }
> >
>
> This would have a bigger race.
> A breakpoint might be hit by which time the node is removed and we
> have no way to find out the uprobe. So we deliver an extra TRAP to the
> app.

Gah indeed. Back to the drawing board for me.

> > int mmap_uprobe(...)
> > {
> > spin_lock(&uprobes_treelock);
> > for_each_probe_in_inode() {
> > // create list;
> > }
> > spin_unlock(..);
> >
> > list_for_each_entry_safe() {
> > // remove from list
> > ret = install_breakpoint();
> > if (ret)
> > goto fail;
> > if (!uprobe_still_there()) // takes treelock
> > remove_breakpoint();
> > }
> >
> > return 0;
> >
> > fail:
> > list_for_each_entry_safe() {
> > // destroy list
> > }
> > return ret;
> > }
> >
>
>
> register_uprobe will race with mmap_uprobe's first pass.
> So we might end up with a vma that doesnot have a breakpoint inserted
> but inserted in all other vma that map to the same inode.

I'm not seeing this though, if mmap_uprobe() is before register_uprobe()
inserts the probe in the tree, the vma is already in the rmap and
register_uprobe() will find it in its vma walk. If its after,
mmap_uprobe() will find it and install, if a concurrent
register_uprobe()'s vma walk also finds it, it will -EEXISTS and ignore
the error.