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

Srikar Dronamraju
2011-06-17T04:58:08Z

>
> 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.

> 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.

--
Thanks and Regards
Srikar