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

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Date:	Fri, 17 Jun 2011 10:20:00 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 7/22]  7: uprobes: mmap and fork hooks.

> 
> 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
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