linux-kernel - Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

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Date:	Tue, 29 Nov 2011 16:05:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>, tulasidhard@...il.com
Subject: Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

On Tue, 2011-11-29 at 12:48 +0100, Peter Zijlstra wrote:
> There's 2 main cases, 
>         A) vma_adjust() vs unregister_uprobe() and 
>         B) mmap() vs unregister_uprobe().
> 
> The result of A should be -1 reference in total, since we're removing
> the one probe.

This might not be correct for A[23], please double check.

>  The result of B should be 0 since we're removing the
> probe and we shouldn't be installing new ones.
> 
> A1)
>         vma_adjust()
>           munmap_uprobe()
>                                 unregister_uprobe()
>           mmap_uprobe()
>                                   delete_uprobe()
> 
> 
>         munmap will to -1, mmap will do +1, __unregister_uprobe() which is
> serialized against vma_adjust() will do -1 on either the old or new vma,
> resulting in a grand total of: -1+1-1=-1, OK
> 
> A2) breakpoint is in old, not in new, again two cases:
> 
> A2a) __unregister_uprobe() sees old
> 
>         munmap -1, __unregister_uprobe -1, mmap 0: -2 FAIL
> 
> A2b) __unregister_uprobe() sees new
> 
>         munmap -1, __unregister_uprobe 0, mmap 0: -1 OK
> 
> A3) breakpoint is in new, not in old, again two cases:
> 
> A3a) __unregister_uprobe() sees old
> 
>         munmap 0, __unregister_uprobe 0, mmap: 1: 1 FAIL
> 
> A3b) __unregister_uprobe() seed new
> 
>         munmap 0, __unregister_uprobe -1, mmap: 1: 0 FAIL

There's more cases, I forgot the details of how the prio_tree stuff
works, so please consider if its possible to also have:

  __unregister_uprobe() will observe neither old nor new

This could happen if we first munmap, __unregister_uprobe() will iterate
past where mmap() will insert the new vma, mmap will insert the new vma,
and __unregister_uprobe() will now not observe it.

and

  __unregister_uprobe() will observe both old _and_ new

This latter could happen by favourably interleaving the prio_tree
iteration with the munmap and mmap operations, so that we first observe
the old vma, do the munmap, do the mmap, and then have the
find_next_vma_info() thing find the new vma.

> B1)
>                                 unregister_uprobe()
>         mmap()
>           mmap_uprobe()
>                                   __unregister_uprobe()
>                                   delete_uprobe()
> 
>         mmap +1, __unregister_uprobe() -1: 0 OK
> 
> B2)
>                                 unregister_uprobe()
>         mmap()
>                                   __unregister_uprobe()
>           mmap_uprobe()
>                                   delete_uprobe()
> 
>         mmap +1, __unregister_uprobe() 0: +1 FAIL 
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