linux-kernel - Re: ptrace and pfn mappings

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Date:	Tue, 10 Oct 2006 04:58:21 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickins <hugh@...itas.com>,
	Linux Memory Management <linux-mm@...ck.org>,
	Andrew Morton <akpm@...l.org>, Jes Sorensen <jes@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: ptrace and pfn mappings

On Tue, Oct 10, 2006 at 12:47:46PM +1000, Benjamin Herrenschmidt wrote:
> 
> > Switch the mm and do a copy_from_user? (rather than the GUP).
> > Sounds pretty ugly :P
> > 
> > Can you do a get_user_pfns, and do a copy_from_user on the pfn
> > addresses? In other words, is the memory / mmio at the end of a
> > given address the same from the perspective of any process? It
> > is for physical memory of course, which is why get_user_pages
> > works...
> 
> Doesn't help with the racyness.

I don't understand what the racyness is that you can solve by accessing
it from the target process's mm?

> > What if you hold your per-object lock over the operation? (I guess
> > it would have to nest *inside* mmap_sem, but that should be OK).
> 
> Over the ptrace operation ? how so ?

You just have to hold it over access_process_vm, AFAIKS. Once it
is copied into the kernel buffer that's done. Maybe I misunderstood
what the race is?
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