linux-kernel - Re: ptrace and pfn mappings

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Date:	Tue, 10 Oct 2006 12:47:46 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <npiggin@...e.de>
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


> 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.

> > That means that the ptracing process will temporarily be running in the
> > kernel using a task->active_mm different from task->mm which might have
> > funny side effects due to assumptions that this won't happen here or
> > there, though I don't see any fundamental reasons why it couldn't be
> > made to work.
> > 
> > That do you guys think ? Any better idea ? The problem with mappings
> > like what SPUfs or the DRM want is that they can change (be remapped
> > between HW and backup memory, as described in previous emails), thus we
> > don't want to get struct pages even if available and peek at them as
> > they might not be valid anymore, same with PFNs (we could imagine
> > ioremap'ing those PFN's but that would be racy too). The only way that
> > is guaranteed not to be racy is to do exactly what a user do, that is do
> > user accesses via the target process vm itself....
> 
> 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 ?

Ben.


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