linux-kernel - Re: [PATCH 03/17] perf, x86: Add copy_from_user_nmi

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Message-ID: <20120725172500.GI1173@somewhere.redhat.com>
Date:	Wed, 25 Jul 2012 19:25:03 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	acme@...hat.com, a.p.zijlstra@...llo.nl, mingo@...e.hu,
	paulus@...ba.org, cjashfor@...ux.vnet.ibm.com, eranian@...gle.com,
	gorcunov@...nvz.org, tzanussi@...il.com, mhiramat@...hat.com,
	robert.richter@....com, fche@...hat.com,
	linux-kernel@...r.kernel.org, masami.hiramatsu.pt@...achi.com,
	drepper@...il.com, asharma@...com, benjamin.redelings@...cent.org
Subject: Re: [PATCH 03/17] perf, x86: Add copy_from_user_nmi_nochk for best
 effort copy

On Wed, Jul 25, 2012 at 07:16:43PM +0200, Jiri Olsa wrote:
> On Wed, Jul 25, 2012 at 06:11:53PM +0200, Frederic Weisbecker wrote:
> > On Sun, Jul 22, 2012 at 02:14:26PM +0200, Jiri Olsa wrote:
> > > Adding copy_from_user_nmi_nochk that provides the best effort
> > > copy regardless the requesting size crossing the task boundary.
> > > 
> > > This is going to be useful for stack dump we need in post
> > > DWARF CFI based unwind, where we have predefined size of
> > > the user stack to dump, and we need to store the most of
> > > the requested dump size, regardless this size is crossing
> > > the task boundary.
> > 
> > What does that imply when we cross this limit? Are we still in the
> > task stack?
> 
> We store all we could from 'stack pointer' to 'stack pointer' + dump size.
> 
> I discussed this with Oleg and we could probably find vma for the 'stack pointer'
> and check for its size and narrow the dump - maybe more complex, but probably faster
> in comparison with dumping pages we're not interested in.

Ah, that's because the user stack can be larger than TASK_SIZE, right?

Ok then.
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