linux-kernel - Re: [PATCH v2 7/11] Uprobes Implementation

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Date:	Thu, 13 May 2010 15:25:01 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>, Mel Gorman <mel@....ul.ie>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH v2 7/11] Uprobes Implementation

On Fri, 14 May 2010, Andi Kleen wrote:
> 
> But isn't text usually shared?  I don't see how you could set any
> break points or jump probes on text pages with that restriction.

Text is usually private, and read-only. Not generally MAP_SHARED. The 
pages end up getting shared because nobody writes to them, but that's 
almost accidental.

If you write to them, you get a nice clean COW fault, and you are 
_supposed_ to get a nice clean COW fault. It's not changing any semantics: 
the write is not visible to outside users, and those "get a private page" 
semantics were what the mmap() was all about.

In contrast, if it's a MAP_SHARED mapping and writable, the write would 
actually be _visible_ outside the process. And that's clearly totally 
wrong on all levels. Tracing a process should _never_ cause visible damage 
outside that process (you'd hope it wouldn't be all that visibel to the 
tracee either, but that's still secondary).

The alternative, ie a MAP_SHARED but read-only mapping (which looks very 
much like a private mapping) if you use get_user_pages(.force=1), the 
kernel will actually end up forcing a COW break, because making the write 
visible outside would be a security issue (you don't even have the right 
to write to the thing).

Notice how the MAP_SHARED case - writable or not - ends up doing the wrong 
thing. Arguably it does the _even_worse_ thing in the writable case, but 
in either case it's not good. 

			Linus
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