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Date: Sat, 10 Mar 2012 16:58:01 -0800
From: Linus Torvalds <>
To: Djalal Harouni <>
	Andrew Morton <>, Al Viro <>, 
	Alexey Dobriyan <>, "Eric W. Biederman" <>, 
	Vasiliy Kulikov <>, Kees Cook <>, 
	Solar Designer <>, WANG Cong <>, 
	James Morris <>, Oleg Nesterov <>,,, 
	Alan Cox <>, Greg KH <>, 
	Ingo Molnar <>, Stephen Wilson <>, "Jason A. Donenfeld" <>
Subject: Re: [PATCH 1/9] exec: add a global execve counter

On Sat, Mar 10, 2012 at 4:36 PM, Linus Torvalds
<> wrote:
> I wonder if the number part of exec_id would even have to be 64-bit. I
> think I can do about 10000 execves per second if I make the program a
> small static one - and that's on a fast CPU. And it's a per-thread
> counter, so you can't scale it with lots of CPU's. So it would take
> something like four days to wrap. Hmm..

Actually, using a pure counter is horrible, because even if it takes
four days to wrap, it *will* wrap, and the attacker can just count his
own execve's.

If, instead, you were to use a counter that counts *independently* of
execve's, you're much better off.

And if you use one that is free - because the CPU implements it
natively - you're even better off.

IOW, why is the exec-id just the time stamp counter (on any random cpu
- we really don't care)? That should be safe even in just 32 bits
exactly because it's not under the control of the user.

And it's zero cost for us to update.


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