linux-kernel - Re: [PATCH 1/9] exec: add a global execve counter

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Date:	Sun, 11 Mar 2012 15:48:02 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Djalal Harouni <tixxdz@...ndz.org>, linux-kernel@...r.kernel.org,
	kernel-hardening@...ts.openwall.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Kees Cook <keescook@...omium.org>,
	Solar Designer <solar@...nwall.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	James Morris <james.l.morris@...cle.com>,
	linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg KH <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...e.hu>, Stephen Wilson <wilsons@...rt.ca>,
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH 1/9] exec: add a global execve counter

On Sun, Mar 11, 2012 at 10:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> It should be per-process, signal_struct makes more sense. Or may be
> mm_struct.

I do wonder if we shouldn't just consider the "struct mm_struct"
pointer to *be* the unique exec ID. It's what /proc/pid/mem does, and
it works fine, and allows us to just use a normal pointer as the
unique ID.

Just increment the mm_count for the thing, and hold a reference to it,
and now you're all done.

By definition an execve() will change the mm struct, and if you have a
refcount to the old one, it won't be re-used. And it's not a huge
allocation, although it would definitely be good to put that thing on
a diet.

And as long as we use 'mm_count', not 'mm_users', it won't pin
anything else in memory. That's part of the whole point of the
doubly-refcounted thing.

                      Linus
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