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

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Date:	Sun, 11 Mar 2012 16:42:37 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	Oleg Nesterov <oleg@...hat.com>, 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 4:32 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
>>
>> Just increment the mm_count for the thing, and hold a reference to it,
>> and now you're all done.
> Please Linus have you checked the:
> [PATCH 9/9] proc: improve and clean up /proc/<pid>/mem protection
>
> That keeping the mm struct wont work, since it will eat memory and the
> OOM-killer will kill some innocent processes, and the abuse can only be
> catched by the VFS.

That's the point. I made the mistake of using mm_users initially, but
ysing mm_count - which is what I said to use (and what Oleg fixed
things to in commit 6d08f2c71397) should *not* have that problem. It
just keeps the 'struct mm_struct' itself around.

> What's your opinion on it ?

What's the advantage? You replace it with *another* allocation, and a
64-bit thing that is much less useful.

The size of the patch also speaks for itself:

 fs/proc/base.c |   99 ++++++++++++++++++++++++++++++++++++++++++++------------

and it's more complex and uses more memory on average (the refcount
thing is *free* for usual cases).

I do agree that it would be nicer if mm_struct was a bit smaller, but
at the same time, I really don't see the advantage of replacing it
with another allocation entirely that makes the code just more
complicated.

                 Linus
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