linux-kernel - Re: x86: randomize brk() and RLIMIT

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Date:	Thu, 25 Oct 2007 16:46:26 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Sami Farin <safari-kernel@...ari.iki.fi>,
	Linux kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86: randomize brk() and RLIMIT_DATA

On Thu, 25 Oct 2007, Arjan van de Ven wrote:

> > Would be neat if randomized brk and setrlimit(RLIMIT_DATA, ...)
> > worked in a predictable way:
> this isn't a valid case afaics; even on "traditional x86" (before we 
> changed the address space layout, or even today if you have an unlimited 
> stack rlimit) this isn't going to work. applications really shouldn't 
> use (s)brk() but malloc(); you have to be able to fall back to mmap 
> regardless of what you do.

I tend to agree here with Arjan. However it probably would make no harm to 
make at least a little bit consisten behavior of setrlimit(), though it 
has a little use in such cases.

Sami, does the patch below work for you?

diff --git a/mm/mmap.c b/mm/mmap.c
index facc1a7..c7ade18 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -251,7 +251,8 @@ asmlinkage unsigned long sys_brk(unsigned long brk)
 	 * not page aligned -Ram Gupta
 	 */
 	rlim = current->signal->rlim[RLIMIT_DATA].rlim_cur;
-	if (rlim < RLIM_INFINITY && brk - mm->start_data > rlim)
+	if (rlim < RLIM_INFINITY && brk - mm->start_data -
+			(mm->start_brk - mm->end_data) > rlim)
 		goto out;
 
 	newbrk = PAGE_ALIGN(brk);
-
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