linux-kernel - [PATCH] [RFC] brk randomization: compute RLIMIT_DATA properly (was Re: x86: randomize brk() and RLIMIT

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Message-ID: <Pine.LNX.4.64.0710261235270.18815@twin.jikos.cz>
Date:	Fri, 26 Oct 2007 12:44:23 +0200 (CEST)
From:	Jiri Kosina <jikos@...os.cz>
To:	Sami Farin <safari-kernel@...ari.iki.fi>,
	Andrew Morton <akpm@...ux-foundation.org>
cc:	Linux kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Arjan van de Ven <arjan@...radead.org>
Subject: [PATCH] [RFC] brk randomization: compute RLIMIT_DATA properly (was
 Re: x86: randomize brk() and RLIMIT_DATA)

On Thu, 25 Oct 2007, Sami Farin 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?
> Thanks, Jiri, now RLIMIT_DATA works as expected. Using only RLIMIT_AS to 
> limit processes' memory usage is not very easy.  It includes also 
> libraries mapped read-only, I have to check/modify the limits when I 
> update/add libraries,...

This patch is [RFC] as I am not sure whether it is worth it (see Arjan's 
comment above).


From: Jiri Kosina <jkosina@...e.cz>

brk randomization: compute RLIMIT_DATA properly

In cases of heap area placed at randomly-generated offset from 
mm->end_data (arch_randomize_brk()), we need to subtract the value of the 
offset for setrlimit(RLIMIT_DATA) to work properly -- otherwise we count 
the unoccupied memory between mm->end_data and mm->start_brk as occupied.

Signed-off-by: Jiri Kosina <jkosina@...e.cz>

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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