linux-kernel - Re: [PATCH v3] ipc,shm: disable shmmax and shmall by default

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Date:	Fri, 18 Apr 2014 11:26:02 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <davidlohr@...com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	Michael Kerrisk <mtk.manpages@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>, aswin@...com,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v3] ipc,shm: disable shmmax and shmall by default

Hi Davidlohr,

On 04/18/2014 03:25 AM, Davidlohr Bueso wrote:
> So a value of 0 bytes or pages, for shmmax and shmall, respectively,
> implies unlimited memory, as opposed to disabling sysv shared memory.
That might be a second risk:
Right now, a sysadmin can prevent sysv memory allocations with

     # sysctl kernel.shmall=0

After your patch is applied, this line allows unlimited allocations.

Obviously my patch has the opposite problem: 64-bit wrap-arounds.

> --- a/include/uapi/linux/shm.h
> +++ b/include/uapi/linux/shm.h
> @@ -9,14 +9,14 @@
>   
>   /*
>    * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
> - * be increased by sysctl
> + * be modified by sysctl. By default, disable SHMMAX and SHMALL with
> + * 0 bytes, thus allowing processes to have unlimited shared memory.
>    */
> -
> -#define SHMMAX 0x2000000		 /* max shared seg size (bytes) */
> +#define SHMMAX 0		         /* max shared seg size (bytes) */
>   #define SHMMIN 1			 /* min shared seg size (bytes) */
>   #define SHMMNI 4096			 /* max num of segs system wide */
>   #ifndef __KERNEL__
> -#define SHMALL (SHMMAX/getpagesize()*(SHMMNI/16))
> +#define SHMALL 0
>   #endif
>   #define SHMSEG SHMMNI			 /* max shared segs per process */
>   
The "#ifndef __KERNEL__" is not required:
As there is no reference to PAGE_SIZE anymore, one definition for SHMALL 
is sufficient.


--
     Manfred
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