linux-kernel - Re: [PATCH 2/3] ipc/sem.c: increase SEMMSL, SEMMNI, SEMOPM

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Date:	Tue, 7 Oct 2014 16:09:15 -0400
From:	Rafael Aquini <aquini@...hat.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Rik van Riel <riel@...hat.com>, 1vier1@....de
Subject: Re: [PATCH 2/3] ipc/sem.c: increase SEMMSL, SEMMNI, SEMOPM

On Mon, Oct 06, 2014 at 08:32:42PM +0200, Manfred Spraul wrote:
> a)
> SysV can be abused to allocate locked kernel memory.  For most systems, a
> small limit doesn't make sense, see the discussion with regards to SHMMAX.
> 
> Therefore: Increase the sysv sem limits so that all known applications
> will work with these defaults.
> 
> b)
> With regards to the maximum supported:
> Some of the specified hard limits are not correct anymore, therefore the
> patch updates the documentation.
> 
> - SEMMNI must stay below IPCMNI, which is 32768.
>   As for SHMMAX: Stay a bit below this limit.
> 
> - SEMMSL was limited to 8k, to ensure that the kmalloc for the kernel array
>   was limited to 16 kB (order=2)
> 
>   This doesn't apply anymore:
>    - the allocation size isn't sizeof(short)*nsems anymore.
>    - ipc_alloc falls back to vmalloc
> 
> - SEMOPM should stay below 1000, to limit the kmalloc in semtimedop() to an
>   order=1 allocation.
>   Therefore: Leave it at 500 (order=0 allocation).
> 
> Note:
> If an administrator must limit the memory allocations, then he can set the
> values as necessary.
> 
> Or he can disable sysv entirely (as e.g. done by Android).
> 
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
> ---
>  include/uapi/linux/sem.h | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/sem.h b/include/uapi/linux/sem.h
> index 541fce0..dd73b90 100644
> --- a/include/uapi/linux/sem.h
> +++ b/include/uapi/linux/sem.h
> @@ -63,10 +63,22 @@ struct  seminfo {
>  	int semaem;
>  };
>  
> -#define SEMMNI  128             /* <= IPCMNI  max # of semaphore identifiers */
> -#define SEMMSL  250             /* <= 8 000 max num of semaphores per id */
> +/*
> + * SEMMNI, SEMMSL and SEMMNS are default values which can be
> + * modified by sysctl.
> + * The values has been chosen to be larger than necessary for any
> + * known configuration.
> + *
> + * SEMOPM should not be increased beyond 1000, otherwise there is the
> + * risk that semop()/semtimedop() fails due to kernel memory fragmentation when
> + * allocating the sop array.
> + */
> +
> +
> +#define SEMMNI  32000           /* <= IPCMNI  max # of semaphore identifiers */
> +#define SEMMSL  32000           /* <= INT_MAX max num of semaphores per id */
>  #define SEMMNS  (SEMMNI*SEMMSL) /* <= INT_MAX max # of semaphores in system */
> -#define SEMOPM  32	        /* <= 1 000 max num of ops per semop call */
> +#define SEMOPM  500	        /* <= 1 000 max num of ops per semop call */
>  #define SEMVMX  32767           /* <= 32767 semaphore maximum value */
>  #define SEMAEM  SEMVMX          /* adjust on exit max value */
>  
> -- 
> 1.9.3
> 
Acked-by: Rafael Aquini <aquini@...hat.com>
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