linux-kernel - Re: [PATCH 0/4] ipc/shm.c: increase the limits for SHMMAX, SHMALL

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Date:	Fri, 2 May 2014 15:16:39 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>, aswin@...com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 0/4] ipc/shm.c: increase the limits for SHMMAX, SHMALL

Hi Manfred,

On Mon, Apr 21, 2014 at 4:26 PM, Manfred Spraul
<manfred@...orfullife.com> wrote:
> Hi all,
>
> the increase of SHMMAX/SHMALL is now a 4 patch series.
> I don't have ideas how to improve it further.

On the assumption that your patches are heading to mainline, could you
send me a man-pages patch for the changes?

Thanks,

Michael


> The change itself is trivial, the only problem are interger overflows.
> The overflows are not new, but if we make huge values the default,
> then the code should be free from overflows.
>
> SHMMAX:
>
> - shmmem_file_setup places a hard limit on the segment size:
>   MAX_LFS_FILESIZE.
>
>   On 32-bit, the limit is > 1 TB, i.e. 4 GB-1 byte segments are
>   possible. Rounded up to full pages the actual allocated size
>   is 0. --> must be fixed, patch 3
>
> - shmat:
>   - find_vma_intersection does not handle overflows properly.
>     --> must be fixed, patch 1
>
>   - the rest is fine, do_mmap_pgoff limits mappings to TASK_SIZE
>     and checks for overflows (i.e.: map 2 GB, starting from
>     addr=2.5GB fails).
>
> SHMALL:
> - after creating 8192 segments size (1L<<63)-1, shm_tot overflows and
>   returns 0.  --> must be fixed, patch 2.
>
> User space:
> - Obviuosly, there could be overflows in user space. There is nothing
>   we can do, only use values smaller than ULONG_MAX.
>   I ended with "ULONG_MAX - 1L<<24":
>
>   - TASK_SIZE cannot be used because it is the size of the current
>     task. Could be 4G if it's a 32-bit task on a 64-bit kernel.
>
>   - The maximum size is not standardized across archs:
>     I found TASK_MAX_SIZE, TASK_SIZE_MAX and TASK_SIZE_64.
>
>   - Just in case some arch revives a 4G/4G split, nearly
>     ULONG_MAX is a valid segment size.
>
>   - Using "0" as a magic value for infinity is even worse, because
>     right now 0 means 0, i.e. fail all allocations.
>
> Andrew: Could you add it into -akpm and move it towards linux-next?
>
> --
>         Manfred



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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