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

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Date:	Mon, 21 Apr 2014 10:25:06 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Michael Kerrisk <mtk.manpages@...il.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>,
	gthelen@...gle.com, aswin@...com, linux-mm@...ck.org
Subject: Re: [PATCH 0/4] ipc/shm.c: increase the limits for SHMMAX, SHMALL

On Mon, 2014-04-21 at 16:26 +0200, Manfred Spraul wrote:
> Hi all,
> 
> the increase of SHMMAX/SHMALL is now a 4 patch series.
> I don't have ideas how to improve it further.

Manfred, is there any difference between this set and the one you sent a
couple of days ago?

> 
> 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.

Sorry but I don't quite get this. Using 0 eliminates the need for all
these patches, no? I mean overflows have existed since forever, and
taking this route would naturally solve the problem. 0 allocations are a
no no anyways.

I do agree with the series iff we endup taking this 'increase the limit
size approach'. But I just don't see the need.

Thanks,
Davidlohr

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